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EFV-8 Club Forum / 1940 Ford Discussion / Slight miss at higher speed

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Discussion Topic: Slight miss at higher speed

classiclover

01-31-2022 @ 11:45 AM

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Posts: 79
Joined: Aug 2021

Greetings

I just started having a slight miss at 45-50mph. Feels like fuel, but not sure. this is my first older car and would appreciate any advice in terms of diagnosing. I put new plugs in it a month ago, NGKs, so I dont think the plugs are the issue. I am thinking fuel pump, carb adjustment, timing, but not sure where to start. Any advice from you experts

Thanks

Dale

carcrazy

01-31-2022 @ 3:14 PM

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What car are you working on?

Your problem could be caused by either marginal fuel delivery or intermittent ignition function. If the miss feels like a soft miss, it is most likely a fuel system problem. A hard miss is usually caused by an ignition problem.

Check fuel delivery by cranking the engine with the ignition switch turned off. Disconnect the fuel line at the carburetor and place the line into a can. One pint of fuel should be pumped within 45 seconds.

Check for adequate spark by disconnecting one of the plug wires from a spark plug and holding it a small distance away from the engine head while the engine is running. You should see a strong spark jump between the end of the wire and the head. Be sure to wear insulating gloves or use an insulated pliers to hold the wire to avoid getting shocked while doing this test.

Be aware that carbon core wires, as they age, can break down and permit high speed misfires while performing adequately at idle and low speeds.

classiclover

01-31-2022 @ 3:55 PM

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Hi again

I apologize for not stating my 1940 ford stock coupe. I assumed that because of the 40 ford form we are on. I think I have a fuel delivery issue. I have ruled out bad gas, It is very noticeable at higher speeds. What do the experts think fuel vs elect.

Dale

alanwoodieman

02-01-2022 @ 5:25 AM

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higher speed, higher ign load. helmet distributor? check for shorting on the shaft going thru spark plug rotor

TomO

02-01-2022 @ 7:09 AM

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To determine if it is fuel related, pull the choke knob out slowly while you have the miss. If the miss improves, check for a bad flex line or a plugged fuel line.

You can check for sufficient fuel delivery by using this process:
To check fuel delivery, you need a graduated 16 ounce or larger container to determine the amount of fuel delivered in a measured time. Disconnect the fuel pump line from the carburetor and direct it into a graduated container. Crank the engine long enough to get 2 ounces of gas in the container and then continue cranking for 15 more strokes of the pump. The pump should have delivered 6 more ounces of gas.

Insufficient fuel delivery can be caused by a vacuum leak, plugged flex line, worn pivot in the pump arm, stretched diaphragm, plugged fuel line or tank pickup, plugged tank vent or wrong gas cap or a worn push rod. The push rod should be 8 7/8" long for the cast iron manifolds and 7 7/8" for the aluminum manifolds. It travel is about 3/8".

If you fuel delivery passes the above test, your problem is most likely an ignition problem.

Check for a good 1/2" BLUE spark when a plug wire is held near a good ground like a head nut without paint and the engine is idling. Weak spark can be caused by a bad coil, condenser or low voltage to the coil. Low voltage to the coil can be caused by the ignition switch or ballast resister.

If you have a good spark, the problem could be carbon tracking in the distributor caps or the rotor leaking spark to the distributor shaft.

Tom

This message was edited by TomO on 2-1-22 @ 7:18 AM

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