| 31hotrod | -- 10-22-2010 @ 10:54 AM |
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What causes backfire through the carb? This has blown out 2 power valves on my Ford 94 carb.
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| supereal | -- 10-22-2010 @ 12:39 PM |
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The common cause is crossed (incorrect) placement of plug wires. It is also true that crossfiring can result from worn or cracked wires, often inside the metal conduits between the distributor and the plugs. Occasionally we find that a carbon path inside the distributor cap is the culprit. Backfiring is the usual cause for failure of the power valve diaphragm due to a sudden pressure rise inside the intake manifold. The power valve is actuated by vacuum, and can't withstand application of pressure. If these places don't show the cause of your problem, it is probably due to excessive carbon deposits in a cylinder or two causing preignition. If you are using a stock distributor, excessive timing advance usually isn't possible to a degree that causes premature firing. Aftermarket ignition systems are a different story.
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| TomO | -- 10-23-2010 @ 9:23 AM |
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If you describe your set up, maybe we can give you further help. In your other post you said that you had a 7.5 power valve, that would make for a rich mixture that could cause gas pooling in a cool intake manifold. Tom
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| carcrazy | -- 10-23-2010 @ 12:15 PM |
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Two typical causes of backfiring through the carburetor are the fuel/air mixture is too lean or the ignition advance is too great. Does the engine only backfire when accelerating cold or does it do it under all conditions? With more information we can determine the cause of the problem.
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| Old Henry | -- 10-23-2010 @ 4:17 PM |
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I changed my carburetor this week and had rough running including backfiring until I discovered that the gasket between the carburetor and the intake manifold had stuck to the carb I'd removed so I had none. I think the air leaking in under the new carb was causing the rough running and backfiring- too lean of mixture - just as carcrazy said. I installed the gasket and it took care of the problem. I have a follow up question for this thread. How do you know if your power valve is damaged? I'm not sure I have any symptoms of such even though I had quite a bit of backfiring for a short time. Still Old Henry
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| 37RAGTOPMAN | -- 10-23-2010 @ 4:31 PM |
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make sure your plugs are clean and gapped correctly.and the wires are attached in the proper order, check for vacuumn leaks. any loose wires on the ignition switch,? coil ? resister? did it back fire before you worked on the carb,? explaining in more detail would help. let hear how you made out 37RAGTOPMAN, and KEEP on FORDIN!!!
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| supereal | -- 10-24-2010 @ 10:46 AM |
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You can test a power valve a couple of ways. You can place it between your lips and try to suck or blow thru it without much effort. Or, after being sure the gasket is OK, set the carb on paper, with the bowl full, and see if it will leak gas. Of course, you can substitute another valve to see if it changes the operation of the carb under load. Sooty plugs are a good indication of a bad valve or leaky gasket. The valve is made to open in the 8.5-9" (HG) vacuum level. Most standard valves are unmarked. If you find a higher number than 6 stamped on the valve, it is probably a non-standard replacement, in most cases and probably installed after someone changed main jets.
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| ford38v8 | -- 10-24-2010 @ 11:28 AM |
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Old Henry, To elaborate on Supereal's advice, suck on the valve to see if it will adhere to your tongue by suction. If it doesn't adhere, turn it around and try the other way. If the diaphram is damaged, it won't adhere. Alan
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| Old Henry | -- 10-24-2010 @ 4:16 PM |
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My carb is already installed in the car. Any symptoms of a damaged power valve in the engine's performance that would suggest that it is damaged without removing it and disassembling it to test the valve alone? I checked the plugs and they weren't sooty. Still Old Henry
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| Old Henry | -- 10-25-2010 @ 8:30 AM |
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I think I got my answer. First, I wondered what the power valve does. Then I wondered how to know whether it was working right or damaged by backfiring. Here's what I learned: (The figure referred to is attached.) The power valve J (see Fig 9) is operated by the vacuum below the throttle plate through passage L, and the power valve spring K. At idle, the vacuum is the highest and decreases as the load increases. The diaphragm (actuated by vacuum) holds the power valve on it’s seat until the vacuum drops to from 81/2" to 9" of mercury where it is not high enough to resist the action of the spring. This point, at level road running at a constant speed is approximately 3800rpm. Under load as in climbing hills, etc., the vacuum drops as it becomes necessary to open the throttle wider in order to maintain speed. When the vacuum drops to from 81/2" to 9" of mercury, the power valve is opened by the spring, as when the engine speed exceeds 3800rpm on level road, and the fuel then flows into the power valve and the channels and through the high speed gas restrictions into the centre or main vertical well M (see arrows, Fig 9); this gives the additional fuel required for high speeds and for heavy loads at full throttle and low speeds. So, if I have plenty of power at high speeds and hill climbing, my power valve must be working OK. Does that sound right? Still Old Henry This message was edited by Old Henry on 10-25-10 @ 9:56 AM
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| supereal | -- 10-25-2010 @ 12:59 PM |
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If it ain't broke, don't fix it!
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