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Stumble at speed

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  • Fuel System: Stumble at speed

    This problem has been bugging me for several months. My 62 Hawk has a constant stumble (or hesitation) above about 35 mph. It gets worse as you increase speed, and also gets worse as the car warms up fully. This happens as you cruise. When I step on the gas at any time it accelerates smoothly, no problems. The car has a Mallory electronic distributor which has worked fine for well over a year now. Did have some glitches with it when I put it in. I first looked at ignition problems. I replaced all the one year old plugs. Plug wires seems fine, one year old as well. No arcing and distributor cap looks good. I also notice that idle slightly surges somewhat when engine is hot. Car does not stall.

    I think that this a fuel rather an ignition problem. I had the Stromberg two barrel rebuilt by a local Napa guy last year. Carb seemed fine until the stumble just came up one fine day as I was cruising. A professional old car mechanic told me to try a couple of 93 octane gas tanks plus a quality octane booster (I am estimating I am running about 95 octane), with no change so far in the car. He said that this kind of problem is almost always gas and not fire. I have also tried adjusting the carb which it did not need before this started. No effect. I replaced the fuel pump in late 2013. Car has an electric fuel pump that I turn on only on cold starts. I bypassed it but no change in problem. Turning it one while driving makes no difference. Cleaned needle valve and used carb cleaner also to no effect.

    Vacuum seems fine--about 18 inches.

    I have had many Studes over many years. Am a fair shade mechanic, but diagnosis sometimes stumps me. Making zero progress on this is frustrating to say the least.

    I really appreciate the invaluable help I constantly find in this Forum. Thank you for any advice you can offer.

  • #2
    I know you must have changed the fuel filter.......but I once had a 56J that did this, and it turned out to be a restricted fuel filter.

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    • #3
      I was thinking float level, and/or main jet.

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      • #4
        It sounds like a low speed/high speed transition problem, usually due to a plugged or partially plugged circuit in the carb. Disconnect the fuel line, take off the air cleaner, take a can of carb cleaner with the plastic extension tube, try squirting it into all the slots and jet holes in the carb bores, trying to force it back into the float chambers. See if that works, if it does, pull the carb top, drain the float chambers and do it again with the top off, wipe and clean the bowls dry. Might be a small speck of junk, or you might have picked up a tiny amount of water, and it's preventing fuel atomizing out of a small passage. Also a very minute vacuum leak can do the same thing. Check your base gaskets and try smeering (a technical term) some thick oil or light grease all around the edge of the gasket see if that helps. Check your vacuum advance hose and advance unit to be sure it holds vacuum (suck on the hose end, put your wet finger tip over the end to hold the vacuum, should stay stuck to your finger tip (or use a cheap vacuum tester).
        Last edited by karterfred88; 01-02-2016, 11:59 AM.

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        • #5
          You said you didn't think it was ignition related, but I would check the coil. I have had similar problems that seemed fuel related, but replacing the coil cured the problem.

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          • #6
            I have seen this before. There was a large chunk of debris in one of the main jet intakes. When I removed the large slotted plugs from the bottom of the fuel bowl that cover the main jets, there was the culprit. It (the dirt) would settle to the bottom, but would float up and cover 1 jet while under way.
            Bez Auto Alchemy
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            • #7
              I would use a fuel pressure gauge in line just before the carb to be sure you don't have a restriction in the pickup tube or a bad fuel pump.

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              • #8
                Thanks guys for all the help. It turned out to be a bad carb base gasket. After I pulled the carb the problem looked pretty obvious. The old fiber gasket (little over a year old) was slightly deteriorated on the inner edge. I used a metal reinforced gasket with some Permatex 81781 Ultra Copper gasket sealant. Car is back to running great.

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                • #9
                  I'd kind of expect spraying carb cleaner, or propane around the carb at idle might have helped find that.

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                  • #10
                    Originally posted by Dan Timberlake View Post
                    I'd kind of expect spraying carb cleaner, or propane around the carb at idle might have helped find that.
                    Or simply tightening the base bolts shoulda improved it. Never had a gasket go that bad. The newer ones must be made of ground up newspaper or something similar.

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                    • #11
                      I'm betting on electrical...

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                      • #12
                        I tightened the base bolts back when and forgot to mention that in my original post. I also tried the carb cleaner at base of carb and around possible vacuum leak areas, with no results. Is it possible that the leak showed itself only at higher speeds?

                        Oh--I also put in a spare coil compatible with the Mallory distributor, with no change,

                        I guess the moral of the story is to beware of fiber carb base gaskets.

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