Misfire at Idle

Classics Fan

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Anacortes, WA
1968 350 2V. Yesterday I set timing, idle fuel mixture and idle speed (895 in Park and 640 in Drive). There is still a slight miss in the left bank at idle. I pulled the plugs on that side and they looked good, nice and tan. I checked the ignition wires for good connection at both ends. There is no stumble or hesitation under any throttle position at any speed. The car runs and pulls well. Any ideas what could be causing the slight misfire? The only thing I can think of is a bad wire. TIA!
 

melsg5

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Check the wires with an ohm meter, 1k ohm per foot of wire max.
What did you set the initial timing to with the vacuum advance disconnected?
What procedure did you use to adjust the mixture screws?
 

Classics Fan

SUSTAINING MEMBER
88
3
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Anacortes, WA
I’ll see if I can figure out how to measure resistance. I was shooting for 34* total advance but stopped at 14* initial that yields 32* total, vac adv disconnected and plugged. For idle mixture I started with 4 turns out then adjusted for highest vacuum with gauge attached.
 

melsg5

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If you have a tachometer try adjusting the mixture screws 1/4 turn at a time on each side till you get a drop in rpm and then backout 1/4 turn. If you can do that safely in gear, wheels chocked and parking brake on, setting to 650 in gear.
______________________________
 

Classics Fan

SUSTAINING MEMBER
88
3
8
Anacortes, WA
Issue resolved. In an earlier post I described how the sound of the miss was noticeable at the tailpipe. Today I jacked and braced the rear of the car and inspected the exhaust. I found a large open hole in the left pipe where it bends to go up and over the rear axle. Whoever installed the mufflers did a terrible job connecting and welding two pipes together. The sound I was hearing wasn’t a miss, even though that’s exactly what it sounded like. It was exhaust gas blowing into the tailpipe, uncontained by the big gap, kinda like puffing into a bottle. I fixed the problem temporarily by wrapping aluminum tape around the joint where the gap is. That solved the problem.

I went ahead and fine-tuned the idle mixture to achieve maximum RPM and reset idle RPM to 650 in Drive / 900 in Park. I also checked resistance on all wires and found them good. The car is running practically like new. Thanks for your help guys!
 
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