The Stovebolt Site Forums The Shop Area Engine Shop engine backfiring hard.

I took the trip down to indy yesterday to help my sister rebuild her house. Every time I let off of the accelerator my engine (454) would backfire quite badly. Luckily the trip was at night and I could go almost the entire way without altering my speed. If I let off the accelerator very slowly I wouldn't backfire very much. But it is popping now even at idle (at least it was after the trip, maybe now that it isn't so hot it would be alright). I really don't know what causes backfires other than timing, I have new adjusted points, recently adjusted valves, new timing chain and recently adjusted timing.
Known problems, exhaust leak at head (bad bolt hole), choke is stuck open, air filter housing pinches throttle body (I have ordered the spacer).
Thanks a lot

the mixture could be too rich in your carb, or the float could be stuck open or you could have a vacumm leak. Mine used to do that, and it turned out that it was sucking air between the carb and the intake manifold.

You could have sticking valves.
Check it with a vacuum guage.

The cause of the backfire is the exhaust leak. What causes it is when you let off on the throttle there is a lean mixture of fuel going into the exhaust, this mixes with the raw air coming through the exhaust leak at the manifold, when there is enough of this, and it hits the hot exhaust pipe or muffler it will explode. If you fix the exhaust leak it will stop the backfire.

Racecarldad is right. Fix that leak and the problem should go away.
One other thing might cause it. If you have a AIR (Smog) pump, the diverter valve might be bad or unhooked. It will cause similar problems.

Check the EGR valve that it is closing fully,It should shut when you let off the gas if it does not you get a super rich mixture going down the tail pipe. Blew up a couple of mufflers before i found it. (Guys at the parts store could not figure how the seam could split the full length :rolleyes: )

The leak is between the heads and the headers, not the manifold and the heads, but this wouldn't make a difference, would it? Still raw air and lean fuel mixing.
Thanks again.

Any exhaust leak you can hear that is up close to the engine[before the exhaust pipe goes 2 or 3 ft.] will suck raw air in and cause the backfire. I have seen people use a product called [muffler mend] or something like that to stop the raw air from getting mixed with unburnt fuel. This [muffler mend] comes in a tube like calking. You put it on when the exhaust is cold and then start up the engine and let idle till the exhaust is hot then shut it off and let it set for about 30 minutes. This will let it set up like cement and it should stop the air leak. I just thought , I believe it is called muffler cement.