| MICHV8 | -- 06-15-2024 @ 6:48 AM |
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I'm cleaning my Holley 94 as something is causing the engine to surge from idle with no intervention on my part. I suspect either power valve or float level...anything else I should focus on? Thanks
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| carcrazy | -- 06-15-2024 @ 8:35 AM |
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If by "engine surge at idle", you mean "rough idle" here are some items to check: Dirty idle jets and passages Rich idle adjustment Dirty air cleaner Float level too high Choke not fully open Fuel volatility too high or too low Substantial air leak into intake manifold Fuel pump pressure too high or too low Clogged carburetor jets Ignition point gap too narrow or much too wide Sticking ignition point breaker arm Weak spark which permits some missing Leaking engine valve Sticking engine valve Broken valve spring Insufficient tappet clearance Contaminated fuel
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| MICHV8 | -- 06-15-2024 @ 8:52 AM |
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no, not rough idle, but significant engine acceleration as though the throttle is engaged (but it isn't)
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| carcrazy | -- 06-15-2024 @ 9:14 AM |
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I would look for an intermittent vacuum leak such as a hose going to the windshield wiper motor. The flexible fuel line going from the fuel pump to the carburetor could also be leaking causing an inconsistent fuel supply.
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| MICHV8 | -- 06-15-2024 @ 1:41 PM |
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yep...the wiper connection was not tight, but that didn't fix the issue although it seemed to help. I completely disassembled the carb and thoroughly cleaned it with carb cleaner. I also adjusted the float which was not correct. The power valve is the correct flat version, but the diaphragm looks thin. I do get some seepage from the throttle shaft, but not excessive. What I now find is that the car will idle about the same as it has it the past when running more smoothly and then backfire (not huge, but moderate) and then accelerate, return to same idle, backfire and accelerate, etc. Continues like that until I shut it down. I will double check the distributor, but it looked good when I first checked it when this current problem started. My current theory is that the backfire is creating a vacuum that is causing the power valve to open...is that possible? appreciate any ideas... This message was edited by MICHV8 on 6-15-24 @ 4:08 PM
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| carcrazy | -- 06-15-2024 @ 7:22 PM |
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Likely causes of a backfire through the carburetor are: Fuel/air mixture is too lean. Too much spark advance. Leaky or sticking intake valve or weak or broken intake valve spring. Leakage of current across distributor cap by allowing spark to occur in a cylinder which is on its intake stroke. Two mixed-up spark plug wires
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| RAK402 | -- 06-15-2024 @ 7:28 PM |
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Could the mechanical advance be hanging up in the distributor?
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| carcrazy | -- 06-15-2024 @ 7:41 PM |
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What year engine are you working on? Early Ford Flatheads up to and including 1948 passenger car engines have centrifugal (mechanical) spark advance mechanisms which use a vacuum brake to reduce the spark advance under some conditions. 1949 and later Flatheads do not have a mechanical advance feature in the distributor but rely solely on a vacuum advance mechanism.
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| 40 Coupe | -- 06-16-2024 @ 5:44 AM |
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Backfire can rupture the power valve. Once ruptured car will act flooded at start, very long start time and very poor idle. I use only rebuilding kits from Daytona in Fla check their web site. Do check the fuel pressure also.
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| MICHV8 | -- 06-16-2024 @ 6:49 AM |
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thanks for the input 1939 59AB will get new power valve will double check timing some advise removing distributor plunger plate entirely???
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| carcrazy | -- 06-16-2024 @ 7:40 AM |
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Do not remove the vacuum break plunger completely. Just back off the adjusting screw so that it is no longer effective.
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| MICHV8 | -- 06-16-2024 @ 12:09 PM |
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Some progress. I re-worked the distributor timing and the car idles fairly well considering some cylinders have low compression. It had one slight backfire when it was cold, but didn't repeat as it did before. However, it did "surge" (significant increase in RPM) on its own a couple times and does not immediately return to idle when giving it accelerator gas...it seems to surge for a few seconds before it returns to a more normal idle. Additionally, I turned the idle screws in all the way and it had no affect on the idle, so something is not right...
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| carcrazy | -- 06-16-2024 @ 1:10 PM |
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If the idle mixture screws have no effect on the running of the engine, then either the throttle plates are not at the closed throttle position or you have a leaking power valve and massive vacuum leaks. Have you measured your manifold vacuum at idle? It should be 19 or 20 inches of mercury at sea level.
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| MICHV8 | -- 06-17-2024 @ 7:00 AM |
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will do, but will get new power valve and throttle shaft...who is the best supplier?
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