Back Fire

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.

On 11/28/2010 12:22 AM, FC Bill wrote:
>
> 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.

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

---
Olli

"FC Bill" wrote in message
...

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...

On Sat, 27 Nov 2010 23:22:43 +0100, FC Bill > wrote:

> 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.


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

J.

--
Using Opera's revolutionary e-mail client: http://www.opera.com/mail/

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