I am having some backfire's in the muffler only at high rpm? I have a supercharged z28 with 8lbs of boost. Recently, one of my cat's went bad and plugged the muffler. Today, i had the cat and the muffler replaced and now under acceleration, i am getting some backfiring in the muffler. Fuel psi is normal and i have an fmu. vacume at idle is 20mmhg. Help, anybody!

 

20mmhg or 20inhg? Only 20mm would be pretty bad...20" would be normal for a stock motor.

 

20inches of vacume. It's backfiring in the higher rpms?

 

Bad exhaust valves cause exhaust backfiring. Either they aren't seating right, or they're floating. Could also be running real rich.

[This message has been edited by ATOMonkey (edited August 17, 2001).]

 

ATOmonkey,

Thanks for the reply! I was thinking it was running lean because backfires on the top end usually indicate a lean condition.... anyway, I took my 8:1fmu off and it runs fine. However, it does need more fuel. When i first put the fmu on the car would shift hard and pull hard all the way through the rpm range. Since the fmu is off, it does not pull as hard. I'm going to get a kit to change my fmu to a smaller ratio. Thanks!