Engine miss when cold (low compression?)

• Sep 26, 2021

• #1

mountain33

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My 71 RR with 383 4bl has an annoying miss. Car starts right up Idles fine but you can detect a miss. I pulled all of the plugs today they look great, gap good. The weird thing is once i drive car about 2 to 3 miles it goes away. If i try to accelerate when its cold miss is really bad.Once car is hot everything is fine. Im looking for confirmation on what i think is wrong. Will low compression on a cylinder cause this symptom? I always assumed that low compression would result in smoke but it doesn't smoke at all. I do have a cylinder with low compression. Im assuming that once engine is hot everything is tight and cylinder is firing. Any thoughts?

 

• Sep 26, 2021

• #2

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R413

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Carbureted cars need some heat to run right. No performance analysis when cold. Sounds like your car is normal, they all have their own character. That fact that it runs fine with some heat in it is telling you it’s happy.

We are spoiled with the computer controlled multiport fuel injected cars of the last few decades.

 

• Sep 26, 2021

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Billccm

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Check the intake manifold bolts for being loose. Check the carb for being tight.

 

• Sep 26, 2021

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steve340

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You may have a faulty spark plug lead.

 

• Sep 26, 2021

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turbine68rt

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Are you running points or electronic?
Is your choke pulling off too much during warmup (if one is used)?

 

• Sep 26, 2021

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mountain33

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Thanks for the replys. will check the manifold and carb. Running everything original. Basically pulled car out of garage after 40 years of sitting. New plugs, points, condenser, cap and wires. had heads redone, lifters and rods. Kept everything stock. Has thermostatic choke. I believe thats what they call it. It starts on 1st turn of key, just a weird miss when cold. I ran car after pulling plugs and getting hot ran great. Going to see if i can identify the cylinder with the problem when cold.

 

• Sep 26, 2021

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turbine68rt

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mountain33 said:

Thanks for the replys. will check the manifold and carb. Running everything original. Basically pulled car out of garage after 40 years of sitting. New plugs, points, condenser, cap and wires. had heads redone, lifters and rods. Kept everything stock. Has thermostatic choke. I believe thats what they call it. It starts on 1st turn of key, just a weird miss when cold. I ran car after pulling plugs and getting hot ran great. Going to see if i can identify the cylinder with the problem when cold.

Click to expand...

Do you know what your dwell is set at currently?

 

• Sep 26, 2021

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steve340

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It could also be something like a slightly obstructed jet in the carb. Or the calibration is very lean.
If it is lean somewhere it will miss when cold as it needs a richer mixture and is close enough to be OK when hot.

 

• Sep 30, 2021

• #9

mountain33

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I put vacuum gauge on car today. The vacuum gauge needle would move from 20 to 18 very slowly. Ive seen on web results sticky valve, fouled plug, bad wire, cracked cap. If its a valve is that something that come and goes? If valve could if be a bad lifter effecting the valve?

 

• Sep 30, 2021

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steve340

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I would try plugging off all vacuum sources or potential issues and start there.
Plug off the PVC, brake booster etc.

 

• Oct 1, 2021

• #11

mountain33

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Well started from the beginning. Figured lets check everything. Pulled all the plugs and checked gaps, checked points, checked cap. Checked rpm. Checked to make sure carb was tight. When i put timing light on found it almost a 0. looked to be a couple of degrees btdc. When i loosened nut for distributor and grabbed cap to adjust got nailed good. Number 5 wire was bad. Replaced wire with the original that was in trunk, reset timing to 10 BTDC. Only reason i used 1 was seen it posted somewhere on this forum. Adjusted idle screws. Went for a ride. Ran so much better. I probably will benefit from having carb rebuilt professionally. Winter project. Thanks to all of you who took your time to help me try and figure this out. So glad I joined this group. Only social media i use.