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Thread: "micro jerking" at highway speed

  1. #1
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    "micro jerking" at highway speed

    Greetings,

    At low speeds, or around town it's fine. No hot rod mind you, sluggish actually from a dead stop and while I've always been unhappy with that part of it, I've lived with it.

    But at highway speed (approx. 45 and above), there's a constant hesitation or "micro jerking" that really takes away from the driving experience. Looking for a starting point on what to do -- carb rebuilt professionally about 10 years ago, plugs have about 6k on them, cap and rotor I want to say also about 6k on them.

    Any thoughts or advice on where to start in diagnosing would be really helpful. I'm not the greatest mechanic, so I'd appreciate the opportunity to ask follow-up questions/request details.

    Thanks!
    James
    -James

  2. #2
    President Member TWChamp's Avatar
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    Do you have a hot blue spark from the coil wire to ground?

    Faster speed takes more voltage to fire the plugs.

    Not likely a problem, but this test only takes seconds, and if the spark is good, I'd look for a fuel problem.

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    President Member christophe's Avatar
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    Hi James,
    I'm stumbling onto the very same problem since my 63S-K6 is back on the road. I've licked most of it by checking carefully the ignition system. New points set at 37° partly took care of this but I also found that Standard Products ignition cap and rotor are close to crap now. So I reverted to NOS parts. Nevertheless, each time I'm going up a hill the problem is there again. It is really annoying and I think that the rear axle like does not like it too. To me, the engine acts as if, under heavier loads, it was unable to ignite correctly the gases in the combustion chamber or as if the gasoline was bad (no pinging, though). I still have two more tests to make. I bought a gasoline additive to enhance the octane level and a performance coil. I will report the results. Oddly, I noticed in the recent years that most of my carburetted cars had almost the same trouble (no jerking, but a lack of power at highway speed). I found one day that this disappeared when I closed the choke a little. So, I'm inclined to think that the problem is mostly related to the gasoline we have now. Unfortunately, I couldn't try this on the Hawk as the choke is automatic. Please, let us know if you find something.
    Best of luck and nice day to all.

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    Yes, could be some sort of a "lean surge" sort of a situation.
    The much leaner metering rods that I used to put in the AFB on my GT for summer driving, gave great mileage but would cause a slight surge when loping along on only the primaries. Felt like a slight jerk but was really a surge. If a bit of choke helps, I'd look into richening up the mixture?

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    Try adjusting choke hot so it remains slightly closed at speed. sounds like lean condition. if this helps raise the metering rods slightly. Luck Doofus

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    do you have any "popping" while accelerating ? I did and found the points quite used up. I'm also changing plugs. And I've also found that "choking" the carb a bit helps too. I've tried the mixture screw, but I need to check off things one at a time. The engine is on the stand now (install tomorrow)

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    Thanks to all for the replies. I have to work for the next several days so won't get to it but wanted to add a few points of clarification in case it helps guide the guidance.
    While at highway speed, it is very responsive and has no problem accelerating (the jerking is still there, of course but there is no loss of power).
    A few years back, I converted to Pertronix and it became too sensitive to heat and would die when it was 80 or above outside. So I pulled it out and put the points and condenser back in with no problems since. I did however keep the coil which is a "flame thrower" .6 ohms is all I can see on it at the moment. I thought this one was better. Should I reinstall the stock coil?
    Fuel pump is also relatively new, being a few years old and having about 3-4k miles on it.
    No popping during acceleration.
    I'll look in the manual in the meantime but is it relatively easy to adjust the choke? And how about the air/fuel mixture screws to fix the possible lean condition? Anything there -- I've long been vexed by the lack of change in idle when turning the screws all the way in/out...?

    Thanks so much, guys!
    -James

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    speedster#201
    By your id are we correct in assuming you have a 259 V8 with a carter CFB carburetor.
    By you saying that the idle screw do nothing; there is trouble with the carburetor. When you turn them all the way in the engine should stall.
    It is easy to overhaul a carburetor unless someone has previously screwed with it.
    Time to try another carburetor.
    You could also have a vacuum leak or a worn out distributor.
    A bad distributor or coil does not explain the idle screws doing nothing.
    I would solve the idle screw problem first and see what happens.
    Sometimes the easiest way to fix a problem is to throw parts at it.
    Ron

  10. #10
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    Thanks, Ron.
    We are talking original carb. The idle screws issue - possible fix if I rebuild or will I need a new carb? Thoughts appreciated!

  11. #11
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    speedster#201
    I don't understand how the engine stays running with the idle screws all the way in. Fuel is coming from someplace. Are the throttle plates too far open; defeating the idle screws and then maybe the main jets are too small. I just don't know. Something is not right.
    Send the carburetor to Dave Thibeault. He does a beautiful job. Just make sure who ever you give the carburetor to is told what is going on.
    The way I do a carburetor to set the idle is to connect a vacuum gauge and shoot for the highest vacuum ; while continuing to close the throttle and adjusting the idle screws for the smoothest idle at the lowest rpm.
    Ron

  12. #12
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    By you saying that the idle screw do nothing; there is trouble with the carburetor. When you turn them all the way in the engine should stall.
    You could also have a vacuum leak
    I don't understand how the engine stays running with the idle screws all the way in. Fuel is coming from someplace. Are the throttle plates too far open; defeating the idle screws.
    X3, if the air idle screws have no effect, there's a vacuum leak somewhere.

    1. The idle air screws control the air admitted. Back them out 1-1/2 turns and slow the idle with the throttle stop screw.
    2. Check for worn throttle shaft bores by putting a drop of oil at all four bores. Does the idle change?
    3. Disconnect and plug the distributor vacuum advance line. Most older advance diaphragms are rotted out and many new Chicom repops leak.
    4. Squirt oil all around the intake manifold-to-head joints. I've found intake gaskets totally rotted out causing major vacuum leaks.
    5. Often on 4-bbls, the secondaries don't return fully closed. Pry/tap the linkage arm lightly to insure they're fully closed.

    jack vines
    PackardV8

  13. #13
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    Jack;
    Learn something new every day or relearn it It has been many years since I worked in a carburetor electric shop as a teenager. I thought the idle screws controlled fuel and not air. I do vaguely remember some other carburetors that had what I thought was a large air bled screw along with the idle screws.
    I have seen many carbs with worn throttle shafts and the idle screws still worked.
    Anyway a vacuum leak could explain his other troubles.
    Ron

  14. #14
    President Member TWChamp's Avatar
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    Quote Originally Posted by rstrasser View Post
    Jack;
    Learn something new every day or relearn it It has been many years since I worked in a carburetor electric shop as a teenager. I thought the idle screws controlled fuel and not air. I do vaguely remember some other carburetors that had what I thought was a large air bled screw along with the idle screws.
    I have seen many carbs with worn throttle shafts and the idle screws still worked.
    Anyway a vacuum leak could explain his other troubles.
    Ron
    My Model A Zenith carb idle mixture screws control the air, but most all the other carbs I ever worked on, they control the fuel.

    Ron, you are sure right about your comment about someone preciously working on (screwing up) a part.
    It's a joy to restore generators that someone else hasn't messed with, but that's pretty rare.

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