Hi guys, maybe someone has an answer, i have 3 vintage 1973 OS40sr engines with the rear rotary valve and the rear carb, i am getting back into the sport some 32 years later.
I have replaced the bearings and cleaned up the engines, but they all have the same problem they had 30 years ago.
If you peak out the engine, and even go 1 -2 clicks rich they will run through a tank of gas on the ground, even if held up verticle, but if you take off, just as you climb out about 75-100 feet they will intermittanlty suddenly stop.

I am running an A3 OS plug with 5% nitro, i nearly totalled a new model after a dead stick argument with a fence, the fence one, so some repairs needed.
The engines will not transition well and almost refuse to increase speed from idle, once you get past 1/2 throttle transistion seems ok.
They do not have any idle mixture adjustment just one needle for all the range, and they do not idle well at all.
I seem to remember running a 9x 6 prop back then, as i still have a heap of them, i fitted a 10x 6 and the slow idle improved hugely but the engine does not pull as well with the bigger prop, any thoughts, its as if the motor leans out with flight speed, i suspected the carb was not getting enough air at the back so i enlarged the hole on the firewall, but it made no difference the rear of the carb is about 5mm from the firewall.
I have fitted a OS46ax rear half of the muffler to the engines to reduce the noise, i would not be able to fly the engine as it was built 30 years ago, but even with the original muffler it still cuts out at about 75-100ft.

I am running pressure from the exhaust, but the engine did also have a crank case nipple on one of the carb attachment holes, would crankcase pressure be a better way to run.
Does anyone have a factory spec sheet for this motor ie prop size etc. Sorry about the long post but i really am running out of ideas and i like the performance these old engines give when they run right.

Best Regards
Lambo1