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Engine basics => Engine set up tips => Topic started by: Motorman on July 31, 2016, 09:39:19 PM
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What was the size of the original Cox TD .049 venturi? As I recall it was .110" does that sound right?
MM
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What was the size of the original Cox TD .049 venturi? As I recall it was .110" does that sound right?
MM
Mine come out at .117" I.D.
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I made one that's .078" for sport flying. I'm trying to run it on 10% fuel so I don't have to maintain 1/2a fuel. I've noticed it likes to flame out if the needle's not fully tweeked out. I was wondering if I open it up to stock diameter if that would help so I could run it a crack rich. Already got the 1702 head with one gasket and running a 5-3 prop.
MM
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I'd never even run a Cox engine until my son gave me some he'd bought at a "going out of business" sale". One of them was a TD 049 so I gave it a try on the test stand with my usual zero nitro fuel. Once I found the right needle setting it would start with one flick hot or cold and would easily richen out to a cute little 4 stroke then back to an ear piercing 2 stroke. What a delightful little engine :).
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I made one that's .078" for sport flying. I'm trying to run it on 10% fuel so I don't have to maintain 1/2a fuel. I've noticed it likes to flame out if the needle's not fully tweeked out. I was wondering if I open it up to stock diameter if that would help so I could run it a crack rich. Already got the 1702 head with one gasket and running a 5-3 prop.
MM
You don't need high nitro or "1/2A fuel" to run 1/2A engines. I have run Cox engines on just about everything. They run the BEST on higher nitro fuels, but not absolutely necessary. Going that small on the intake is what is giving you problems. The stock venturi is intended for suction fuel feed. The way to get the TD to run like you want is head gaskets. Pile them on and take them off one at a time to get what you like and play with props. For 1/2A stunt, I think the Medallion .049 is preferred, but some like the TD because they are easier to find sometimes. At the KidVenture circles, we ran everything, Norvel and cox engines, on donated SIG Champion 15%, but 10% will work just fine.
Type at you later,
Dan McEntee
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At the speed contests, we run 10%. My sweepette has a TD 049 in it and has been with us, and flown at each one. Now, it's only won when no one else entered 1/2A proto, but it runs exceptionally
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My rule of thumb is if you can't drop the butt end of a 1/8" drill bit through it, it's stock. I run all of mine on pressure bladders, so I use a 9/64" bit to open it up. I'm not contradicting y'all's opinions on the fuel, however I'm a nitro junkie, so I run 35% in all of them. Makes it easier to needle and if you want screaming speed, it really does perk it up. But you would have nothing but trouble drawing fuel with a unpressurized tank.
Rusty
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I forgot to add that they made shims for the Cox cylinders also, that fit under the cylinder between it and the case. Either a Cox part or a Dale Kirn item. It adjust the deck height plus lets you align the exhaust so it runs fore and aft. It all depends on what you want out of the engine.
Type at you later,
Dan McEntee
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I forgot to add that they made shims for the Cox cylinders also, that fit under the cylinder between it and the case. Either a Cox part or a Dale Kirn item. It adjust the deck height plus lets you align the exhaust so it runs fore and aft. It all depends on what you want out of the engine.
Type at you later,
Dan McEntee
Hi Dan, I don't know where you can find the cylinder shims anymore. I used to get them from Joe Klause. Their sole
purpose was to shim the cylinder to be in blueprint specs; not to adjust the deck clearance. The blueprint spec shows
that at TDC the top of the piston must be dead even with the head gasket shelf. The deck clearance is set by adding
the squish band height to the head gasket dimension. Most folks do not know the Cox's mfgr tolerances were all over
the place when it came to the squish band depth of each 1702 glowhead. Dale said "They" found the squish band height
varied from .003" to .021"! ~^
I learned a bit when listening to Dale Kirn, who helped me set up a (Blueprint) stock TD .049 to hit over 100mph with a two-line
model on 30% fuel. I also paid close attention to him about setting up propellers for 1/2A speed and 1/2A proto.
Well that's enough of this...
Tony ;D
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I put on a stock venturi and got the plane in the air. It's a uniflow tank and the engine is very sensitive to flying height. Fly at 8' runs great, go down goes rich go up sags off. never seen anything that touchy.
MM
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I put on a stock venturi and got the plane in the air. It's a uniflow tank and the engine is very sensitive to flying height. Fly at 8' runs great, go down goes rich go up sags off. never seen anything that touchy.
MM
Make sure your vents are nice and open, and maybe pointed into the direction of travel/prop wash. Maybe try crank case pressure to even it out. Look up the ballon method of 1/2A fuel tanks, as that may be what you need. Larry Renger swears by it and so do others. I haven't done that much 1/2A stunt to get to that yet.
Type at you later,
Dan McEntee
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Tee Dees are just not much good in stunt unless you use pressure feed. They were Free Flight engines designed to run in just one attitude, pointed way up! And not for long, so they would not tend to overheat.
Try a smaller prop to boost the rpm to speed up the air flow through the venturi. I agree with high nitro, because again, you are running higher RPM to improve the fuel draw.
Otherwise use pressure. That little nipple on the side of the case gives just the right amount of pressure feed. You turn the crank so the port is open where the nipple is, and drill with a .015 drill through the crankcase. Works great! If you can find one, Kirn Kraft and others made pressure nipple backplates. Or use a bladder tank.
The Cox of my choice for stunt is the Medallion.
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Larry, what size hole do you need for a backplate tap? I understand you can fill the nipple with JB Weld and a sooty or waxed wire through it, then pull the wire out after the JB sets. .015" or less?
Rusty
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Just a side note. The timed crankcase tap will be allot more pressure than a back plate tap. Both systems will benefit from the 128 tpi KK NVA but with timed pressure or small surgical tubing bladder you need it. For CC pressure you also should use the KK check valve. They made an in line version and one built right into the back plate. You need to flush out the check valve after every flying session with carb cleaner spray.
MM
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I've run a lot of Cox .049's, both reed and TeeDee. Running under 20% nitro is a pain in the butt and not at all satisfying.
Small venturis smooth them out and don't reduce power very much.
Over-propping is death. Let the engine wind up on 5-3 or less.
Low compression heads - useless. You would be better of beating YOUR head against a wall.
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You're right, got to go full bore on Cox .049 to get joy.
MM
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The Cox of my choice for stunt is the Medallion.
Mine, too. The way they run is almost miraculous. Stock, with Cox Racing fuel, you can turn your back to the circle and aside from the Doppler effects, you couldn't tell whether it was doing stunts or not. By far the steadiest-running engine I have ever seen of any type, it's uncanny.
Brett
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All the Tee Dee .049/.051 venturii I had came with a wire mesh screen, and lots of owners seemed to remove the screen. Not a good idea, as the venturi is already too big for decent suction feed. You might put on a couple layers of pantyhose material with an o-ring.
Another thing is that uniflow requires more suck than an "open" tank...as the 4cycle guys found out 15 years ago when they were working with the Saito .56 and .72. Uniflow tanks just didn't work, and apparently muffler pressure didn't help enough.
I recall running as small as a .008" hole in a backplate pressure tap, using the sooted up wire trick, but on Tee Dees only ran a penbladder...a real pen bladder, from Bill's Miniature Engines....which is long gone. D>K Steve
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Motorman....I did an experiment on Cox Engine Forum CEF a year or so back with the so called high zoot TeeDee crank shaft
at the time I was trying to get the TeeDees I had hauling butt with lite hawk and lil hacker combat wings....simply because I had more TeeDee .049s and .051 than Norvel, Cyclon, Profi or Fora 1/2a engines...I played a lot with different venturi openings
I always ended with a bladder and Texas Timer NVA as TeeDees on suction just are too hard for me to tame...Ken Cook has great success with hard tanked TeeDees but I must suck at it...but a bladder run is no chit reliable and repeatable
As stated above by someone, I much prefer the Medallion bone stock for a trouble free stunt experience... and they actually can accommodate a larger variety of props depending on the line length and plane weight
I have a Skyray, RM bipe and Lil Jumpin Bean all with Cox medallion .051s and balloon tanks
BTW I have (too many too) given up in Cox reed engines long ago...I would rather fly than fuss
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TD's on pressure bladder from Texas timers is the way to go. I run a stock venturi for ease of starting. I also use a Texas Timers fine NVA.
High compression heads or drop in inserts. 1 gasket + 1 gasket for every 10% nitro is a good rule of thumb to start with. (35% gets 4, 25% gets 3) tune from there by adding/removing gaskets till peak RPM.
Phil
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For both the Medallions and TeeDees I really like the Galbreath head and Nelson plugs
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Hi Dan, I don't know where you can find the cylinder shims anymore. I used to get them from Joe Klause. Their sole
purpose was to shim the cylinder to be in blueprint specs; not to adjust the deck clearance. The blueprint spec shows
that at TDC the top of the piston must be dead even with the head gasket shelf. The deck clearance is set by adding
the squish band height to the head gasket dimension. Most folks do not know the Cox's mfgr tolerances were all over
the place when it came to the squish band depth of each 1702 glowhead. Dale said "They" found the squish band height
varied from .003" to .021"! ~^
I learned a bit when listening to Dale Kirn, who helped me set up a (Blueprint) stock TD .049 to hit over 100mph with a two-line
model on 30% fuel. I also paid close attention to him about setting up propellers for 1/2A speed and 1/2A proto.
Well that's enough of this...
Tony ;D
I just started researching Cox 049s. Building a couple from some old parts and ran across these guys, EX Model Engines. Apparently when Cox sold off, half the inventory went to the guy in Canada but he only got the Baby Bee / Golden Bee stuff. The high performance stuff went to EX models. They have those shims in stock, I believe.
http://www.exmodelengines.com/product.php?productid=17603&cat=266&page=2
P.S. If anyone has a box of old reed valve stuff they'd be willing to part with, please let me know. I got some from Ty but trying to avoid dropping a bunch of money in new reed valve parts like stunt tanks, back plates, etc.
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I just started researching Cox 049s. Building a couple from some old parts and ran across these guys, EX Model Engines. Apparently when Cox sold off, half the inventory went to the guy in Canada but he only got the Baby Bee / Golden Bee stuff. The high performance stuff went to EX models. They have those shims in stock, I believe.
http://www.exmodelengines.com/product.php?productid=17603&cat=266&page=2
P.S. If anyone has a box of old reed valve stuff they'd be willing to part with, please let me know. I got some from Ty but trying to avoid dropping a bunch of money in new reed valve parts like stunt tanks, back plates, etc.
Hi Bill, those cylinder shims from ex are not what should be used. The cylinder shims that "We" used
were cut from brass, not copper and they came in sets of much smaller thicknesses.
Our timing shims came in sets of several .001", .003" and .005" pieces. The real trick
was to find an adequate fitting cylinder and piston that gave the correct amount of
piston showing above the shelf after lapping(lapping Not TDC). All my 1/2A TD Combat engines were propped
to turn 24,800 to 25,400 on the ground using Tornado Black Combat props, stock venturi and
30% nitro fuel. There is some good news; there is a seller on ebay that sells the correct
cylinder shims, I'll edit in his name later. Good luck; :)
Tony
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Hi All, the ebay seller that has Cox 1/2A cylinder shims is : combatalbert
Tony
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Hi Bill, those cylinder shims from ex are not what should be used. The cylinder shims that "We" used
were cut from brass, not copper and they came in sets of much smaller thicknesses.
Our timing shims came in sets of several .001", .003" and .005" pieces. The real trick
was to find an adequate fitting cylinder and piston that gave the correct amount of
piston showing above the shelf after lapping(lapping Not TDC). All my 1/2A TD Combat engines were propped
to turn 24,800 to 25,400 on the ground using Tornado Black Combat props, stock venturi and
30% nitro fuel. There is some good news; there is a seller on ebay that sells the correct
cylinder shims, I'll edit in his name later. Good luck; :)
Tony
Thanks for that! I was wondering as those shims are listed as .010 and .015, I believe. I have 2 Black widows inbound right now, both from ebay. The first one, I looked at it, and it was clocked perfectly from the factory. The second I thought was the same but on looking closer to the picture, see a shim under the cylinder. Both are older with metal backplates so they should have the correct BW cylinder. The only thing to see is if the shimmed one still has some SPI.