Join Date: Nov 1999
Location: Central California, USA
If the mixture gets too lean you can get a lean backfire up the intake. Also this can happen if you have an intake valve that's too tight.
However, my best guess is that your intake manifold gasket is leaking, probably sucking oil and air in from the valley area. The leak is probably on one or two cylinders maybe near the distributor.
On some intake manifolds the bolt holes on the ends are smaller than the other bolt holes. when you tighten the manifold down, it sits lower and lower as you tighten the manifold bolts, eventually the corner bolts can bind and stop the manifold from coming down any more.
Manifold misfit is caused by milling the block and heads and causes the intake surfaces to move away from the intake manifold faces.
The solution is to get special thicker intake gaskets.
If I were standing there listening to it it would be a lot easier, but that's my guess.
Try spraying a little window cleaner or water around the manifold and carb base while it's running and see if you hear a change. Slurping or difference in speed would indicate a leak. Of course you could have a leak on the bottom of the manifold only, that wouldn't show up.
And, if your carb is very black inside you are getting backfiring.
The older I get, the faster I was!
[This message has been edited by davidpozzi (edited 06-03-2000).]