Hoping this will help. My car ran fine with a 750 race Demon for years. A few weeks ago, I drove it around, came home, parked in front of the garage. When I went to pull the car in the garage, it wouldn't stay running. Popped, and backfired. Rebuilt the carb. Cleaned everything, and put in a new set of gaskets, and power valve. Same problems. I read this thread, and went out, and put in a new power valve, and it started up, and ran fine. So, I had 2 power valves in a row go bad. The quality of products is really bad anymore.