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  1. #1

    Default Rough Idle on start then smooths out 81 Fairmont

    So this thing has been happening. I took a long ride the other day and suddenly I got a terrible rough idle at stop lights, but ran fine on highway for me to get home just bad sitting at a light.

    The plugs, cap, wires are a year old at most. So I changed the ignition coil as that was old and it seemed to fix the issue at first. I let it sit for a few days.

    I go to start it up again, sure enough very rough idle, almost like a the engine wobbling struggling to spin, but with my foot on the gas it stays spinning and then ultimately smoothes out to a nice high idle, then once it warms up, I give the gas a tap and were back in business.

    What gives? What do I do next?

    Thanks.

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  2. #2
    FEP Super Member xctasy's Avatar
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    Jun 2012
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    Leaking two bolts on the carb to log head adapter. Use an unlit propane torch, and check its not increasing engine speed on cold start. If its leaking, butane or propane will act as a=extra fuel in a high vacuum area, and the engine speed will increase.

    These old n line engines are amazingly simple, but the log head 1-bbl and also the 1-bbl 4.9 truck and van engines...same deal, the carb is bolted up with four bolts, two diagonals diametrically opposed, and leak prone if not used a lot. The 1-bbl 258 Jeep and AMC 232 have the same kind of carb arrangement, and those engines have a torque wrench requirement of 12 lb-ft. Ford uses another set of bolts in addition to one diagonal. This is leak prone because all four bolts are never torqued the same due to the way the carb is placed. If the adapter has each bolt torqued to the right amount, there are no leaks.

    The adapter looks like this





  3. #3
    FEP Super Member erratic50's Avatar
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    Dec 2016
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    Nebraska
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    I was going to guess some form of carb problem...

    We had a 78 E100 van and an 83 Club Wagon van back in the 80’s and 90’s that had 300 I6’s and did the same crap. Dad rebuilt the carb and resealed everything on the intake side on them when it happened each time and solved it.

    i never really knew the issue for sure, but remember dad spraying starting fluid checking for leaks and found several to start with. I like the propane torch idea a lot better, far less dangerous

    love how precise of an answer is listed above, I hope it works

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