Backfiring in the 40

[CruiserStuff]

When I'm going down hill (with foot off the gas), I'm getting small to large backfires (depending on the rpms). Also, when cold, getting some backfiring when I push the clutch in between gears. Any help?

 

[Pin_Head]

Idle mixture is too lean at high manifold vacuum to ignite, so the mix detonates when it hits the hot manifold. Either the idle circuit is plugged or misadjusted or if it is a smog era carb, the throttle positioner is non functional.

 

[Downey]

Additionally, the anti-backfire valve on end of air injector manifold could be bad. Also, backfiring is more often than not an electrical problem, although Pin Head could be spot-on!!!!!

 

[CruiserStuff]

Thanks guys. Any suggestions on where/how to start diagnosing? How can I tell, for instance, if it is the throttle positioner? Forgive my ingnorance....

 

[mano002]

Mine used to do that when the timing was off a bit. I later found the advance diagragm was damaged, which was making my timing off a bit.

 

[trucruiser]

Another reason for deceleration backfire could be an exhaust leak Manifold or donut.