| Dustbowl | -- 05-05-2012 @ 3:51 PM |
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I have asked this question long before but not satisfied with replies, so will try again. Why does my 36 backfire when engine is de-accelerating? I have original engine and carb but dual almost straight through mufflers. I understand this is common; I actually like the noise but very curious how unburned fuel gets into exhaust with carb closed, assuming that is the root of the "problem".
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| Stroker | -- 05-05-2012 @ 4:14 PM |
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Dustbowl: Check your ignition timing. A retarded spark will often result in a "trailing throttle" backfire because the mixture (having been lighted late) is still burning when it enters the exhaust system, plus the high vacuum tends to enrich the mixture, creating a "wet" exhaust. The fuel gets into the intake through the idle circuit, even though the throttle plates are closed. So....you have fuel, very little air, and a late spark. Together, they will produce a nice "backrap" or popping in the exhaust. Bump the timing a few degrees, and see if that doesn't cure the problem, if you consider it to be a "problem". There's nothing you can do about the idle enrichment when the throttle is snapped shut, while the motor is still revving. All you can do is bump the timing to attempt to burn the mixture before the exhaust valve opens. Just don't overdo it, as obviously too much spark "lead" can bring-on pre-ignition ("knock" under loaded conditions). Aftermarket dual's and glass-pack mufflers allow you to hear this after burn, as the stock system pretty much muffles it.
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| supereal | -- 05-06-2012 @ 9:54 AM |
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Look for a vacuum leak, such as a cracked windshield wiper hose, or leaking carb or manifold gasket. Be sure the throttle plates are fully closing to the throat of the carb. Air getting into the fuel stream under high vacuum allows the unburned fuel to ignite, causing a backfire. If left unrepaired, you can bet on destruction of the power valve.
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| Stroker | -- 05-06-2012 @ 4:37 PM |
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Dustbowl: Super's right about air leaks, but I figured you would have checked those out after your previous posting. As for the power valve, you don't say what you are using for a carb, but Super's also right if you are running a Chandler-Groves/Ford/Holly 94-(OEM for late 38 through 53). Those power valves have a delicate vacuum-operated diaphragm that is sensitive to lean mixtures that cause a backfire in the intake manifold ("coughing-back") as opposed to a nice exhaust "grumble". If you are still running a Stromberg 48 or 97, I don't believe you can hurt it with the "backfire" that you apparently savor; as the mechanical power valves in those old "leakers" are pretty stout. Unfortunately, we tend to use the term "backfire" to describe two different phenomena. Me and my ancient tribe of Left Coast contemporaries tend to refer to a trailing throttle exhaust snapping as "backrap". To me, and the aforementioned septuagenarian contemporaries, "backfire" describes a coughing-back through the carb; or what we used to call "Fuel Rejection". Methinks you should simply jerk your foot off the throttle and enjoy it.
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