Author Topic: ST 46 ABC (Read 2868 times)

Paul Allen · « **on:** January 18, 2008, 01:58:51 AM »

Having fitted a Bri Stunt ABC piston/liner and hemi head
to my ST46 it was bench run then fitted in a model to see
how it went in the air. Initial runs were at my home field which
is at sea level,and had it spinning a Zinger 11x5 wood prop at 9,200 rpm
and the motor run was impressive.
For its next outing I had removed one 8 thou head shim to try for a
quicker break into 2 cycle,flying was done at another field which is above
sea level and requires the needle to be screwed in a bit to maintain the
set rpm.
My first flight I had to screw the needle out some to obtain 9,200rpm after
take off it went dead rich.
Rpm was checked again on the next flight with same result.
This had me a little confused removing a head shim had made the motor run
rich?So after the motor had cooled, off with the head and back in went the shim
this was a total of 25thou in head shims.
Motor was started and straight away it was rich so in with the needle to 9,200rpm on release it was back to normal, a level flight switching from 2to4 then 2 cycle
when the nose went up.
Three flights all good.
Venturi is a restrictor type of 0.285" dia,5% nitro.
Any idea's as to why it would run rich with a shim removed.

tom hampshire · « **Reply #1 on:** January 18, 2008, 06:25:24 AM »

Hi Paul - Might it be that the ambient tremperature went down for the second set of flights? Denser air could cause higher manifold vacuum and more fuel draw. Your situation certinly seems counterintuitive. Tom H.

don Burke · « **Reply #2 on:** January 18, 2008, 09:34:47 AM »

Sounds like dirt in the fuel line/NVA that cleared out.

RandySmith · « **Reply #3 on:** January 18, 2008, 09:58:36 AM »

"""My first flight I had to screw the needle out some to obtain 9,200rpm after
take off it went dead rich.
Rpm was checked again on the next flight with same result.
This had me a little confused removing a head shim had made the motor run
rich?"""

HI Paul

Several other things could happened here, however the motor could have gone rich because you turned the needle out and made the engine richer.
When adding or taking away a head shim, many times you *cannot* set the engine with a tach to the same RPMs. The motor is now setup differantly and will need a differant setting.
It sounds as if the motor was a little richer at the ground launch , then when it unloaded in the air again went richer. try the same setup with the gasket removed but set the engine to *sound the same*, that is sound like the same setting you had to start with.
Also compressing that engine on most setting will make the engine run in a stronger 4 cycle with much less breaks, and decompressing will make the engine run in much more of a 4-2 break. That is it will break earlier and more often with the less cpmpression.
You also would benefit from using a .003 or .005 shim differance, rather than using .008 , that sometimes, is a pretty coarse adjustment on a st 46.
The last comment is if the ABC piston sleeve set is not completly broken in it could have very easily heated up on the ground from the extra compression, this would require a richer needle setting on the ground, which would have gone even richer as it went up in flight , unloaded ,and cooled down

Regards

Randy

Paul Allen · « **Reply #4 on:** January 18, 2008, 06:41:23 PM »

Randy,
The last part of your reply about the ABC sleeve
may very well be the answer.It had run time on the bench
but not as much as the Aerotiger.
Will try your suggestion about it sounding the same on release.
PS the Aerotiger is one strong stunt motor!
Thanks guy's for the responses,much appreciated.

Brett Buck · « **Reply #5 on:** January 18, 2008, 09:05:53 PM »

Quote from: paul allen on January 18, 2008, 01:58:51 AM

Rpm was checked again on the next flight with same result.
This had me a little confused removing a head shim had made the motor run
rich?So after the motor had cooled, off with the head and back in went the shim
this was a total of 25thou in head shims.
Motor was started and straight away it was rich so in with the needle to 9,200rpm on release it was back to normal, a level flight switching from 2to4 then 2 cycle
when the nose went up.
Three flights all good.
Venturi is a restrictor type of 0.285" dia,5% nitro.
Any idea's as to why it would run rich with a shim removed.

Because it had too much compression? Randy and the guys may well be right, all of those things are possible. But the sort of behavior you saw used to happen from time to time with stock engines. In fact that very frequently happened when the engine had a lot of hard running and the stock head gasket compressed and extruded, reducing the clearance. When that happens, some engines, sometimes, ran "funny" like this. The fix was to replace the head gasket with a new one, bingo, back the way it was. It was more prone to this with larger props, so that doesn't quite fit the theory, but it sure sounds familiar.

Brett

RandySmith · « **Reply #6 on:** January 18, 2008, 09:41:07 PM »

Brett wrote "It was more prone to this with larger props, so that doesn't quite fit the theory, but it sure sounds familiar. "

Hi Brett

This fits perfectly with this behavoir, Larger props "will" load an engine more on the ground, after it gets airborne a large prop will unload and the motor will go richer. I have seen dozens of times people using props that are too large and it will many times make the engine harder to set on the ground, make the engine hotter on the ground and cause the very behavior we are speaking of. However in this case it seems like the unbroken in ABC P/S maybe the culprit.

Regards
Randy

Brett Buck · « **Reply #7 on:** January 19, 2008, 12:01:17 AM »

Quote from: RandySmith on January 18, 2008, 09:41:07 PM

Brett wrote "It was more prone to this with larger props, so that doesn't quite fit the theory, but it sure sounds familiar. "

Hi Brett

This fits perfectly with this behavoir, Larger props "will" load an engine more on the ground, after it gets airborne a large prop will unload and the motor will go richer. I have seen dozens of times people using props that are too large and it will many times make the engine harder to set on the ground, make the engine hotter on the ground and cause the very behavior we are speaking of. However in this case it seems like the unbroken in ABC P/S maybe the culprit.

I meant that the dinky prop he is using is a lot less prone to suffering from overcompression OR excess ground load/overheating. It would make more sense if he was using a 13-6 than an 11-5. It should be pretty happy at pretty high compression with an 11-5.

Brett

EddyR · « **Reply #8 on:** January 19, 2008, 01:38:17 PM »

I have the very first ST/46 ABC combo. I worked with Brian when he was developing them and I did almost all the testing before he released them. I found that the ABc setup he was selling needed very little break in. I bench ran my first one 15 minutes on the stand and then put it in a plane and it runs perfect every time.Every run is like the best ST/46 run you ever had. I had problems in the start if I used to much oil. The motor would go RICH. I am now running as little as 12-15% oil with 5% of it castor. That seems like a small amount of oil but the motor runs strong all the time. I also use a 11/5 prop . It will turn a 12.5/6 if you need it to but my plane is small and light. I have never seen the hemi head supplied by Brian as I made my own before he was including them with the ABC P&L . When I got my ABC set up for theST/35 I broke it in and ran it with this same low oil fuel. I have another ST/46 set up the same in another plane and it had problem from the start as described by Paul. I set it aside for a year and when I got it out I changed to the fuel with less oil and it runs fine. I suspect that my motors are higher in compression than Brian's with the head he supplies.I did a lot of testing in 1980 with head shape for the ST/46 and it is not a true hemi as a lot of people use to make them. What Randy said about jumping too much might have a lot to do with it and your fuel. Also I am using .292 intake. In the past when I used a .280 the motor seemed weak. The .280 intake may be holding you in a 4 cycle to long. There is almost no difference in fuel consumption between the two. (85% of every flight I have made since 1980 has been with a ST/46.
Ed Ruane

Paul Allen · « **Reply #9 on:** January 19, 2008, 02:47:28 PM »

Ed
Fuel mix is 11% castor,11% klotz,5% nitro,no
info with piston/liner on amount of oil to run.
Paul

RandySmith · « **Reply #10 on:** January 19, 2008, 05:29:43 PM »

I meant that the dinky prop he is using is a lot less prone to suffering from overcompression OR excess ground load/overheating. It would make more sense if he was using a 13-6 than an 11-5. It should be pretty happy at pretty high compression with an 11-5.

Brett

YEP that can and does happen too, if many engines are lightly loaded with high compression, the engine will also go rich when it unloads in the air. I have seen this so bad at times the engine shuts off rich. Normally the smaller props are much easier to set the needle on the ground though

Randy

EddyR · « **Reply #11 on:** January 20, 2008, 10:14:30 AM »

Paul When I ran 22% oil I had problems unless I ran very low compression. Even in my ringed version's I still have I run low castor,5-8%. Many motor rebuilders recommend the use "all castor " or 1/2 castor. All I am giving you is what has worked for me. I also found had problems with inconsistency with my Rustler 40's until I switched to low oil. Many years ago when Tom Dixon introduced the Double Star-50 he recommended low oil and I had problems until I changed to low oil fuel.
Ed

RandySmith · « **Reply #12 on:** January 20, 2008, 10:49:34 AM »

"I bench ran my first one 15 minutes on the stand and then put it in a plane and it runs perfect every time.Every run is like the best ST/46 run you ever had. I had problems in the start if I used to much oil."

Hi Ed

His may or may not be as loose as yours was, I have had many of Brian's PS in my shop fitting motors some were looser , some tighter, You just won't know unless you have his in your hands. Its tough to tell over the internet

I would use my standard blend fuel with this motor, I use 20 % total with 75 % synthectic oil. The ST 46 is a dual ball bearing engine so your not going to hurt it by going down to 18% ,as long as the mixture is kept a little on the rich side. I also have seen these , and ST 60s ABC run with SIG Champion very well and it is 50-50 with 1/2 being Bakers castor. Sounds to me like he just needs to do a little more flying and test out a couple more setting on his 46.

Randy

Richard Edwards · « **Reply #13 on:** January 21, 2008, 03:05:24 AM »

Can someone tell me where I can get head shims shims for Super Tigre G21/40 G21/46 and 60 engines I see them being mentioned but cannot find a supplyer.
Richard Edwards

Paul Allen · « **Reply #14 on:** January 23, 2008, 02:31:47 AM »

Thanks Ed and Randy,more testing to be done.

Ray · « **Reply #15 on:** January 23, 2008, 03:32:52 AM »

Quote from: Richard Edwards on January 21, 2008, 03:05:24 AM

Can someone tell me where I can get head shims shims for Super Tigre G21/40 G21/46 and 60 engines I see them being mentioned but cannot find a supplier.

Perhaps Frank Bowman ?

.

Bob Reeves · « **Reply #16 on:** January 23, 2008, 11:04:43 AM »

Paul and Ray.. Please go to your profile and remove your Zip code.. You are slowing down the forum.. For more info see the zipcode threads in the open forum.

Thanks

Dave Adamisin · « **Reply #17 on:** January 26, 2008, 07:37:58 PM »

A little note to add to this discussion. Oil lowers the octane of the fuel. It makes sence that high compression engines run better with lower oil contents.

RandySmith · « **Reply #18 on:** January 27, 2008, 12:28:27 PM »

Quote from: Dave Adamisin on January 26, 2008, 07:37:58 PM

A little note to add to this discussion. Oil lowers the octane of the fuel. It makes sence that high compression engines run better with lower oil contents.

Yes , And also sense it is a thicker mixture it tends to make the compression of the engine act even higher than it is, this can, and many times does make for a more ratty sounding run.

Regards
Randy

Dave Adamisin · « **Reply #19 on:** January 28, 2008, 07:54:37 AM »

Quote from: RandySmith on January 27, 2008, 12:28:27 PM

Yes , And also sense it is a thicker mixture it tends to make the compression of the engine act even higher than it is, this can, and many times does make for a more ratty sounding run.

Regards
Randy

Absolutly. The dynamic compression (displacement minus leakage) can go up due to less blow by. All of these subtle effects are what make some tunning mods less than what you expected.