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Re: A little bit of a backfire

On 28 Feb 98 at 1:18, Soap Duck wrote:

> Since I have got my carb put on it has been doing a little bit of backfiring.
> I know not why.  However, I do know that there are no vacuum leaks.  Could it
> perhaps be the wrong amount of Fuel/air mixture?  Would bigger jets on the
> carb solve this?  If it helps I think I have a few exhast leaks.  

Backfire usually occurs under deceleration and is caused by the 
under-rich mixture which usually fails to burn in the combustion
chamber being passed on into the exhaust where the gas condenses out 
and waits for a source of ignition (a bit of hot carbon, or expelled 
hot gasses from a combustion cycle that fired.)

There is not a lot you can do for this unless it is happening in 
normal operation (not under deceleration.)  If it is happening under 
acceleration or cruising, then your mixture is wrong, usually too 

If it is happening under deceleration then the best you can do is 
make sure your throttle valve is closing completely when you let up 
on the pedal.

There is no truth to the common belief that exhaust leaks can cause
backfireing since it is clear that the positive pressure in the 
exhaust means that any leak will be from the inside out rather than 
the other way around.

True backfires, meaning firing backwards through the intake manifold, 
are rare, and are usually the result of grossly misadjusted valve 
timing or burned intake valves, neither of which happens in air 
cooled VWs.. 

Jim Adney, jadney@vwtype3.org
Madison, Wisconsin, USA

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