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Author Topic: Low RPM of modified Jett60  (Read 13101 times)

Offline Matt Piatkowski

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Low RPM of modified Jett60
« on: April 25, 2016, 07:09:46 AM »
Hello,
Please see the attachment.
Your comments will be appreciated.
Thanks,
Matt

Offline Dave_Trible

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Re: Low RPM of modified Jett60
« Reply #1 on: April 25, 2016, 08:40:02 AM »
Matt by looking at the photo this engine has been run very hot.  Could be it needs another rebuild.  Two things I'd do;  drop to a 12" prop so the engine isn't loaded so hard and pitch the fuel.  Get something better and with 23-24% min. oil content.  I use about 60-40 castor/Klotz in all my Jetts.  I like Byrons fuel which I add castor to.  Power master, Sig or just about anything is better than your current fuel.  I ruined several engines years ago on that stuff.

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Offline Tim Wescott

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Re: Low RPM of modified Jett60
« Reply #2 on: April 25, 2016, 10:18:04 AM »
He mentions that it still has a lot of pinch at the top -- that sounds like maybe it needs more breaking in.

I agree that trying a smaller prop would be a Good Things -- a big prob would both lug it down and make it run hot.

Brett Buck has also made some sensible comments on how uncowled engines don't cool as well as properly-cowled engines.  I've never experimented with it, but baffles to channel the air around the back of the cylinder may help.

Oh -- good looking plane.  I like the natural wood finish on the fuselage.
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Offline FLOYD CARTER

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Re: Low RPM of modified Jett60
« Reply #3 on: April 25, 2016, 12:05:29 PM »
The typical RO_Jett engine will almost always have a "pinch" at TDC if turned over slowly, even after many hours run time.  I hope you followed the break-in procedure recommended by RO.  Burn marks all over cylinder and muffler might indicate a fuel leak in cylinder head, and certainly at the muffler connection.  The muffler flange needs to be lapped straight.

You must try different fuels.  Even SIG 10-10-10 is close to the recommended mix.
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Offline Randy Cuberly

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Re: Low RPM of modified Jett60
« Reply #4 on: April 25, 2016, 12:41:34 PM »
I have and run several RO Jett engines and have discovered ,like others that they do not like a lot of castor oil and actually tend to run hot when a lot of castor is used.

I agree with Floyd that the burned oil appearing on the outside of the engine indicates a leak, probably in the muffler interface but would carefully loosen the head bolts and re-torque them in a cross pattern just as insurance that the leak is not there.

I also agree that the pinch at the top of the stroke remains on a god RO Jett pretty much for the life of the engine.  I have a 65 that has been run for many hours over a period of many years with 15% nitro, 15% Klotz and 3% castor oil and the pinch still remains and the engine runs and feels as strong as it did when just broken in.

I also agree that the 13 inch prop is likely too much to allow the engine to achieve the RPM it needs to develop power in the proper range.

RO Jetts are not "Old School" engine technology and need to run in a slightly higher range.  My 61 seems to run best on a pipe with a three blade 12.25 X 4 narrow blade Medjlic Prop at a launch RPM of 9,900 to 10,000 RPM.  The 65 turns a slightly larger prop at about 9700 RPM.

Randy Cuberly
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John Leidle

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Re: Low RPM of modified Jett60
« Reply #5 on: April 25, 2016, 06:43:44 PM »
  I'm going to say the prop is more like a club than a prop.. unless you have reworked it. I tried a Zinger 13-6 just last Thursday on my ROJETT 67 I had the same results. I suggest a Master Airscrew or APC 12-6, 12-7 or a 13-6 in the same flavors  ...
                        John

Offline Matt Piatkowski

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Re: Low RPM of modified Jett60
« Reply #6 on: April 28, 2016, 04:12:31 AM »
Guys,
Thank you for your thoughts.

I am running APC 12x7 on this Jett today and will try to get better fuel - most likely Byron that is available from time to time but only in some hobby stores in the Greater Toronto Area.

Burn marks are from the leak between the header and the cylinder before I put the aluminum shim, cut from the beer can, there. The engine also spits the fuel from venturi from time to time so some of it lands on the cylinder and burns. When I run this engine with Windy's pipe (2014 and 2015), there was also a leak between header and pipe. I fixed it in 2015 but shortly after that I smashed my Great White with this Jett. I had so many different problems with the piped setup that I temporarily gave up and will run this engine using the header muffler only.

This sideways mounted Jett60 is now hard to start and requires priming. The glow plug is good so I started thinking that perhaps the clunk tank I am using in my RToucan is the reason. When the model stands on the ground, the clunk is about 1.25" below the spray bar hole level. Flipping the prop while closing the venturi inlet with the thumb draws fuel in ( I see it in the silicone fuel feed tube) but, when the thumb is removed and the prop is flipped fast, the engine remains dry - there is no kick from the glow plug and no burp. Only after priming and flipping the prop a few times I can get a kick from the glow plug and some burps.

My other plane (Parrot) has Evo36 mounted also sideways. This engine uses a typical wedge style soldered metal tank in which the fuel feed tube end is almost on the level with the spray bar hole while the plane stands on the ground and the engine starts easily and without priming.

Question: perhaps for the side mounted engines using the clunk tank, the tail of the plane must be temporarily raised while starting the engine to eliminate the pressure difference between the clunked end of the fuel feed tube and the spray bar hole?

Thank you,
Matt


Offline Balsa Butcher

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Re: Low RPM of modified Jett60
« Reply #7 on: April 28, 2016, 01:49:06 PM »
Is this the cast case GMA Jet from the 90's? They did have a lot of pinch up top but that should not be a problem. I had the exact same issue with a cast-case 51, same era. As suggested, the cure was a smaller prop. Settled on an APC 11.5 x 4, tuned pipe, 10,200 launch RPM. 12x7 is too much pitch, try a 12x4  and let 'er rip. While not extreme high RPM engines, they run well in the high 9's to low 10's, rpm wise. Not luggers like Stalkers or ST-60's.

Fuel wise, manfacturer  recommends is 17% synthetic, 5% castor. Powermaster used to make that blend. Although it is discontinued it can be approximated by adding 2 ounces of castor and 2 ounces of synthetic to good quality (powermaster, sig, byron) R/C 10% nitro fuel. This suggested by a top level flier who uses these engines. It works. Good Luck.
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Offline Matt Piatkowski

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Re: Low RPM of modified Jett60
« Reply #8 on: April 28, 2016, 03:39:12 PM »
Hello guys,
It is a crankcase from 1999 - George Aldrich made some changes to this particular Jett60 and I bought it from him.

I cannot use this engine with the pipe as the plane I am using it in ( modified Score; new name RToucan) cannot accommodate pipes - this Jett 60 must be muffled and it is.

I run it today on the bench with 12x7 APC, 10% nitro fuel ( ~21% total fuel; 75% synthetic, 25 castor) and got max. stable RPM = 8,500.

The engine was in pure 2 stroke. I know the pitch is too high according to the stunt community opinion but I need more speed and hopefully I will get it.

I am trying this engine with this prop and this fuel tomorrow flying RToucan on 66' long lines.

If I can get 5.1-5.3 s laps and good lines tension, the problem will be solved as I will be able to practice square figures. If the plane is still sluggish and there is no enough lines tension, I will replace Jett60 with Ro-Jett76 that will give me all the power I need.
Thanks,
M


Offline Balsa Butcher

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Re: Low RPM of modified Jett60
« Reply #9 on: April 28, 2016, 08:05:55 PM »
You would be better off running a 12x5 at around 9600 RPM on muffler. Should be doable.  8)
Pete Cunha
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Offline Brett Buck

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Re: Low RPM of modified Jett60
« Reply #10 on: April 28, 2016, 09:57:07 PM »
Hello guys,
It is a crankcase from 1999 - George Aldrich made some changes to this particular Jett60 and I bought it from him.

    This is straight from GMA, unmodified from when he was working on it?  If so, I think I see the issue. It might be able to turn any prop from a 10-6 to a 15-6 at 8500 rpm.

    If you can do it safely, I suggest measuring the head clearance as-supplied. If it is an engine set up by GMA, it will very likely have an enormous head clearance, and will likely require LARGE props at low revs, and probably a larger venturi. I think the original GMA-Jetts used something around a #1 drill size with no spigot (.228), or maybe larger.  It will *not* work like a conventional stunt engine.

   Before changing anything, check the clearance and check back. Alternately. although this isn't the greatest or most reliable way to measure, if you have a compression pressure gauge, that will work in a pinch. For reference, my PA and RO-Jett 61s send the pressure up to about 115-120 psi, and the first GMA-Jett we tested was more like 35 psi - and it was not from leakage, it was from the 1/16" of head gaskets.

    Brett
« Last Edit: April 28, 2016, 11:19:16 PM by Brett Buck »

Offline Matt Piatkowski

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Re: Low RPM of modified Jett60
« Reply #11 on: April 29, 2016, 05:57:38 AM »
Pete,
I will try 12x5 APC or/and MAS and if I can get 9,600 RPM perhaps it will be enough.
By the way: my old Magnum PRO 45 turns 11x7 APC propeller at 10,250 on the ground using the same fuel as Jett60 so either this Magnum is quite "powerful" or my Jett60 is rather "weak". Again, only 8,500 RPM from this Jett on 12x7 propeller yesterday. I know that "weak" and "powerful" are not precise technical terms but this is the best description at this moment.

Brett,
Jett60 was straight from GMA in 2000 when I bought it and it had white, short, "plastic" venturi with the throat airflow diameter of 0.226" and the fuel delivery hole of 0.100". The spray bar fuel delivery hole was (and still is) 0.080". There was only one shim below the cylinder head but, unfortunately, I did not measure it so I do not know if it was "thick", "thin" or "normal".

When I got this engine from George Aldrich sixteen years ago, I knew very little about stunt and the complexity of two cycle ICE. I run this Jett only once in 2002 or 2003 using 12.75x4 Zinger propeller with Windy's carbon composite pipe, having straight end with a very short stinger. I did not measure the ground RPM at that time.

Then, the engine sat on the shelf for 11 years and I was working hard to build my retirement fund.

In 2014 the engine was dusted, inspected, cleaned and I started learning flying my IntrepidXL renamed Great White. I used 12.75x4 Zinger (cut from 14x4 therefore it was wide), Windy's pipe and 10% nitro Omega with some castor added. in 2014,I could not get launch RPM higher than 10,000  using this setup - the best was 9,300-9,600 so I was never sure if the pipe was actually doing anything except of muffling the noise. One hot day in 2015 Spring, the engine slowed down suddenly while the plane was overhead. The guy flying the plane, a very good stunt pilot, regained the control of the plane falling vertically on him and landed safely.

Jett60 lost almost the entire compression and it was very easy to turn the prop using pinky.

I mailed the engine to Dub Jett who replaced the worn out piston and installed the new Ro-Jett liner. I also got the brand new, black "plastic" (what is this material...is it Rilsan?) stock venturi with the throat airflow diameter of 0.186" and the fuel delivery hole of 0.060".

I partially broke this hybrid Jett flying my Great White but, in the end of 2015 Summer, I destroyed the plane. Jett60 survived with minimal damage and I am using it now to fly RToucan ( former name Score).

I know now much more about stunt and ICE and hope to move in small steps to fine tune this engine. I have finished the engine break process, compression is excellent ( Dub installed only one thin shim) and I will be experimenting with venturis and props. Like I wrote before, I need more launch RPM therefore the first step is to open up the venturi airflow - I will use again GMA white venturi with 0.226" opening and we will see.

Personally, I like 12x7 APC setup with the muffled Jett60 as I will be able to compare it to my Magnum Pro45 turning 11x7 APC but, again, I need more launch RPM.

If GMA venturi gives me this 1000 RPM I am looking for ( 9,500 RPM on 12x7 APC will be enough for sure as RToucan weights RTF dry 65 oz. and has more than 700 sq.in. wings area with flaps), the problem will be solved for a while. I like to have a lot of lines tension everywhere as this gives me this good feeling that the plane will not fall from the sky when there is a wind gust. Only when I learn the entire pattern, I will be looking for true fine tuning of lap times, tension and so on.

Best Regards,
M







   


Offline Brett Buck

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Re: Low RPM of modified Jett60
« Reply #12 on: April 29, 2016, 11:10:04 AM »

If GMA venturi gives me this 1000 RPM I am looking for ( 9,500 RPM on 12x7 APC will be enough for sure as RToucan weights RTF dry 65 oz. and has more than 700 sq.in. wings area with flaps), the problem will be solved for a while. I like to have a lot of lines tension everywhere as this gives me this good feeling that the plane will not fall from the sky when there is a wind gust. Only when I learn the entire pattern, I will be looking for true fine tuning of lap times, tension and so on.


    Matt, the reason I ask is that your numbers  (as in the previous thread) are so astronomically off what any other RO-Jett does, that I have no idea what is going on here. There appears to be something seriously wrong with a RO-Jett 61 that wants to run at 8500 RPM. A Zinger is a poor prop that would have to be extensively reworked to get a decent flight, but I am sure I could get my engine to spin a dinky prop like that at 11000 rpm on open exhaust. Ted ran his RO-Jett 61 on a header muffler (maybe the same one) and it was 4-stroking at 9000 or so with a 12.5-5 3-blade. My piped version can easily spin up a 12.5-3.75 Eather at 12000 rpm on the ground and it's set up to try to drop off dramatically at 11,000.

   So I really don't understand, at this point, what could be wrong with it. If GMA set it up, I would understand, because his engines *did* run like yours, due to extremely low compression ratio. It required a large prop with a large venturi, and lots of pitch.  But that would be with his head and compression setup.

   If you have something like a conventional stunt prop, (APC or Rev-up 12-4, or similar) we might be able to figure something out but you are far off normal performance.

    Brett

   

Offline RandySmith

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Re: Low RPM of modified Jett60
« Reply #13 on: April 29, 2016, 12:42:43 PM »
Have you ran this engine on a 12 x 4 prop? it would help to see the peak RPM numbers from that. Also do you have a set of calipers so you can measure the deck height , instead of guessing?

Both will help a lot in helping you

Randy

Offline Matt Piatkowski

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Re: Low RPM of modified Jett60
« Reply #14 on: April 29, 2016, 03:20:48 PM »
Randy, Brett,
Please see the attached and I will follow your advice.

Randy,
What is "deck height"?
I do have a decent quality and precise electronic caliper.

Thanks,
M

Offline Tim Wescott

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Re: Low RPM of modified Jett60
« Reply #15 on: April 29, 2016, 03:52:43 PM »
Matt:

Deck height is the distance from the center of the crankshaft to the top of the cylinder.  If you measure the distance from the piston top to the sleeve top at both top-dead center and top-bottom center, Randy will (probably) know how the engine compares to what it's supposed to be.

Your title says "modified" but your text says that the liner has been replaced.  Is the engine a repaired stocker, to your knowledge?  Or is it truly modified?
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Offline RandySmith

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Re: Low RPM of modified Jett60
« Reply #16 on: April 29, 2016, 03:55:46 PM »
Randy, Brett,
Please see the attached and I will follow your advice.

Randy,
What is "deck height"?
I do have a decent quality and precise electronic caliper.

Thanks,
M

The height of the piston, at top dead center, from the head, just measure with head off, put the piston at top dead center before removing the head, measure the distance from the piston top to the top of the sleeve, the measure the plug -in depth  of the head, subtract to get the clearance or deck height.

Run the MAS and APC 12x 4   and  check peak RPMs on these  2 props

Randy

Offline Tim Wescott

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Re: Low RPM of modified Jett60
« Reply #17 on: April 29, 2016, 04:00:33 PM »
The height of the piston, at top dead center, from the head, just measure with head off, put the piston at top dead center before removing the head, measure the distance from the piston top to the top of the sleeve, the measure the plug -in depth  of the head, subtract to get the clearance or deck height

Randy

I said "WHAT?" to this, then went wiki-ing.  I found this (from here):

"First, know that there are two deck heights on a motor. Block deck height is the measurement from the centerline of the main bearing bore to the flat part of the block deck where the heads bolt on. On a small block Chevy, this dimension is usually given as 9.025" from the factory. The block deck height is not to be confused with piston deck height, which is the measurement from the piston crown to the block deck where the heads bolt on, with the piston at top dead center."

So, I was giving the definition of block deck height, while you're using the definition for piston deck height.  I think most automotive gear heads, when they say "deck height" mean "block deck height".
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Offline RandySmith

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Re: Low RPM of modified Jett60
« Reply #18 on: April 29, 2016, 04:35:40 PM »
Hi Tim

The block deck height of a "model airplane engine" is not really used, the helpful deck height is the top of piston to head clearance. and to be
done correctly , for block deck height...you would need to remove the sleeve , measure the block deck, sleeve lip, take into account any sleeve shims etc  and then what?  HB~> #^
Deck height of the piston top , to bottom of head, is the thing we are using, and it is the most useful measure

regards
Randy
« Last Edit: April 29, 2016, 07:22:29 PM by RandySmith »

Offline Tim Wescott

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Re: Low RPM of modified Jett60
« Reply #19 on: April 29, 2016, 05:47:56 PM »
@Randy: yup.  At work I'm the guy integrating the work of a bunch of other disciplines, so I've learned that some word that means one thing in one discipline means something entirely different somewhere else.  Of course, I regularly forget this, but I have learned it.

@Matt: Is this the same Jett60 that has the fuel draw problems?  Is it stock?
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Offline Brett Buck

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Re: Low RPM of modified Jett60
« Reply #20 on: April 29, 2016, 06:01:01 PM »
Randy, Brett,
Please see the attached and I will follow your advice.

Randy,
What is "deck height"?
I do have a decent quality and precise electronic caliper.

     I would use the 12.25-3.75, since we have a lot of experience with those, and, it is a *very* light load for this engine. I would expect a 4-stroke at around 10,000, and a peaked-out lean setting near 13000. If

     Brett

Offline Matt Piatkowski

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Re: Low RPM of modified Jett60
« Reply #21 on: April 30, 2016, 05:26:50 AM »
Tim,
This is the one and only Jett60 that has the fuel draw problem but the fuel draw problem is becoming a secondary issue compared to low launch RPM.
I am driving now to our club and will be testing the engine with all the propellers selected during my latest discussion with you, Randy and Brett.

The engine will be tested in my RToucan, using the SS-8 clunk tank installed just behind the firewall. When the plane is held parallel to the ground, the mid-plane of the tank is on the level of the fuel hole in the venturi spray bar. I am using the shortest possible fuel lines and the tank installation is as follows: fuel draw silicone tube has the clunked end, fuel intake tube is plugged after fueling and the shape of this tube inside the tank is such that it reaches to the very top of the tank, allowing to fill the entire available volume with fuel. The third tube, short inside the tank, is connected via the silicone tube to the brass "S" bent metal tube that allows the ram pressure to "enter" the tank in flight. I am not sure if such tank is uniflow or not.

Please feel free to comment/advise re: this tank.

There are fuel and air filters installed.

Brett,
I will test the engine with APC 12.25x3.75.

Randy,
After the test, I am going to remove the engine from the plane, disassemble it and measure the deck height according to your description. I will also provide the photos of old GMA liner (or sleeve), old GMA piston and will measure the GMA shim, that I found (hurray!!) in one of my junk boxes.

Wish me luck but you know what? I truly enjoy all this trouble shooting and digging deeper and deeper because I believe that the truth is out there (this is of course from the X-Files but they, whoever they really were, dealt with UFOs and not with the C/L planes that seem to be much harder)
Cheers,
M

Offline Matt Piatkowski

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Re: Low RPM of modified Jett60
« Reply #22 on: April 30, 2016, 12:57:27 PM »
Hello again,
Just came back from the circle.
For the ground tests, I have used 10% nitro fuel with ~21% total oil(75% of it synthetic, 25% castor) and GMA original, white venturi, ID of the airflow 0.226".

Results:
Prop 1 - APC  12x7, max.RPM 8,700-8,600, full 2 cycle. Time of run: ~5 minutes
Prop 2 - MAS 12x4, max.RPM 11,300, full 2 cycle. Time of run: ~5 minutes
Prop 3 - APC  12x4, max RPM 11,250, full 2 cycle. Time of run: ~5 minutes
Prop 4 - APC 12.25x3.75, max RPM 10,900, full 2 cycle. Time of run: ~5 minutes ( Brett....I could not get more from this engine with this prop).

Then, I flew my RToucan with 12x7 prop. on 66' long lines for ~ 6 minutes. Laps: ~5.4 s, adequate lines tension but not enough to start seriously practicing the square figures so I made several normal and inverted loops, OH 8 and inverted flight and landed.

M

 

Offline Brett Buck

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Re: Low RPM of modified Jett60
« Reply #23 on: April 30, 2016, 02:27:34 PM »
Prop 4 - APC 12.25x3.75, max RPM 10,900, full 2 cycle. Time of run: ~5 minutes ( Brett....I could not get more from this engine with this prop).
 

   Take the muffler off, and try it again (wear ear protection!).

     Brett

Offline Tim Wescott

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Re: Low RPM of modified Jett60
« Reply #24 on: April 30, 2016, 02:37:14 PM »
I'm not familiar with the engine, but it just sounds to me like there's something wrong with it.  It's why I keep asking about whether it's stock.

Did you check to make sure that it's got good primary compression -- i.e., is the crankcase seal good?  Air could be leaking at the backplate, the crankshaft, and the piston/liner seal.  In extreme cases (you DID say you replaced the liner) it could be leaking between the liner and block at the exhaust port.

While you are taking it apart, make sure that the liner is in the right way around.  It's moderately easy in a baffle-piston engine to put it in backwards -- it's a little harder with a Schnuerle-ported engine.  Either way, if you get it backwards the engine might run, but it'd be sick.  Anyways, the exhaust should open first, then the bypass port(s), then (if there is one, I think) the boost port.

When you have it apart, take pictures of the bits & pieces.  That won't help me to help you, but I bet that it'll fire off some thoughts in the heads of competent people like Randy and Brett.
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Offline Lauri Malila

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Re: Low RPM of modified Jett60
« Reply #25 on: April 30, 2016, 02:56:01 PM »
...also, you should measure the head volume with piston in tdc. It's easy with a small syringe with fine needle, from plug hole.
Just the stack of shims tells nothing if the head has been modified.
In general we should make a more clear difference between head volume and squish clearance, both are important.
I don't know exact values when fuel with nitro is used, but with no nitro 80/20% fuel the head volyme of a .60 should be around 1,15..1,20cc.

Lauri

Offline Brett Buck

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Re: Low RPM of modified Jett60
« Reply #26 on: April 30, 2016, 03:04:49 PM »
I'm not familiar with the engine, but it just sounds to me like there's something wrong with it.  It's why I keep asking about whether it's stock.

  Something is indeed wrong with it. On open exhaust, a 40VF will spin the APC about 13,000 rpm with a similar choke area. My RO-Jett, with no modificatons and with a pipe set to regulate at about 11,000, would do about the same, and much faster on open exhaust. I haven't tried it but I would expect Matt's level of performance with the liner in backwards.

   It's not a stock RO-Jett in terms of functional parts, or something is broken.  What makes me very suspicious is that it was originally a GMA engine, and it is operating about like how George set them up to run - no power/very limited RPM range. But even the stock GMA engines could fairly easily be improved by setting the head clearance to normal (about -.025 vice about .080-.085) and reducing the venturi to something more normal. They had other issues but they would run about like a normal engine then.

    Brett

Offline RandySmith

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Re: Low RPM of modified Jett60
« Reply #27 on: April 30, 2016, 06:39:54 PM »
Hello again,
Just came back from the circle.
For the ground tests, I have used 10% nitro fuel with ~21% total oil(75% of it synthetic, 25% castor) and GMA original, white venturi, ID of the airflow 0.226".

Results:
Prop 1 - APC  12x7, max.RPM 8,700-8,600, full 2 cycle. Time of run: ~5 minutes
Prop 2 - MAS 12x4, max.RPM 11,300, full 2 cycle. Time of run: ~5 minutes
Prop 3 - APC  12x4, max RPM 11,250, full 2 cycle. Time of run: ~5 minutes
Prop 4 - APC 12.25x3.75, max RPM 10,900, full 2 cycle. Time of run: ~5 minutes ( Brett....I could not get more from this engine with this prop).

Then, I flew my RToucan with 12x7 prop. on 66' long lines for ~ 6 minutes. Laps: ~5.4 s, adequate lines tension but not enough to start seriously practicing the square figures so I made several normal and inverted loops, OH 8 and inverted flight and landed.

M

 

Hi Matt

It is weak.. when you take it down to measure, pictures may help a lot in diagnosing the problem, take one of the head, from the plugin side

Randy

Offline Matt Piatkowski

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Re: Low RPM of modified Jett60
« Reply #28 on: May 01, 2016, 01:00:44 PM »
Randy, Brett and everybody else,
Please see the attached.
I hope the measurements of Deck Height were taken correctly.
Thanks,
M

Offline Brett Buck

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Re: Low RPM of modified Jett60
« Reply #29 on: May 01, 2016, 01:20:34 PM »
Randy, Brett and everybody else,
Please see the attached.
I hope the measurements of Deck Height were taken correctly.

    No, I don't think so. Measure the distance from the "shoulder" of the head button (the flange that rides on the liner) to the edge of the button insert.

     What we want is how far the internal part of the button clears the piston at TDC. For a properly running engine it could be anywhere from .005 to about .030, for your engine we suspect that it is much larger than that. But it is not .338.

      The "head"/clamp ring does not have anything to do with it. It's how far the button sticks down into the cylinder.

      Brett

Offline RandySmith

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Re: Low RPM of modified Jett60
« Reply #30 on: May 01, 2016, 01:51:58 PM »
Randy, Brett and everybody else,
Please see the attached.
I hope the measurements of Deck Height were taken correctly.
Thanks,
M

Hi Matt
Just FYI , the ports your calling transfer ports are the 2 main Schnuelre ports , the single intake port you identify  as the intake port is the intake Boost port.
Looking at the engine, I do not think you will  ever get a decent run out of it with the 2 good mounting holes used, you will have to use a Clamp type mount across the luggs, or a backplate mount, or combo of both.
Can you tell me how you are mounting the motor?
please do as Brett asked and find the measure of how much the head plugs into the sleeve, and the measure of the sleeve at TDC to the top lip of the sleeve

Randy

Offline Steve Hines

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Re: Low RPM of modified Jett60
« Reply #31 on: May 01, 2016, 02:32:35 PM »
Don't know if this will help but I got a 76 that was used, and the cylinder 180 deg off. I did not think a motor would run like this, but it did. It had no power and you could not get any rpm. I could not tell just looking in the exhaust port.

Steve

Offline Brett Buck

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Re: Low RPM of modified Jett60
« Reply #32 on: May 01, 2016, 03:12:25 PM »
Don't know if this will help but I got a 76 that was used, and the cylinder 180 deg off. I did not think a motor would run like this, but it did. It had no power and you could not get any rpm. I could not tell just looking in the exhaust port.

     They will definitely run that way, as you say, no power and usually very hot.

   Brett

Offline Matt Piatkowski

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Re: Low RPM of modified Jett60
« Reply #33 on: May 01, 2016, 04:02:19 PM »
Randy,Brett and everybody else,
Please see the attached.

Brett wrote: "...It's how far the button sticks down into the cylinder."
In this engine the button sits on the 0.010-0.011" shim, the shim sits on the top flange of the liner(sleeve) and the liner sits on the surface of the "step" machined in the cylinder cast. If the shim is removed, the volume of the combustion chamber will slightly decrease and the compression will increase.

Perhaps this will help in increasing the RPM?

Randy,
You wrote:"...you will have to use a Clamp type mount across the luggs, or a backplate mount, or combo of both.".

I am using two 1/8" thick Al 2024 T6 plates to clamp the engine using all four mounting screws. The crutch is R/C style, with two reinforced nylon "T" mounts.
There is no excessive vibration.

M



Offline Steve Hines

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Re: Low RPM of modified Jett60
« Reply #34 on: May 01, 2016, 04:10:03 PM »
Was told that after a crash, it was cleaned up and never ran good after.  He was going electric, I new dub would fix it if I couldn't. Don't know if this one had been cleaned at sometime.

Steve

Offline Brett Buck

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Re: Low RPM of modified Jett60
« Reply #35 on: May 01, 2016, 04:11:33 PM »
Randy,Brett and everybody else,
Please see the attached.

Brett wrote: "...It's how far the button sticks down into the cylinder."
In this engine the button sits on the 0.010-0.011" shim, the shim sits on the top flange of the liner(sleeve) and the liner sits on the surface of the "step" machined in the cylinder cast. If the shim is removed, the volume of the combustion chamber will slightly decrease and the compression will increase.

  Does the button extend down into the cylinder?  It certainly appears to, but your drawing doesn't show that.  How far?  We don't care about how the liner fits the case, or the measurements of the flange on the liner, or the clamp fits the button or case, we care how close the piston comes to the lowest part of the button inside the cylinder.

   If it does fit as your drawing shows, we certainly have an answer, but it almost certainly doesn't.

     Brett

Offline Brett Buck

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Re: Low RPM of modified Jett60
« Reply #36 on: May 01, 2016, 04:13:31 PM »
Was told that after a crash, it was cleaned up and never ran good after.  He was going electric, I new dub would fix it if I couldn't. Don't know if this one had been cleaned at sometime.

Steve

  Not very well, if so, because after 9+years mine has only the lightest of light browning of the head, piston and liner. Another unusual item is that the intake ports show significant varnishing, which is also unusual.

    Brett

Offline Matt Piatkowski

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Re: Low RPM of modified Jett60
« Reply #37 on: May 01, 2016, 05:28:09 PM »
Sorry Guys...my sketching mistake.
Please see the attached.

By the way, Brett...you used the term "cylinder" and the button goes down 0.080" inside the liner(sleeve).

Now what?

Thanks,
M

Offline RandySmith

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Re: Low RPM of modified Jett60
« Reply #38 on: May 01, 2016, 05:37:45 PM »
HI  Matt
I am missing the info?? we need the measure from the top of piston, to the top of the sleeve, at top dead center.
and the measure of the depth of the head button insert, ..how much does it go into the sleeve
forget any shims or gaskets
forget the case
forget the head clamp

just need those two measurements

Regards
Randy

Offline Brett Buck

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Re: Low RPM of modified Jett60
« Reply #39 on: May 01, 2016, 07:52:43 PM »
HI  Matt
I am missing the info?? we need the measure from the top of piston, to the top of the sleeve, at top dead center.
and the measure of the depth of the head button insert, ..how much does it go into the sleeve
forget any shims or gaskets
forget the case
forget the head clamp
Randy

  Piecing it together from the new post, the previous post (where the piston came up 0.100 below the rim of the liner), I think his clearance is around .030, which is on the large side but not enough to cause this issue.

   Maybe the button has much less volume than any of mine, but I have run mine with similar clearance, and nothing surprising. I think it is something else.

    Brett

Offline RandySmith

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Re: Low RPM of modified Jett60
« Reply #40 on: May 01, 2016, 08:14:35 PM »
I wanted to be certain, he wrote dim A  is  80 thou ?  I thought that was  about 100 thou, also he is using what looks to be either a 10 or 11 thou head shim
If this is the case I would setup the motor with 20 to 25 thou head to piston clearance and run the engine again. this would give a base line that we know exactly what it is.

One other thing, I have seen too much clearance in the outside sleeve to case fit cause low power and erratic runs
maybe you want to measure the case bore and the outside sleeve diameter. It has .008 thou clearance (way too much) , and is low in power and erratic running

Regards
Randy

Offline Steve Hines

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Re: Low RPM of modified Jett60
« Reply #41 on: May 01, 2016, 09:27:17 PM »
Matt take a minute and check to see the cylinder is 180 deg off. Brett said there was varnish on the intake ports,  this could be getting hot from the hot gases blowing down thru what should be the exhaust port, which is now the transfer port. Exhaust port will be higher port, this will be easy to check. Crazier thing have happened.

Steve

Offline RandySmith

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Re: Low RPM of modified Jett60
« Reply #42 on: May 01, 2016, 09:56:53 PM »
Don't think the liner was in backwards, he knows what port is the exhaust, and you can clearly see the much heavier  burn on outside and inside of the exhaust ports of both liners, looks like it has been ran very hot, or for a long time with heavy castor

Randy

Offline Matt Piatkowski

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Re: Low RPM of modified Jett60
« Reply #43 on: May 02, 2016, 07:37:53 AM »
I have decreased the castor component but apparently not enough.
As the engine seems to be more or less broken now, I will run it on fuels with synthetic oil only.

What about replacing the 0.010-0.011" shim with 0.003 or 0.004" and torquing the head screws really hard?

When the compression goes up the RPM should go up too.

I need perhaps 9,000 RPM on 12x7 APC prop. to have enough lines tension with still decent lap times.
Again, I know 12x7 is considered not good for competitive stunt but I am not competing on the high level only still learning to fly the pattern.

Anything else to try?

I can fly the plane with my EVO60 NX that is brand new and has the C/L venturi.

The exhaust timing of EVO60 NX is 150 degrees but if it runs faster than Jett60 on the same prop I will be ok.

If not EVO60 NX, I can mount the brand new Ro-Jett76 in my RToucan but this would require longer landing gear.

Any other ideas?

Regards,
M


 

Offline RandySmith

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Re: Low RPM of modified Jett60
« Reply #44 on: May 02, 2016, 08:02:20 AM »
I have decreased the castor component but apparently not enough.
As the engine seems to be more or less broken now, I will run it on fuels with synthetic oil only.

What about replacing the 0.010-0.011" shim with 0.003 or 0.004" and torquing the head screws really hard?

When the compression goes up the RPM should go up too.

I need perhaps 9,000 RPM on 12x7 APC prop. to have enough lines tension with still decent lap times.
Again, I know 12x7 is considered not good for competitive stunt but I am not competing on the high level only still learning to fly the pattern.

Anything else to try?

I can fly the plane with my EVO60 NX that is brand new and has the C/L venturi.

The exhaust timing of EVO60 NX is 150 degrees but if it runs faster than Jett60 on the same prop I will be ok.

If not EVO60 NX, I can mount the brand new Ro-Jett76 in my RToucan but this would require longer landing gear.

Any other ideas?

Regards,
M


 

Hi Matt
try it with the head set at 20 to 25 thou  from the piston top, looking at what you have there maybe use 1 .004 gasket only, see what the RPMs are then
I also have to ask, is the engine really free turning? the prop should have no drag off compression and does it pop back and forth when flipping it?  I am wondering if it got more damage in the crash?

Randy

Offline Matt Piatkowski

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Re: Low RPM of modified Jett60
« Reply #45 on: May 02, 2016, 08:48:52 AM »
Hi Randy,
I will use the 0.004" shim and run the engine with 10% nitro fuel with 17% synthetic oil and only few drops of castor using the same props as before.

The engine is turning loosely (the prop pops back and forth 3-4 times when flipped by hand) when wet after extra priming using a squirt of fuel directly into the venturi opening. This kind of extra priming solved the problem of difficult starts: after refueling the tank to full, I turn the prop slowly 5-6 times with my finger on and off the venturi opening, then I flip the prop fast 6-7 times to spread the fuel, then I inject ~ 0.5 ccm of fuel into venturi with the prop turned CCW for the fuel to reach the fuel inlet in the shaft, then I plug in the starter, flip the prop hard and the engine starts. It starts slower at first but I keep the starter hooked up for 2-3 seconds more and it reaches the RPM. When the starter is removed then, there is no dip in RPM.

I could not check the bearings after the crash as I do not have the tools required to completely disassemble the engine and put it back together.

It is possible that one or both bearings have some internal damage to the rings or/and the balls retainers (baskets) as there is a very weak clicking sound coming from inside but only from time to time. This sound is very weak and there is no resistance associated with it.

There is also a little play ( ~0.03") on the shaft along its axis. Simply: when the shaft is pulled and pushed, the end moves ~0.03" back and forth. This play is not present when the propeller is installed - only when the shaft threaded end is held by fingers and pushed or pulled.

Question: how hard can I torque the head mounting screws in order not to damage the Aluminum cast cylinder? I wonder if the RPM problems are not caused by the fuel leak between the button and liner. I cannot see anything leaking there when the engine is running but perhaps the shim was not squeezed enough?

Thanks for trying to help,
M




Offline Tim Wescott

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Re: Low RPM of modified Jett60
« Reply #46 on: May 02, 2016, 09:49:13 AM »
I could not check the bearings after the crash as I do not have the tools required to completely disassemble the engine and put it back together.

Didn't you have the piston out of it?  Most engines of that type will cough up a crank if you just lightly tap it out (but -- I don't have a Jett-anything so I may have my head someplace dark and warm).

It is possible that one or both bearings have some internal damage to the rings or/and the balls retainers (baskets) as there is a very weak clicking sound coming from inside but only from time to time. This sound is very weak and there is no resistance associated with it.

If you can get it down to the point where the piston is out (which you have) you should be able to do a quick functional test -- spin the crank, and feel to see if it turns smoothly.  If it feels like there's gravel in there, or if it wants go to some preferred position, then the balls or races are damaged.  If the bearings are the culprit, the nose of the engine would be getting hot-hot-hot.

There is also a little play ( ~0.03") on the shaft along its axis. Simply: when the shaft is pulled and pushed, the end moves ~0.03" back and forth. This play is not present when the propeller is installed - only when the shaft threaded end is held by fingers and pushed or pulled.

This sounds normal to me for this general kind of engine, but I've never had a Jett, so...

Question: how hard can I torque the head mounting screws in order not to damage the Aluminum cast cylinder? I wonder if the RPM problems are not caused by the fuel leak between the button and liner. I cannot see anything leaking there when the engine is running but perhaps the shim was not squeezed enough?

Oh noooooo!  I couldn't give you a torque number, but if you reef on the bolts at all then you'll warp something.  If the thing won't seal correctly with the screws put in so they're just barely tight, then reefing down on them won't make it seal.  With metal to metal seals like this you're more likely to make them leak by over-tightening than by under-tightening.

You're clearly approaching this with the attitude of wanting to fix the engine yourself, so forgive me if I step on your toes here -- should you be considering sending it to Randy, or to some guy who's last name is Jett?  It may cost you a few bucks, but you'd get back a known-good engine, or a good story on why it's not worth it to pursue the matter.
AMA 64232

The problem with electric is that once you get the smoke generator and sound system installed, the plane is too heavy.

Offline RandySmith

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Re: Low RPM of modified Jett60
« Reply #47 on: May 02, 2016, 06:37:15 PM »
 " Question: how hard can I torque the head mounting screws in order not to damage the Aluminum cast cylinder? I wonder if the RPM problems are not caused by the fuel leak between the button and liner. I cannot see anything leaking there when the engine is running but perhaps the shim was not squeezed enough?

Thanks for trying to help,
M
"

You should not go past 15 inch pounds, 12 to 15 inch pounds would be plenty tight enough for th3 head bolts

Randy

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Re: Low RPM of modified Jett60
« Reply #48 on: May 02, 2016, 10:28:29 PM »
    An   APC 12-7   @ 9,000 revs?   Hang on,, I used that prop on a PA.65 SE & set it at 7,800 ton of pull.
  John

Offline Brett Buck

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Re: Low RPM of modified Jett60
« Reply #49 on: May 02, 2016, 11:13:10 PM »
    An   APC 12-7   @ 9,000 revs?   Hang on,, I used that prop on a PA.65 SE & set it at 7,800 ton of pull.
  John

   The fact that it goes so slow on  conventional stunt prop (APC 12.25-3.75, originally designed for a mere 46VF) tells me there is something seriously wrong with it. As I recall, the 46VF should launch around 11,000 RPM with that prop, even with a pipe keeping it from going a lot faster.

    Apparently it is not due to extremely low compression (like Randy and I both suspected) - in fact, I ran mine in the same range (assuming the button hasn't been modified) with nearly no change in the performance from the more nominal settings around .022. The fact that the case was effectively destroyed in a crash certainly one pause as well. But I expect the crankshaft to stay together and straight. Mine was driven about 2" into pretty hard blacktop and only the prop stud was bent.

   I am at a loss. Probably something we could figure out if it was in our hands, but I don't see us getting anywhere the way we have been going.  I would just send it back for a complete rebuild or replacement at this point.

     Brett


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