Help with slight engine miss

[NoForAnAnswer]

I have a 1971 Chevy K10 with a 350/350.

Last year I put in a new (not rebuilt) distributor, new blue streak points, new condenser, new MSD plug wires, new MSD cap and rotor. Ran great.

A few months ago I overhauled the Quadrajet carb with new jets, new needles, new float, new choke pull off, new power piston bushing, new gaskets, new vacuum hoses, new fuel filter.

I adjusted it using a vacuum gauge and it read bouncing between 17-18in.

I did a complete tune-up about 2 months ago, the cap is still good, the rotor is good, the points are good, and I changed the plugs and adjusted everything.

I only got the intermittent miss on cold startup and went away when it was warmed up, then it missed a little at just idle. When I drove home from work yesterday it started missing under load, but not constant.

I went out and checked for vacuum leaks today and I didn't find any.

I use this board alot and have learned a ton just by reading. My dad bought this truck in 1983 and he gave it to me in 2000 and at first I neglected it but now I'm spending all my time and money trying to fix it up. I feel a great pride working on and driving this truck. So please help me button down this engine miss.

My dad and I know you good folks will be able to help us out.

[WIDESIDE72]

Sounds like ignition to me. Have you checked your plug wires? Be sure one isn't touching the exhaust somewhere. I would also pull the plugs to look at them. In the past with the engine off I have also pulled the plug wires off the plugs then lightly seated them. I then started the engine and using insulated pliers pulled the wires one at a time to see if there is any change. (be careful when working on a running engine!) You can also by an inline tester and plug it in line with the wire and plug. His is faster if you have a helper to start the engine and not have to run back and forth.
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[WIDESIDE72]

Originally Posted by WIDESIDE72

Sounds like ignition to me. Have you checked your plug wires? Be sure one isn't touching the exhaust somewhere. I would also pull the plugs to look at them. In the past with the engine off I have also pulled the plug wires off the plugs then lightly seated them. I then started the engine and using insulated pliers pulled the wires one at a time to see if there is any change. (be careful when working on a running engine!) You can also by an inline tester and plug it in line with the wire and plug. His is faster if you have a helper to start the engine and not have to run back and forth.
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[68gmsee]

Slight miss can be caused by several things from exhaust/intake leaks to ignition. If it's a bad or constant miss it will be easier to diagnose but an intermittent one is harder to diagnose.

Adjust the idle air screws to see if that will minimize it.

Check the vacuum advance to insure it's not leaking.

Check modulator valve hose and brake booster for leaks.

Carb butterfly valve shaft for looseness.

Place a timing light on each of the plug wires to see if the ignition firing is consistant.

Check and double check for leaking gaskets on carb and intake and exhaust.

If all check okay then you may have to start by swapping out spark plugs and other ignitions parts again. I've had new plugs and wires cause misses and it can be frustrating since we assume that's not the problem.

HEI can cause intermitten problems also and

[68gmsee]

One other thing that can cause intermittent sputtering or misses is fuel. Don't overlook that possibility. Contamination or just bad gas can give you all sorts of problems.

I've gone so far as getting some good fresh gas in a gallon container, place it under the vehicle with a long fuel hose and run the engine that way to make sure it's not contamination.

[allchevy58]

Did it start after you rebuilt the carb? You could have it too rich or too leak. Found my Monte Carlo was too rich at start up and too leak at 2500 RPM's. Can be checked with smog machine.

[cdowns]

i would put in a new fuel filter first

[NoForAnAnswer]

Originally Posted by allchevy58

Did it start after you rebuilt the carb? You could have it too rich or too leak. Found my Monte Carlo was too rich at start up and too leak at 2500 RPM's. Can be checked with smog machine.

Yes, it did start after I rebuilt the carb. It was running fine with the old carb on. But I decided to overhaul another quadrajet we had and put it on there because the one that was on before, I could bottom out both idle mixture screws and nothing would happen.

[NoForAnAnswer]

Originally Posted by cdowns

i would put in a new fuel filter first

Right now I have the filter in the carb and an inline filter. Should I take the in carb filter out? Would that be OK?

[geezer#99]

You could try by removing the carb filter.
First thing I do to find a miss is unhook and plug off the vacuum to the distributor. That eliminates too much timing as a possibility.
On the original carb that ran right and bottoming the screws changed nothing there wasn't anything wrong with the carb. Curb idle was too high and uncovered the idle slots in the carb.
Is your vacuum advance hooked to ported or manifold vacuum?

[NoForAnAnswer]

Originally Posted by geezer#99

You could try by removing the carb filter.
First thing I do to find a miss is unhook and plug off the vacuum to the distributor. That eliminates too much timing as a possibility.
On the original carb that ran right and bottoming the screws changed nothing there wasn't anything wrong with the carb. Curb idle was too high and uncovered the idle slots in the carb.
Is your vacuum advance hooked to ported or manifold vacuum?

I believe it's ported vacuum. I hooked up the advance to the same port it was on the previous carb. We tried to put the old carb back on and it wouldn't run at all. From what it sounded like it now has a massive vacuum leak. Maybe I can transfer all the new parts to the old carb and try and run it. The one I rebuilt had been sitting on the shelf for close to 30 years befor I overhauled it.
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[NoForAnAnswer]

I forgot to mension that I bought a new accel super stock coil and when I hooked it up I reversed the terminals for few months. Could I have ruined my coil?
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[geezer#99]

Try a different coil.
Check for exactly which. Ported or manifold. Manifold you'll feel vacuum at idle. Ported you won't. Manifold vacuum can cause a miss at idle.

[NoForAnAnswer]

Good thing I didn't throw away my old coil. I'll check on those on my day off tomorrow and post an update.
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[WIDESIDE72]

Originally Posted by geezer#99

Try a different coil.

x2
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[NoForAnAnswer]

Got outside today in 95 degree weather and took a good look at my engine. I found my cap and rotor were pretty worn so I replaced both. Looked at the points and they were in great shape so I put em back in. I replaced the accel coil with my old ac delco coil. Fired it up and still have the same miss as before. With it running I figured I'd check for vacuum leaks again. When I sprayed carb cleaner near the base of the carb, the rpms went down. So I might have a leak around the carb base gasket.
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