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Discussion Topic:
Mystery Mis-Fire
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Mercman |
07-29-2023 @ 6:54 PM
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Member
Posts: 6
Joined: Oct 2009
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I own a 1951 Mercury M3 (1 ton) pickup. It has the original 8BA engine which has been bored out .060 and it's running on 12 volts, original single Ford carb, stock heads, Chevy distributor on the original Ford distributor shaft, and an electric fuel pump. It has always been reliable but lately it's developed a mystery mis-fire. I can't determine its source. I've replaced all critical electrical components: points, condenser, rotor, distributor cap, plugs, plug wires, coil, ignition switch, electric fuel pump, battery and battery cables. I had an flathead expert rebuild the carburetor last month too. I'm running 91 Octane and I've proved good fuel flow from the tank through two new filters (one on each side of the elect. fuel pp). I just completed a successful compression test today... all cylinders showing uniformly 125 psi +/- 2 psi. The new NGK spark plugs after a test run are carboned up, but all are firing strongly. After all those replacements, I've taken it out on several test runs but it always mis-fires (very short loss of power) then it recovers all within a 1/4 second. I cannot depend on it running on the street with unreliable power like this. It idles nicely with no vibration, and I just checked the timing again... it's spot on! I'm now wondering if there's a short in a critical wire somewhere but I can't think which one to start with. I have missed all the car shows this summer because of this problem. Any ideas? Signed Desparate.
This message was edited by Mercman on 7-29-23 @ 7:19 PM
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carcrazy |
07-29-2023 @ 8:01 PM
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Senior
Posts: 1587
Joined: Oct 2009
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It sounds to me like you have an intermittent electrical problem. Are all of the electrical connections in the primary ignition circuit clean and tight? Are all of the ground connections good and tight? Are you running the original type of solid metal core spark plug wires? Since you are running a points ignition system you shouldn't have a problem with electrical spikes causing a problem. Check the spring tension of the points set to make sure it is within spec. You could be experiencing "point bounce" if the spring is weak. Under what conditions are you experiencing the loss of spark? Hope you get your problem solved soon so you can enjoy driving your car.
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carcrazy |
07-30-2023 @ 8:07 PM
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Senior
Posts: 1587
Joined: Oct 2009
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Check inside of your distributor to see if any of the small wires are either shorting out or causing an open circuit when the point plate moves to adjust the spark advance. When your Chevy distributor was set up to mount on your engine, was the centrifugal spark advance curve modified to meet the requirements for a Ford Flathead? Flatheads are fast burn engines that like about 2 to 4 degrees BTDC of initial spark advance at idle with a maximum (total) advance of about 22 to 24 degrees BTDC at 3000 RPM. This is much less than the advance required for Chevy and other OHV engines. Are you running centrifugal and vacuum advance on your distributor? If you are just running centrifugal advance, it would be good to copy the advance curve of the Ford distributors used from 1937 thru 1948. Does your engine have the stock camshaft? What is your idling manifold vacuum reading? Unless you are experiencing spark knock, your engine should run fine on 87 Pump Octane gasoline.
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ford38v8 |
07-30-2023 @ 9:37 PM
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Senior
Posts: 2736
Joined: Oct 2009
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Mercman, your problem has developed before of after which of the maintenance items you mentioned? Some issues can develop at any time, old or new. Look for a carbon trace inside your distributor cap. Replace the condenser again. Check your power valve, particularly as you say your plugs are carboned up: One single backfire can destroy a power valve, so this is my guess and I'm sticking with it. By the way, an occasional misfire is not a catastrophe, it happens to all of us as we get older!
Alan
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