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Engine Stuttering/misfiring at high revs - 289

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    Originally posted by cokecan View Post
    Vacuum advance? Could it be advancing timing too much (or not enough) causing the engine to run rough/funky.
    I think you're correct. It does seem to run differently, if not better when I play around with that.

    Any advice on further vacuum diagnoses and fixes?



      I guess first thing is to get a vacuum gauge on the carb and check the vacuum is where it needs to be and is stable. Do you know if your using ported or manifold vacuum? You can also check if the advance in the dizzy needs a clean/lubrication.

      My understanding is that vacuum advance should be connected to the manifold port on the carb as oppose to the ported port. When I had my 302 tuned earlier this year, we discovered that it ran smoother and better without the advance connected at all (just plugged the carb and left the vacuum advance can in place). I haven't noticed a difference in gas mileage but the car is certainly smoother all round. The advance was advancing too much at at 3000rpm.


        If you have a good timing light a quick check is to slowly rev the engine and watch the ignition advance with the light. It should advance smoothly and progressively with engine speed and also retard as the engine slows. Over the years the mechanism wears and lacks lubrication then starts to stick. Also you can put a short length of pipe on the vacuum canister and suck on it with the engine idling, again you should see progressive movement of the timing marks.


          Sounds like a breakdown in the HT ignition side of things....wait until itís dark and rev it with the hood open, see if thereís sparking across leads distributor cap and coil ...
          1966 Mustang GT Coupe A Code 289 4V. Guardsman Blue with Diamond White stripes. 15" TT D wheels with BFG 215/60/15F 225/60/15R.

          If it aint broke...Fix it till it is!!


            Had a similar thing years ago, stupidly replaced the 40 year old points with new. Engine would not rev past about 4000 when it just broke into misfires. Replaced the new points with the original Ford 40 year old stuff after a little clean, and back to easy 6200 rpm shifts. HTH
            Cheers, Martin