Backfiring. What causes it and is it bad? Please

If the carburettors are set up correctly, and you have a little air getting in the exhaust pipe it may pop and crackle.

As others say it was too rich before now its correct..

Like my progressive it just does a gentle crackling going down hill as it runs very lean with the throttle shut engine braking, and I just live with it.


Mega banging is unburnt fuel being lit by the exhaust flame from a firing cylinder.. loose sparkplug wire, points, coil or electronic ignition failing . turning the ignition on and off while in gear at 30mph can do it too..


If it gets too rich then it backfires up the inlet and that can split the maifold boots ( one of my dying engines managed this ..)

 

^this.

 

Mine popped and banged when the ignition switch wiring was damaged by the muppets who installed the alarm and immobiliser.

 

When you have the throttle open a bit the speed of the air flowing through the venturi in the carburettors sucks fuel through the main jets sort of in proportion to the amount of air flowing through, so the mixture is about right.
When you shut the throttle at speed, the engine is trying to go fast and suck lots of air through the idle jet path, and until it returns to idle speed there is not enough fuel coming through the idle jet for the amount of air whizzing through. So it runs leaner.
Couple that with exhaust leak and it pops.

 

What Mike said.

Basically, the fuel air ratio is different from low idle to high idle...so the unburnt fuel in the exhaust is also different. The air leak in the exhaust is adding to that change, making it more combustible = Bang. Fix the exhaust leak, and the unburnt fuel in the exhaust will be less combustible = no bang.

Probably.

Have you fixed it yet? You just need to tighten your joints mate.

Probably.