I have an OS LA 40 engine as for as I know it is stock, with a .257 venture, one stock head shim T/Bolt idle bar plug, turning an A P C 11x4 prop, 5-10% n 22% oil, and on the bench it runs well. It turns in the 9500-10,000 R P M range, so I am asking how you have yours set up, and THANKS a One other thing I forgot it has a R Smith P A 40 Needle assy in it also...lot.

I use 11-4 props on my la 40s. If the 257 venturie is bigger than the standard OS venturies you should be all right. I have never had to add a head shim. I launch at a wet two stroke and they work fine on a modifies twister and a Nobler.

  Motor man, it already has an aluminum back plate, but thanks for the suggestion..

I'm not expert on this, but I would think a 11-4 is more than this engine should have. I'd be inclined to try a 10-4 APC, and I'd think that you should get a lot more than 10k.  My 20FP with a 9-4 APC on 10% will make 13,800.

I don't have any mileage with the stock 40LA, but based on my experience with the 46LA, an 11-4 and launch RPM of 9500-9800 is probably about right.  They don't want to turn as fast as the little engines, and they do unload in the air.