OS 15LA Fueling Problem
 

I have a vintage Champ tail dragger with a new fully run in os15la and It's giving me fits. I can't adjust the airbleed screw for idle with the tailwheel on the ground, it goes lean and dies. A richer adjustment makes it idle a little bit longer but then if I lift the tail wheel off the ground during idle the engine goes overly rich and dies. If I adjust the airbleed with the tail up it will idle forever but as soon as I drop the tail back down it goes lean and dies within 5 sec.

1. Idles perfect and forever on a level test bench with the same tank, lines, fuel etc but If I tilt my test bench up at more than a 15° angle it leans and dies within 5 seconds.
2. Back plate and carb have been sealed up with rtv.
3. Tank, fuel lines and high speed needle tested with pressure and vacuum, no leaks. Carb not tested for air leaks but does not appear to leak fuel.
4. All screws checked for tightness.
5. Compression is normal.
6. Tank centerline is level with carb spraybar on the test bench and aircraft.
7. Airbleed screw is screwed in just past halfway across the airbleed hole for good idle on test bench.
8. No bubbles in fuel lines.
9. Pressure nipple is clear and tank pressure at full throttle is a little over 1.0 psi.
10. Runs great at wot in any position.

I say no bubbles in fuel lines but there is a small bubble that forms right where the fuel line plugs into the carb after it dies from an overly lean condition when I tilt my bench up 15° from level.

I'm lost, never had this happen before but then this is my first .15 and my first airbleed carb. Maybe the carb is leaking in air where the black plastic fuel inlet connector meets the carb body?

Sounds like you covered everything already. Smaller motors like to idle a bit higher than larger ones. You shouldn't expect a 2,000 rpm idle. Other than that, an airbleed carb screw is clockwise rich CCW lean. opposite of normal. I am sure you know that. Is the tank close to the motor?

Thanks aspeed. Yes, the tank couldn't be any closer to the engine on the plane and on the test bench.

I went ahead and vacuum and pressure tested the carb. The only leak is around the throttle barrel, which is normal. Put the engine back in the Champ, set the airbleed for a reliable 2500 rpm idle with the tail raised up to level. I guess I'm just going to have to live without being able to idle for more than 5 sec with the tail on the ground. I've never had this problem using larger engines with twin needle carbs but whatever, it still pulls the 2-1/4 lb. plane around nicely.

This will be my first and last OS airbleed carb engine on a tail dragger.

It sounds like you do not have enough pressure feed to the tank from the muffler. Which muffler do you have?
If it is the stock one, check that the baffle has not been altered (I don't know how much pressure there should be at idle, in terms of PSI)

On the .10 FP I can hold the plane upright and it will just keep on running, that is with the same muffler as the .15 engines has.

Thanks Mr Cox. This 15LA is new from Tower Hobbies, no alterations have been made except for sealing the back plate and carb with RTV. The pressure nipple is clear, the muffler seals well to the exhaust port and pressure at wot is approx 1 psi. At idle the tank pressure is too close to zero to accurately read on my 15 psi gauge. I have a new 5 psi gauge on order, should be here soon.<br /><br />My assumption is that once any rc engine is brought down to idle the tank pressure will drop to near zero psi within a few seconds. At idle fuel should be drawn by virtue of crankcase vacuum alone should it not? Older rc engines didn't even have mufflers.

Older engines had a smaller bore in the carb, and would draw fuel fine without the extra pressure from the muffler. The addition of muffler pressure then enabled a larger bore in the carb to be used and thereby gave a higher power at full throttle.

I have never tried to measure the pressure from the muffler at idle, but it is there (even if it is small) and it will help the weak fuel draw from the carb alone at low rpms.

Reading through your posts again, I think perhaps you simply have the throttle set too low. Most .15 engines will not idle reliably at 2500rpm, as you have indeed also found out. I only ever had one .15 engine that would do that and that is the Norvel .15 which has a very restrictive stock carb.

On a .15 engine you'll be fine with anything below 5000rpm, with around 3500rpm at best I would say. First tune the high end needle at full throttle, then tune the airbleed screw for a nice throttle respons. Note that this is richer than what will sound ideal at a stationary low throttle setting, but it a safer tuning that will ensure that the engines doesn't hesitate upon acceleration. After that the idle is what it is, don't change the airbleed setting, just use the radio to set a reliable low rpm. That is from where you can advance the throttle fairly quickly without any hesitation from the engine. Don't try to use a too low throttle setting, just use what is reliable for the engine, i.e. it shouldn't die if the plane is tilted a little (±20° roughly) and it should respond well to the throttle. I don't have any of the .15LA myself, but I would guess that the reliable idle is in the range 4000-5000rpm, and anything lower is just an added bonus (with 2500rpm being unreliable in practice).

Here is an example of a carb tuned for best in-flight performance, it is an OS .10FP (with the same muffler as your's is) I was really surprised at how low and reliable the throttle is, but it is far from 2500rpm:

Huh, OK maybe my expectation that the engine would idle reliably at the OS spec'd 2500 rpm is unmerited. As it turns out, idling at 2500 is not necessary for good landing speed with my Champ anyway, I have to keep it throttled up some for proper rate of descent. I'll try a higher idle, thanks.

My LA and FP .15s are on hand launch planes and the motors work flawlessly. I guess I don't tilt them up like a taildragger. I have to cut the motors at the last minute with the trim so the props are not spinning into the dirt. I would guess 3.500 ripems though. Most of the mufflers I have bored out the baffle and opened up the muffler outlet. That would not be the right thing to do for a reliable idle. You could also experiment with just taking off the muffler pressure tube and richening it just to see if it makes it worse. (I would guess that it does, but it is free to try it out.)

When my 5 psi pressure gauge arrives I'll take some pressure line measurements at idle rpms from 2500 to 4000 and report back.

Tank Pressures:

2500 rpm: .04 psi (idle barely reliable but only with airframe at one angle)
3000 rpm: .06 psi (idle reliable but only with airframe at one angle)
3500 rpm: .10 psi (idle reliable and varies 200 rpm up/down depending on angle of airframe)
4000 rpm: .15 psi (idle reliable and stable at any angle)
WOT: .87 psi

I chose to go with 3500 rpm idle for my little vintage champ. Having never run an engine smaller than a .40 I just didn't realize that these little engines need to be idled at higher rpm than their big brothers.

Problem solved! Special thanks to Mr Cox and aspeed for the help.

Glad to hear that you have it running well, a reliable idle at 3500rpm is great for any .15 engine. :)

Thank you also for the numbers, the rapid increase of muffler pressure with the rpm illustrates the problem very well.

Hi!
A 8x5 APC is recommended! At least at sea level.