I have a Hobbico Twinstar that is running two OS .40la engines. Both of the engines are brand new. One of the engines runs fine, but the other engine will run only at full throttle. If I close the throttle the carburator does close, but the engine will still run full throttle. Eventually the engine does start to slow down, but then it begins to speed up and then slow down over and over again. Some guys at my local field took a look at it. We saw some air bubbles running in the fuel line (not many at all). We checked over the fuel tank and all of it's lines but found no leaks. We found that the needle valve was touching the nacelle, so I trimmed the nacelle so that the needle valve was no longer touching anything. Still had the same problem after that. The other engine is running flawlessly. Both engines when at full throttle synch up well, but when the throttle is brought back one engine will come back to a nice idle while the other one stays at full-3/4 throttle and then speed up and slow down over and over again. Any suggestions???

Thanks
HighGs

HighGs,

You'll need to carefully check the entire setup. It could be vibration-induced fuel foaming. This puts air into the fuel system, and makes the engine go lean. The added air in the fuel also acts like the throttle is farther open than it is.

Put the engine on a test stand to see. The fuel tank should be well-isolated from the engine's vibrations. In the model, it may not be so isolated.

Swap the engines around. If problem persists in the same engine, it's the engine. If not it's the fuel system.

Thanks for all the advice! I actually took the engine to my local RC shop (they have a test stand) to check out the problem and one of them right off the bat noticed that one of the carborater screws was missing. It was letting in air which was the reason I could not get the engine to idle. Needless to say I feel pretty stupid for missing this little $0.85 mistake!!!

We're glad you were able to solve your difficulty!