| 39topless | -- 02-21-2013 @ 12:23 PM |
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I am getting an occational( like every 10 seconds) backfire in the engine of my 239 '40 ford. Would this be a timing problem or something else? This message was edited by 39topless on 2-21-13 @ 12:34 PM
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| supereal | -- 02-21-2013 @ 12:32 PM |
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Is the backfire thru the exhaust or carb? Is it often, or occasional? Does it happen mostly or only on deceleration as you coast? Common exhaust backfire is often due to a vacuum leak. Check your vacuum hose to the wiper for cracks, and the line to the distributor, to be sure it is tight. We check for hard to find vacuum leaks by spraying some carb cleaner on the suspect places. That stuff is very flammable, so be careful. It can be, but rarely, due to a leaky intake manifold gasket, or faulty power valve, or its gasket, in the carb. If your plug wires are old, it can be crossfire, particularly in the plug wire confuits. Same is true of carbon paths inside the distributor caps. I wouldn't suspect the timing.
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| 39topless | -- 02-21-2013 @ 12:39 PM |
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backfire is through the carb. seems to change frequency everytime I run it. The wipers are not hooked up to the vacuum line. should I plug this line? Thanks alot. Great advice.
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| supereal | -- 02-21-2013 @ 4:07 PM |
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Yes, plug any unused vacuum lines. From your description, I would run a compression test, and hook up a vacuum gauge. If you need a diagnostic chart, I 'll post one. I suspect you may have a sticking or burned valve.
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| alanwoodieman | -- 02-21-2013 @ 6:27 PM |
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look into the tail pipes-are they sooty? Pull the plugs and look for one that is a different color or look---sometimes an overly rich mixture can lead to what you are experiencing. Timing problem would most likely cause numerous back fireing, check you wiring- is on of the wires on the wrong plug? a loose wire in the inner cap can cause a backfire by "leaking" the coil charge to another plug.
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| 39topless | -- 02-22-2013 @ 8:50 AM |
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Great suggestions. I will get on this this weekend and post my results. Thanks so very much.
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| oldford2 | -- 02-22-2013 @ 5:18 PM |
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If you have been getting backfires thru your carb your power valve may be blown too and leaking gas down the throat. Check it also. John
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| 39topless | -- 02-27-2013 @ 5:47 PM |
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Ok, figure this one out. I checked for vacuum leaks,none. I pulled all the plugs, 5 were very black, cleaned and replaced and did a compression check. All between 55-75. Started up the engine, NO backfire. I am guessing maybe a hanging valve decided to be good.??? Thanks for all your help. Phil
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| supereal | -- 02-27-2013 @ 6:51 PM |
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A sticking valve wouldn't affect all five cylinders. Sooty plugs usually indicate an over rich fuel mixture. In the carb, the only part that would affect more than four cylinders is the power valve because each side of the carb feeds four cylinders only. It is probable that either the power valve is leaking, or its gasket is the source of excess fuel, and also causing the backfire. It can also be due to carb flooding caused by overfilling. In that case, suspect the float.or the float is hanging up. I'll bet your problem will return. It only takes one backfire thru the carb to blow out the diaphragm in the power valve. This message was edited by supereal on 3-1-13 @ 8:38 AM
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| 37RAGTOPMAN | -- 02-28-2013 @ 3:52 PM |
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39topless I had the same problem with a MODEL A, the plugs were dirty,it was poping though the carb, This was in a rebuilt engine,I think breaking it in, caused to it to soot up,maybe to much slow speeds.? and I disected the carb,points timing,etc, there is not alot to look for in a simple system like a MODEL A I found solution in a old time repair manual, make sure you have the correct plugs [heat range] in for your type of driving, I can tell you I had a problem with original CHAMPIONS, in my 37 ,they were very old,and one or 2 were burnt away,from many years of use, I tried a set of NGK, Let me tell you that my 37 runs very well with them it it, I was Amazed how much more power it has, and seems like it gets better MPG, My 3 cents 37RAGTOPMAN
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| 39topless | -- 03-04-2013 @ 12:09 PM |
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Great help, I am going to rebuild the carb and hope that will do it. I don't quite understand the physics behind the backfire but if rebuilding the carb solves the problem, that's all I need to know. Thanks to you all for the excellent advice. This forum is great, Phil
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| Stroker | -- 03-04-2013 @ 5:33 PM |
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Well, the term "physics" scares most of us without a doctor's degree in math, but basically, a lean fuel mixture will burn faster than a stoichometric (read as: "just right") mixture. A very lean fuel mixture can start burning before the intake valve closes, which results in flames from the cylinder going backward into the intake manifold with a resulting minor explosion, which results in a "backfire" through the carb. That's why we always ask if it is backfiring through the intake, or the exhaust.
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| MG | -- 03-05-2013 @ 9:50 AM |
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Stroker, Will pulling the choke out cure the backfire through the carb?
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| Stroker | -- 03-05-2013 @ 1:18 PM |
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MG: Only if it is due to a lean mixture.
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| ken ct. | -- 03-10-2013 @ 6:28 PM |
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It only takes 1 back fire through the carb to blow out PV. Check it for sure. Youve had many according to your statements. ken cy.
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