I was given a OS .40 FSR. The engine looked as if it was never ran.I started the engine with the high speed needle 3.5 turns out for a rich setting. The engine started and I slowly lean it, but the engine continues to stop running when the carb opening is about half way. Also there is no smooth transition from idle to full throttle...it sounds as if the engine is choking . Can this be caused by the o-ring on the high speed needle being worn out or is there something else causing this.

Thanks

It may just be too rich to stay running. Try 3 turns out and try leaning it gradually. If that doesn't work try starting at 2 turns. Or try keeping the glow plug on to see if it will run up. If it's too rich, keeping the plug on will keep it running.

Also your low end mixture might be tto rich and it's stumbling (it will smoke and spit fuel out the exhaust). Or it could be too lean (it will just die with no smoke or fuel spitting).

Is it a new plug? What kind of fuel?

It is a new plug OS #A3 and the fuel is Byron 15%Nitro 18%Oil

The 40FSR is a ringed engine so it might be best to assume it's never been run. That means running it very rich for some time so you won't be able to tune the carb anyway. In the meantime it wouldn't hurt to check out the carb to see if there's anything wrong with it. I'd also use a different fuel to run it in with. For one thing, why use expensive 15% fuel on a test stand for an hour? The other thing is that I believe (and correct me if I'm wrong) that Byron mix by weight so your 18% oil is actually more like 16%, far too little.

If it really has not been broken in like downunder said, then set it for 4000RPM and leave it at 4 turns out with the glow plug on if it needs to in order to stay running. Run a tank through it like this and then try leaning it.

I am unclear as to the throttle position when you start to lean it. Lean the engine with the throttle fully open. If the engine still does not transition at half throttle, then the low speed needle may need adjusted. If it just quits as you advance the throttle, the low speed is too lean. If it stumbles and lags, then it is too rich. Be careful, little adjustments at a time, no more than a quarter turn at a time.

OS produced both ringed and ABC type FSR 40s. You may be running it too rich and wearing it out right away. The ABC engines have an oil retention groove on the piston as compared to a conventional ring. You need to get the carb air opening wide open after starting and control the speed with the high speed needle only. The low speed adjustment is a slot headed screw recessed in the center of the throttle arm.