A new club member showed up to the field today with an old Tower Trainer (10 years) and an engine that came with the plane - a TH .46. I believe the carb is an air bleed and has a black screw with a spring around it for the mixture control. No one really knows how to tune his engine.

So what is different about an air bleed? Is the whole process the same except for the direction you turn the screw? Do you still turn it in 1/16 increments?

Thanks for your help.

You open the needle to lean the engine at idle.

There are usually two screws in a carb like this. One will be on the top of the carb and is the barrel stop. This will have a spring on it as you describe and it keeps the carb barrel from coming out of the carb as well as limit the movement of the barrel. The other screw will come out of the side of the carb on the same side as the throttle arm. There should be a hole in the carb at the end of the screw. You tune this in the same way as you do a low end needle valve, however try to make smaller movements than 1/16". I know that sounds difficult to do, but on an air bleed carb a little bit of adjustment goes a LONG way. I hate air bleed carbs. I've owned one and will never own another one ever again. It took me 5 days worth of working on it to get the idle set correctly with the air bleed. The only thing I can suggest is keep working at it, small movements of the adjustment, and be patient!!!

Ken

arnt the needles reversed. screwing in being richer
or is that only for the HS needle on a airbleed?

Thanks Ken!

On my OS.46 LA, the manual indicates that the Air bleed screw is the one with the spring.

Is this accurate?

I don't want to adjust anything that shouldn't be adjusted.

(edited grammar issue)

main needle valve is adjusted like any other engine screwing it in will lean the engine,the low speed is set by the the barrel stop screw and the low speed mixture is set by the air bleed screw and yes screwing that srew in will richen the low end.usually start with the end of the scrw in the middle of the hole when looking through it and make 1/8 turn adjustments.start the engine ,set the high speed needle,then go to low throttle and set the barrel stop screw for the rpm you want,then you play with the air bleed screw for correct mixture in 1/8 turn movements.if the engine dies as soon as you go to low throtle it usually means too lean turn the screw in,if it idles and slowly slows and dies or hesitates going from low to high throttle with a lot of sputtering it is too rich and then unscrew the airbleed screw in 1/8 turn increments.you need to do this with the engine stopped for saftey reasons.

Too bad. I guess you'll never own a YS Engine then

Yes, I have a YS engine. But I don't want another OS with the air bleed carb on it.

Ken

Ken, I find that really funny, all the OS LAs have the air bleed carb and they are just too easy to get into tune and run great. I think you came upon a real stinker and it has thrown you to the dark side!! Try another one and come back to the light!!

You might be right. It was one of the first engines I had. I messed with that thing what seemed like forever. I just vowed then to not mess with them again.

Ken

Oyy.. sounds like the problems I had with Evolutions. Minnflyer swears by them, but with all the problems I had, I won't go near them any more. I'll just stick with OS.

CGr.

Oyy?

He IS from New Jersey!

How can you tell??? The front yard that's a parking lot???

Ken

I thought people from New Jersey went around saying stuff like "yo!" and "ohhhhhhhhhhh....!!" (I can verbally say what I'm thinking but hard to spell it and get the point across) I'll be there again in June so I need to brush up on the local language.

I've had good luck with air bleed carburetors so long as they are mounted upright. I tried to mount an OS FL-70 inverted and it was absolutely impossible to adjust and get a reliable idle.

If anyone could post a)a pic of an air bleed with the screws labeled, and b) a link to the TH .46 manual with air bleed carb it would be really great.

Sounds like both the barrel stop AND the mixture screw need to be adjusted? I guess I need to find a starting point and know how to adjust these, and in which order.

If it is the same as the 46la please let me know, and I will just look at that manual online.

It's the same thing. I just make sure the throttle barrel closes enough to kill the engine when I hit my kill switch but is opened enough to keep the engine running at idle, it's a trim switch thing. The bleed screw you can just open or close until you get a smooth idle. If you need a starting point on the idle just stick a small pin into the bleed hole until it is closed a little, you will feel it. Read up on the LA and you will know as much about them as most guys. Very simple old school carb. I still have a couple of the LA .46s that haven't had the bleed screw touched in 10 years. I just used the small pin to tell me when I was a bit past half opened and they have been there ever sense.

Thanks Gene.

The air bleed screw is whichever screw that happens to block the air bleed hole in the front of the carburetor when it's turned in. Turning in the screw blocks the air bleed hole and richens the low end adjustment. Turning it out opens up the air bleed hole and leans the low end. This effect isn't as precise as a low speed needle, so you have to kind of "eyeball" how much your adjustment affected the air bleed hole's open area. It's more like setting the crescent area in the carburetor intake for your idle than it is like turning your high speed needle, if that makes any sense.

Air bleed carburetors rely on allowing extra air into the mix to lean out the idle setting. The more you turn out the air bleed screw, the leaner the low end gets. This effect is limited by the size of air bleed hole drilled into the carburetor in the first place, however, so it's almost impossible by design to set an air bleed carburetor too lean on the low end. With some engines, users have reported obtaining best tuning results by actually drilling out the air bleed hole in the carburetor to the next size up to allow for more maximum air intake and to widen the range between full rich and full lean.

Interestingly, like plain bearing engines, only the best engine manufacturers still make air bleed carburetors. You can get air bleed carburetors on new Enya, O.S. Max, Thunder Tiger, and K&B engines. You cannot get air bleed carburetors any longer from GMS, Super Tigre, Evolution, Magnum, Aviastar, SK, JBA, or most other glow engine makers.

I am currently flying a Thunder Tiger GP-61 plain bearing air bleed two-stroke on my Goldberg Protege very regularly. The engine is a pleasure to fly and is very powerful and reliable. It was easy to tune and it seems to require less fine tuning as the weather changes than many of my twin needle carburetor models. It has been my experience that air bleed carburetors make terrific engines for the casual sport flyer because they're not sensitive to climate changes, don't require much in the way of fine tuning, and are very hard to get tuned so out of whack that they won't run at all.

In my opinion, ball bearings are a great option if you can't get fuel with real castor oil in it from your local hobby store. Also in my opinion, twin needle carburetors are handy if you really want to fine tune your two stroke for maximum performance. Neither is necessary for a great sport flying experience, however, and many pilots enjoy season after season of terrific, reliable flying with their O.S. Max LA engines, Thunder Tiger GP series engines, and K&B Sportster engines.

If you need heavy, sensitive, and prone to corrosion in your engine, then twin needle ball bearings are the way to go. For float planes, trainers, and general sport flying however, plain bearing engines with air bleed carburetors are simply a smarter value.

Thanks for clearing that up. Now I know what kind of hole to look for. Maybe it's even clogged (engine is old) and thus could be why we couldn't get the low end leaned out.

Plane was slobbery rich at idle, so much that fuel was splashing out of the carb and engine wouldn't idle lower than 1/3->1/2 throttle.

So how does the throttle stop screw work?

Unfortunately the 46LA manual has pretty lousy instructions for the carburetor, other than the standard OS instructions for tuning the high end.

One picture

As opposed to my 1,000 words?