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1970 with 350 and automatic transmission. I've been having a low idle shudders that sounds and feels like a misfire ever since I redid my intake manifold and valve cover gaskets. It's especially noticeable waiting at stop lights in drive and I can visibly see and feel the rpm dropping slightly for a few moments and then drop 25-50rpm during shudders, comes back up, and repeat. Turning on AC and headlights (electric converted) make it worse. Driving in higher speeds is fine.
Other potentially unrelated symptom is rough cold starts but I believe I've found the issue to that problem: likely bad choke hold off and potentially bad divorced choke spring. The engine springs to life when I manually open the primary blade.
About the car:
Original Quadrajet carb. Mixture screws 3 turns out.
Points ignition with new points, cap and rotor. Dwell at 30 degrees with low variance.
New plugs but haven't replaced with new plug wires
Timing: 12 initial with manifold vacuum advance unplugged and capped. 29 at 2500rpm with vacuum advance unplugged and capped.
Vacuum gauge shows 21in vacuum but goes down to 20in whenever shudder occurs.
Park idle: 950 rpm. In drive: 650rpm - 700rpm with slight 25rpm tachometer needle shake.
I suspected something was off with my points distributor, but since the Dwell was consistent I no longer think it is. I am now suspecting it's a vacuum leak at the Quadrajet? I sprayed some carb cleaner around the shaft bushings and base gaskets but didn't notice any dramatic differences. Please help.
Last edited by LittleGrayBook; Dec 21, 2025 at 08:18 PM.
Reason: Added video
How much timing does your vacuum advance add? 12 initial is a little low since you’re only getting 29 total from your mechanical advance, but that shouldn’t be causing the shaking your describing.
I would remove and cap off every nipple on the carb and intake manifold. It could be possible you have an internal intake manifold leak too, although those are harder to diagnose.
Lastly I would fine tune your idle mixture screws with either a vacuum gauge trying to achieve a maximum value, or with an O2 sensor to get 14.7 on a hot idle.
I disconnected all of the vacuum sources, including ones on the manifold and the vacuum advance, and the shudder still occurred.
I replaced the carb to manifold gasket and it didn’t help. ChatGPT thinks throttle shaft wear is a possibility but the rpm didn’t change when I sprayed carb cleaner around the shaft bushings.
next step might be to individually check the wires as random, intermittent shudders may be ignition? Although dwelling was steady at 30 degrees when I last checked.
Originally Posted by Piersonpie
How much timing does your vacuum advance add? 12 initial is a little low since you’re only getting 29 total from your mechanical advance, but that shouldn’t be causing the shaking your describing.
I would remove and cap off every nipple on the carb and intake manifold. It could be possible you have an internal intake manifold leak too, although those are harder to diagnose.
Lastly I would fine tune your idle mixture screws with either a vacuum gauge trying to achieve a maximum value, or with an O2 sensor to get 14.7 on a hot idle.
I checked for arcs in total darkness and didn't see any. I also unplugged individual spark plugs and noticed some difference in the idle, so I assume they are okay.
The coil tower has some rust and I have a replacement on the way - I'm hoping this is it.
A small random rpm drop like that, sounds like a misfire to me.
I would be checking plugs, wires, cap, rotor, carbon tracks in cap, etc.
This old school spark testor will let you adjust the spark gap and tell you the voltage level that each plug stops firing at.
They should basically all be the same.
If one is way off, you know the cylinder that has the problem.
If all are low, the coil is weak.
A small random rpm drop like that, sounds like a misfire to me.
I would be checking plugs, wires, cap, rotor, carbon tracks in cap, etc.
This old school spark testor will let you adjust the spark gap and tell you the voltage level that each plug stops firing at.
They should basically all be the same.
If one is way off, you know the cylinder that has the problem.
If all are low, the coil is weak.
You can check the resistance in each wire too.
thank you! If the new coil doesn’t work I will definitely try the tester. I also haven’t changed the wires so I may just do that regardless. Many thanks again.
Wires go bad sort of easily.
There's a reason they did away with them in the 80s or 90s!
Mine has those heat shields that are an absolute pain to needle through... but since they hide the wires so well and keep the engine bay looking clean I'll go through the pain, haha. Here's what mine looks like at a recent car show, filmed by a youtuber:
Please help: Low idle shudders
