Backfires on Deceleration

[Enrique]

My 914 backfires on deceleration. It also does so when I back off the throttle then let the wheels turn the engine (as in downshifting and letting the gears slow the car when approaching stoplights). What causes this backfiring?

I've got a 1.8 with Webber 44s. I adjusted the valves but haven't touched the timing or carbs. Where to start?

Thanks!

[ViolentBlue]

sounds like the mixture is a little rich.
but then hey what do I know

[Joe Bob]

Back fire is a classic example of an exhaust leak....somewhere in your exhaust system there is a hole or a leaky gasket...find it, fix it and no more back fire.

[Alpine]

I think backfiring can be a symptom of a motor that is running too lean.

Alpine

[PeteD]

I know this problem well... Here are a few things to check.

What distributor are you using? When I went from an 009 back to the stock 205AA (original with 1.8L), I found all deceleration backfires went away.

But first, look at these things: 1) vacuum leaks. Is your vacuum fitting(s) on the carb(s) well capped if not in use? 2) intake leaks. Carbs and manifolds tend to get loose. You should make sure the gaskets are good and use some locktight on the nuts. 3) mixture and fuel flow. Are your idle mixture adjustment screws in good condition, or have they been ground down by over tightening them? If so, replace them. Also check fuel pressure. It should be around 3 to 3.5 psi. All of these things could give you too much fuel on deceleration (eg too rich).

When setting up your jets, don't forget to consider altitude. The higher you are, the less dense the air and the less O2 content. Thus, you need more air and less gas to get the right mixture the higher up you are.

Also, how crisp is the throttle through 5000 rpm. If it is not sharp and smooth, consider a bigger air jet first. If it already is crisp, start with the idle and main jets.

Good luck.