Hi
I have a week old OS 55AX with a little over half a gallon of fuel through it.
I use it for 3D on a Mojo 40 but it's unusable because it runs very rich from 2/5 through 4/5 of throttle. It even drops rpm from 2/5 to 4/5 of throttle. I can lean it out but then the high end runs way too lean. Looks like a carb problem, maybe the spray bar. Is this a known problem? Send it in or can you send me a new carb?
I broke it in 2 tanks with 12.5% nitro and 20% oil. It's running Power Master 20/20 now. The more prop I use the bigger the problem. With an APC 12.25x3.75 it keeps running, anything bigger and it quits during flight.
Thanks,
Martin

edit: Looking into the carb I see that the spray bar is about 1/16 too short. It already allows maximum fuel flow at around half throttle which explains why it's flooding. Can you send me a new *proper* carb please? This way I don't have to miss a one week old engine for 2 weeks. Thanks!

Although there are other reasons why you may be seeing this poor performance, it is most likely due to the carb being out of balance. I suggest resetting the carb by adjusting the main needle valve to two turns out from fully closed and setting the idle mixture needle to ½ turn out from flush with the carb body. Once you have these settings, start the engine and run it up to full throttle. Adjust the main needle for peak performance at full throttle and then richen it slightly. Then bring the throttle back to idle. Adjust the idle mixture so that your throttle response from idle is crisp. After this process is complete, the midrange should fall into place. If not, then it’s probably best that you send the engine in for service.

David Wolf, Senior Product Support Supervisor
Great Planes Product Support
3002 N. Apollo Dr. Ste. 1 Champaign, IL 61822 USA
Support Phone: 217 398-8970
Email: [email protected]

Thanks David
It's not my first engine :-)

I'll send it in for service. I took the carb apart and all looks fine.

Martin

My problem isn't a carb problem. i have an os 55ax with about two gallons run through it. I put on a pitts muffler last weekend and it started running very lean. I have the high speed needle out close to five turns and it still leans out and wants to quit whenever i go vertical. During straight and level flight at full throttle i dont have this problem. The engine is about eight inches from the fuel tank. Could this be part of the problem?

That is for sure the problem.
It lacks muffler pressure, 8 inch is pretty far with a pitts muffler. Use wider tubing and if that doesn't help you can try to close the muffler exit holes a bit, try an other brand pitts muffler or use a Cline regulator.

how can I close the exit holes? Try pinching them a bit with a pair of pliers? Or is that too drastic?