Power Valve Question - HELP

It could be the power valve, but I've usually seen very poor idle characteristics as well, since a defective one makes the carb run very rich.

The power valve allows you to run lean at part throttle, which sounds like where you're experiencing stumble (1/3-1/2 throttle), and increases fuel flow under power by 8-10 jet sizes. With that being said, your fuel mileage should be hideous if the power valve was shot - to the tune of adding 8-10 jet sizes to the primary side of your carb. Excessively rich symptoms will be an obvious indicator, and your plugs are a good place to start diagnosis.

If you've experienced a recent back-fire, the power valve is probably shot. You can use a Mityvac to test it. Should hold 10-20 In Hg.

I hope this helps, and good luck with the carb.

Southernman