Backfire out the exhaust?

Question

I am working on a 1969 camaro for a customer of mine. The engine is a brand new rebuilt (by somebody else) 350 V8 with HEI ignition, MSD supercoil, large cam, Trick Flow aluminum heads, demon carborator, roller rocker and roller lifters. We have checked the timing, rebuilt the carborator, changed the spark plugs because they were fouled out, adjusted the valves, and tomorrow we will be changing the oil because it is very saturated with gasoline. The car idles great but at high RPMs loses power and consistantly backfires out the exhaust. The backfiring is not one big bang, but a timed series of low pops as if it is even with the ignition. I believe this has something to do with the valves or the cam not being broken in correctly. Everything on this engine is brand new and we did a compression test and the numbers came out 150 or higher. What do we do next? Thanks.

(asking for husband)

Answer 1

maybe try checking the distributer. if any fluid at all gets in there. then the exhaust will pop. also try putting in champion spark plugs. they worked best for me.

Answer 2

OK, some obvious ideas have been discussed like the distributor/timing problem but there are a few more pieces to the timing that this one thing.

If you don't resolve it with the distributor then start with the hardware. Make sure that you are not off 180 degrees either on the mechnical position of the distributor or the timing gears. And always check the position of the #1 cylinder when monkeying around with your timing. Must be top dead center on the compression stroke only.

Also, you need to have some idea of the camshaft and where the timing should be set per the type of camshaft inside the motor. Every camshaft has a different set of timing specks and your top end RPM's and timing settings will be critical.

Last, the intake manifold has to be compatible with the camshaft. Some are rated to only 4,000 Rpm's and others to 8,000 RPM's. And the carburetor has to be compatible with the intake manifold. You might be feeding a low end intake manifold with a high end (higher volume) carb and that would make it run too rich as you pump in a lot of gas at higher RPM's but the motor can't suck in any more air through the more restrictive intake manifold.

Hope it's something simple and easy to fix.

Good Luck!

Answer 3

Your air/fuel mixture is way too rich. You mention a Demon carb and Trick Flow heads but not any upgrade to the intake manifold. This could cause a problem. When you upgrade carb, cam, and heads you should consider an intake upgrade as well because the air/fuel mixture can be restricted by an insufficient intake.

Answer 4

when they put the cam in did they change the springs to match the cam and add the correct shims if any
it sounds like it might be trying to float the valves especially with a roller because they have such a steep ramp

Answer 5

hi there i am ted self employed tech. u may have two pproblems here /one the timing is off is off by a little pinch,and check that the roller rocker r the right one ,annd they maybe to tight. the way to check them is losen them up one side at a time and startthe motor, and them one at a time, but u will have to do it fast or u will be caked in oil. need anymore on it feel free to e mail me .ted

Answer 6

previously i had a customer with the same problem..i changed his dizzy cap n adjusted the crank angle sensor...i would recommended tat the cams to be checked if everything is sittin propelly..probally it has a slight bent valve.

Answer 7

No very inclined with domestic cars. But with Jap. Spec vehicles backfiring is a sign of the car running too rich.

Hope this helps somewhat.

Goodluck.

Answer 8

put new plugs in it and then try a compression test to see if a valve is burnt or out of adjustment

Answer 9

adjust the valves setting are wrong