I was having problems with a backfire when I let off the gas. Everyone agreed it was probably exhaust. Today, I changed the intake manifold, and the valve cover gaskets, and the thermostat and now the car is backfiring under light throttle, the more gas, the louder the backfire.

I switched out the manifold from a edelbrock performer 289 to a edelbrock performer rpm, and everything else is the same. After the new backfire, I also changed out the gaskets at the header collector to fix the first backfire. Anyone know what might be the cause of this one??? Thanks much.

 

did you check the rest of your exhaust for leaks?

 

You have a tuning issue. Ignition timing, a/f ratio etc. Changing manifolds changes the way the engine operates, and changes the fuel requirements. What do your plugs look like?

 

I only took out plug #1 and it looked good. I set the carb using a vacuum gauge, and I have set the timing at 5, 6, 8, and 10 degrees at idle. With my new manifold, what should it be set to?

As for the exhaust, I havent checked the whole system, but is only about 3 months old. I had the whole thing replaced from the collector back.

I guess tomorrow I will check the rest of the plugs, and the exhaust and see what I can find.

 

With that manifold I hope you have other mods, it likes to run with more inital, 12, 14, 16, it really depends on the build.

 

Not to many mods. It has hooker comp full length headers, MSD distributor and wires, mallory coil, edelbrock carb, flowmaster 40's, and a buddy says he believes a MILD cam, but not sure about it. Nothing super though.