QuestionQUESTION: I have a 72 ghia with a 1600cc engine. It has two dual-port kadron carbs. I have been getting consitent backfiring while decelerating in gear. I have no problem while accelerating or during steady-state driving. Sometimes the backfire is a muffled popping noise like you might get through the carbs, but othertimes it is a loud bang. I have adjusted the timing trying to eliminate it with no luck (009 mechanical distributor). I have looked for a leak in the exhaust system, but again no luck. The spark plugs appear to have a slight build-up of carbon, but replaced them with no change. I also reduced the main jet size from 130 to 127.5 to help with the carbon as I felt it was caused by a rich fuel mixture. I have also replaced the distributor points with an electric ignition module, but no improvment occurred. Not sure what to try next...any ideas? I use the Bentley car manual, the Chilton's manual and John Muir's "how to keep your volkswagon alive" manual.
ANSWER: Deccelaration backfire is caused by either too much fuel or 'floating' valves not closing fast enough.
You may do a quick valve check since on your engine it is very simple. Make sure to rotate EACH cylinder to TDC before adjusting it.
If the valves check out, your are going to have to reduce your fuel output or drastically increase your air volume to the carbs. An oversizing of the air filter sometimes helps.
Have you adjusted your gas level in the carbs down about 3/8 of an inch? If they are right on the break point, that can be an issue.
A common mistake is thinking that multi-carbs means more gas to deal with. Properly tuned they actually consume less.
I had a 3 2-barrel set up that got 21 mpg.
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QUESTION: Tazz,
I put in new air filters and unfortunately the problem was not resolved. I went through my old notes and found that the carb idle jets had been slightly enlarged to improve performance from a stand-still. Do the idle jets play any role during deceleration? Is it possible this could be a source of excess fuel? By the way, I am getting better than 23 mpg, so I don't think my main jet size is a problem.
ANSWER: The amount of excess gas we are talking about would be visible to the eye looking in the carb. It is like what you see when the choke is allowed to close with the engine warmed.(make sure your choke flaps are secure).
It would probably behoove you at this point to put a vacuum guage on your engine for diagnostics. It will check the condition of your valves and is a sure fire way of adjusting the air/fuel mixture without guessing.
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QUESTION: Tazz,
Thanks again. One final round of questions (I hope)...
1. Could this be cause by a sticky valve? If so,
2. Would there be other symptoms? (car runs great, no power loss, etc.)
3. How can I verify if a valve is sticking?
4. How do I fix a sticky valve?
AnswerTHIS IS THE FIRST ANSWER I GAVE YOU: Deccelaration backfire is caused by either too much fuel or 'floating' (STICKING) valves not closing fast enough.
You may do a quick valve check since on your engine it is very simple. Make sure to rotate EACH cylinder to TDC before adjusting it.
If the valves check out......
A vacuum gauge will show a valve floating or sticking, and a head job is needed to repair.
