Atwell,
I'm not sure if your car has the gulp (diverter) valve. Early '79s
did not have it (like '78s). ISTR late '79s did have it. This
would be the first culprit to check. Your backfire is due to high
back-pressure in the exhaust manifold generated by the air pump,
coupled with lots of intake vacuum (throttle closed, high rpm).
This causes unburnt fuel to be sucked back into the motor.
The gulp valve was fitted to most NA spec Spits with air pump
to alleviate this condition.
Try temporarily disconnecting the air pump at the check valve.
Plug the check valve with a bit of old rag to stop the diaphram
from rattling when the engine is running. This should cause
the backfiring to cease.
> My '79 Spit 1500 (with a freshly rebuilt Zenith-Stromberg carb, and a
> catalytic converter) will usually backfire when up-shifting over 3000 RPM.
> It is most likely to happen after the car has been idling a minute or so.
>
--
Bob Sykes
"You can't beat your brains for entertainment."
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