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EFV-8 Club Forum / General Ford Discussion / engine surge

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Discussion Topic: engine surge -- page: 1 2

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

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

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)

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.

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

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

RAK402

06-15-2024 @ 7:28 PM

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Could the mechanical advance be hanging up in the distributor?

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.

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.

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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