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1936 Buick Model 80-320 Motor-Rough idle


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The engine has what I would call a "tail pipe miss" Carb has been apart 3 times-the last time sent to shop for rebuild. New Tune Up. I've toyed with idle screw adjustments 1/2 3/4 1 and 1 1/2 turns out. One thing is a constant problem- Once you turn the car off it takes a minute or two but you can see and hear gas leak out of the throttle shaft closest to the motor and drip on manifold like it is percolating-10-12 drops and then it stops. I've always set float level as per book specs but wonder if I should set lower for slightly lower fuel in bowl-or maybe this new gas expands more when hot.. I've tried different spark plugs at reccomendation of Buick club members. I do have slight backfire when I let off the gas and when I pulled the plugs the No's 1 2 and 8 were tannish while others were a more sooty brown. Good compression across all 8 cylinders. Motor has all new intake and exhaust gaskets. Valves recently adjusted and head was redone a while back. Motor pulls strong and never misses under load-just idles bad-especially at tailpipe end as it just spits. The old Delco Remy choke mechanism was disconnected years ago and a cable choke installed . Any suggestions appreciated. I keep thinking it is the carb. I would upgrade to another style carb if it wasn't for the linkages I'd have to come up with

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I'm sure you already thought of this but did you check your spark plug wires for leakage? I would remove the shield if it's installed and then move the car outside to a dark area, and idle it at night while you observe the wires for a leak. You may notice sparks right at the plug , in which case I would put some silicone grease on the plug just around the area under the tip and then install the wires again.

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+1 on the plug wires.

My new-ish Dodge Ram had a low-speed miss that was especially pronounced at idle. I chased down about a million things--plugs, wires, distributor, cap, rotor, O2 sensors, throttle position sensor, just about everything and no joy. Then I discovered a factory TSB about re-routing the plug wires to avoid crossfiring. Problem solved for $0 and 10 minutes of time. Feh.

Anyway, make sure the wires are in good condition and route sequential cylinder wires as far apart as possible (difficult with the straight-8s, especially with the side cover). As JohnD says, look at it at night and you will probably see some arcing.

Hope this helps.

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I had problems with a leaky needle valve causing problems like that. Also had carb kits have the wrong setting info printed in the instructions! I have also had vacuum leaks around the manifold do it too. Those are obvious if the idle screws don't kill the engine when closed down. squirt some gas from an oil can around the intake and see if it quits.You may also pull the choke slightly closed and see if it quits that way if you are worried about setting the engine on fire with the gas. (Carb cleaner works too but a lot of them are nothing more than laqquer thinner and it will trash an enamel engine paint job right now!) Helps to have someone listen at the tail pipe while doing it. Also did you retorque the head and gaskets after running it a while? I always do that when using the old type head and intake gaskets, even if they are NOS or new. Most inline engines will have a slighly leaner mixture at the ends and slightly richer in the center cyls. That might explain your spark plugs somewhat. Good luck with it.

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One more thing, I bet that car had solid core spark plugs when new. If you have resistor wires it may be more prone to crosstalk. I use solid core wires in my old cars and tractors. I am not worried about what it does to the punk kids blasting stereo at the stoplight next to me.

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if you take a shop towel and carfully lay it over the top of the carb with the engine running at idle and slightly smother the engine you can artificially richen the mixture beyond the carb adjustment, If you do this and it smoothes out you may have a vacume leak somewhere ( intake manifold, carb etc) Just do it carefully,

Have you already checked :

Timing, compression, basic unne up, etc

Roger

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