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Engine Acceleration Flat Spot

Home Page Forums Discussion Topics – Ask the Experts Engine Acceleration Flat Spot

This topic contains 1 reply, has 1 voice, and was last updated by  62OF85conv 8 years, 4 months ago.

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    Nov 16 2009, 3:52 AM
    engine acceleration flat spot

    My current ’63 Jetfire is the second one on which i have experienced a quite noticeable dead spot when picking up speed or downshifting at low speeds. When cold, it also will not run smooth unless the idle speed is set higher than normal. The idle mixture also seems too rich, even when screwed in. When it gets up to operating temp, it seems to run a lot better and idles higher than normal. I have adjusted the automatic choke several times. I do not have a depressure valve, and cant find a vacuum leak anywhere, altho i wouldnt rule that out. Any advice? I am at my wits end.

    Nov 20 2009, 10:13 PM
    Engine flat spot

    I have played with this a lot recently. Definitely seemed to improve when I installed Jim’s depressure valve, but I had two solutions on my newly installed engine. First, I adjusted the accelerator linkage. Just kept playing with it until the I got power at the touch of the foot. Next, I retarded my timing just a little bit. My “flat spot” was spark knock – when I put a load on the car I would get a slight knock and not as much power as I thought I should. By retarding the timimg (mine was set a little too fast) by 1-2 degrees, the spark knock went away and now have good power. It’s supposed to cold idle high (mine’s around 1800 rpm) then it kicks down once warm. My experience is that when mine hits 170 degrees of temperature, it’s ready to warm idle, which is best for me about 800 rpm (manual trans), but it doesn’t always want to stay there until really warmed up. I’m working on a manual choke set up that will allow me to fully control it. Hope that helps

    Nov 21 2009, 1:47 PM
    engine flat spot

    Thanks for your thoughts. I have heard that the depressure valve can help. Will pursue that in the near future.

    Nov 24 2009, 11:11 PM
    Jim Noel
    Re: engine acceleration flat spot

    A suggestion would be that your primary idle circuit is plugged or restricted. Back out your idle adjustment screw (located on the top front of the throttle body, hex head with coil spring under it)and blow compressed air down the screw hole. Replace the coil spring and adjusting screw. Turn all the way to closed, then back out 2 full turns. Start her up and fine tune adjust, should be no more than a half turn one way or the other. Give it a try. Jim Noel

    Nov 25 2009, 5:44 PM
    Re: engine acceleration flat spot

    Thanks a lot Jim! I now remember we had discussed this briefly some time ago. Just havent had time to try it, but it is back to the top of the list again. Will let everyone know.

    Nov 30 2009, 7:4 PM
    flat spot/rough idle

    Jetfire Guy, I forgot that I had to do this once. all of the sudden mine would not catch at low idle. I did just as Jim said, removed the idle screw, blew it out with compressed air, and then adjusted it. It works great now. Also, checked out my actual rpm on the idle – cold is more like 2000 rpm, and warm is more like 600-800 (with my cam it has a little lope to it at low idle).



    An alternative to removing adjustment screws (and maybe not getting them reset to perfection). Get the engine good and hot, remove the air cleaner and spray lots of GumOut(TM) down thru the carb. It does tend to stall the engine so make sure youre good and hot first, when the engine stumbles too much wait a second and then when it catches spray again. Into all of the orifices and tubes sticking up – not just thru the air opening. My practice has been to do this every so often.

    Doesnt hurt to also put a can of the same stuff into the gas tank.
    There may well be other good/great cleaners out there but I know GumOut works!! GumOut is the best carb cleaner I have ever found.

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