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Thread: M38A1 Engine Runs Rough

  1. #1
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    M38A1 Engine Runs Rough

    My engine Started running rough recently. Up until that time, the engine had run smooth and was easy to start. After sitting idle for about a week, it started right up but ran rough, like it needed to "clear its throat". I can drive it but it is obvious something changed. I cleaned spark plugs, replaced the condenser, replaced the fuel filter, checked wire connections, disassembled, cleaned and reassembled the carburetor (I believe I cleaned all the ports and passages). I replaced the gasoline with fresh gas in case it had collected some water. The engine starts quickly and idles well. I have not touched the points nor messed with the timing. I am at a loss on what to do next.

  2. #2
    Super Moderator bmorgil's Avatar
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    If it ran great when you shut it down, you may want to check for any carbon tracing in the cap. It could have gotten condensation in the cap and left some "tracks". A good wipe of the cap inside and look closely for any signs of current shorting between the posts and elsewhere. If you have a Dwell/Tach, check the dwell it should be 42 Deg. If it is correct and steady, The points are good. If you cant check the dwell I would definitely look at the points. They need to be clean and not pitted and black or, almost gone. Set the gap and check the timing. If you have an inductive timing light, you can check each wire at the plug for flashes. If they are intermittent or not there, you may have found it. In the dark open the hood when it is running. Look for "Crossfire", arching between the wires. Ozone is a funny thing it affects the insulation in the wires in mysterious ways.

    It will be interesting what you find.

  3. #3
    Super Moderator gmwillys's Avatar
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    Bmorgil is right on the money in his advice. I don't remember if you were running the waterproof military wires, you'll probably have to Ohm out the wires, because that is the most effective way to check them, if you don't see an external crossfire. At this point in time, I haven't found the spec for what the resistance should be. It depends on whether the wires are radio suppression wires or not.
    Last edited by gmwillys; 03-20-2020 at 06:31 PM.

  4. #4
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    I am running the military waterproof wires. I am not an electrical whiz so I may need an expanded explanation.

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    I will wipe out the cap and check the points tomorrow. The engine was running good when I shut it down prior to this problem emerging.

  6. #6
    Super Moderator bmorgil's Avatar
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    Like about 9,000 to 12,000 ohms per foot. However if they are Spiral Core (my favorite) it will be close to Zero to an Ohm or two per foot.

    You will need an Ohm Meter. You won't find checking wire resistance difficult.
    Last edited by bmorgil; 03-21-2020 at 06:37 AM.

  7. #7
    Super Moderator LarrBeard's Avatar
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    Rough Engine - maybe some good tips

    Quote Originally Posted by col_ken View Post
    I will wipe out the cap and check the points tomorrow. The engine was running good when I shut it down prior to this problem emerging.
    After you check the cap and make sure there is no moisture or carbon tracks, start it up again. If it runs OK, you're done. If it still runs rough, try the down home redneck approach; pull one spark plug wire at a time. If it runs a lot rougher with a wire removed, that cylinder is probably OK. If it runs a little rougher (or even no change) that wire is suspect.

    A Harbor Freight digital multimeter meter is a very good way to check those wires - they cost less that $10 and may even be free if you have the right coupon.

    https://www.harborfreight.com/7-Func...ter-63759.html


    Set it up to measure resistance (we'll talk you though that if you need it) and measure the resistance of the wires. The wire that looks a lot different from the others is suspect. If you have the Spiral Core wires, they all should read less than 100 ohms. Radio suppressed wires are a bit different, but if you get one that reads much above 50,000 ohms I'd suspect it has an issue.

    Let us know what you find. We need a success story just now.

    P. S. Do one wire at a time, we recently has a "runs rough" issue where one of the group swapped wires.
    Last edited by LarrBeard; 03-22-2020 at 07:45 AM. Reason: Added P.S.

  8. #8
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    Thank you all. I will keep you posted.

  9. #9
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    Could a vacuum leak be a possibility?

  10. #10
    Super Moderator bmorgil's Avatar
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    Always a possibility! However it seems to idle fine. Usually a vacuum leak will affect idle.

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