tHE vast MAJORITY of problems of OS 40 and 35 fps came from TOO large a venturi, OS swapping the .157 diameter Needle assembly out for a 20 size assembly at .135 or so, and then putting out a muffler (742) that was extremely restrictive . It had nothing to do with over compressed head.
The same problems plagued the LA series , I cannot tell you how many people I have seen trying to use the tiny OS FP Needle assembly with a 290 venturi,The problem with this is it is massive in size and equates more like a .325 venturi with the tiny needle assembly , all the above cause runaway and low fuel suction, and will almost never be stable.
If your going to use the tiny OS 20 NVA you need to use a 250 to 255 venturi max. or use a more standard size , Old OS , Enya, ST, PA etc that are all .157, with this size NVa you can use a .275-.285 , The tweek from there .
Other things that help are a solid motor mount, stiff frontend, well made tank mounted to help minimize vibration, keep the tank close to the engine, stay away from muffler pressure, use a good fresh fuel with mostly synthetic oil, or at least 1/2 synthetic oil, of at least 20% total (22 seems to be used by most) , use nitro that fits what the weather is..don;t under or over nitro your fuel , use what is needed... too much is not better...too little is not better try to alway use fuel filter on the feed line
Randy