ORIGINAL: JohnBuckner
If you are using a typical muffler pressure system and the fuel tank/spray bar (jet) relationship and orientation is reasonable then using a check valve in the muffler pressure line typically will cause excessive head pressure at the needle valve inlet and result in an overly sensitive needle causing inconsistant high speed needle settings and various running problems including flame outs due to leaning in flight.
sonnytex are you trying to fix an actual problem, if so what exactly is the problem. If there is no problem I would suggest you not fix it.
John
On top of what John said, putting a check valve in most engines that run off of normal muffler pressure will cause many problems. Most of which John has addresses. One that he didn't was an excessive waste of fuel. When you put a check valve in a system that wasn't designed for it you will build up pressure in the tank. Most normal glow engines have no way of checking that pressure in the carb/engine. The excess pressure is going to "force feed" the fuel into the engine. Most of that fuel is going to just get blown out of the exhaust without ever being burned.
Ken