Back Fire

Leak in the exhaust system near heads? Exhaust manifold to head seals leaking?

I have a 1970 bug with a1500 motor in it, a new carb, distributor, wires, and plugs all new. Recently my son and I pulled the motor and resealed it; all new gaskets from the case out. Now that it is back together it is running better than ever with one exception; it back fires under load. While driving if you suddenly let off of the gas it will back fire or if you are going down hill without your foot on the gas it will backfire. We checked the intake manifold for leaks at the heads (single port) and checked for vacuum leaks at the base of the carb. and checked the vacuum plugs on the carb.The distributor is a

009 and is set at 9 degrees BTDC. The points are in adjustment and the valves should be but I have not double checked them. Any Ideas? My son and I are befuddled. Thank You PS We have adjusted the timing and it seems to run the best where it sits at around 9. _________________

the afterfire you describe is usually caused by a leak in the exhaust system, not the intake system... from cold, start vehicle, then run your hands along exhaust junctions, you may find one leaking... don't do this if the exhaust is already hot... sometimes you can spot an exhaust leak by black streaks forming from the leak...

Put on a pair of gloves and block the exhaust at the tail pipes and you will hear where the leak is.

J.

Thanks to all for the advice, we will give the exhaust system a good once over.