Thread: Misfire

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I have a SBC with 11.5 :1 compression, which lately misses at idle but clears after 4000 or under load. Runs strong. Under full throttle it smokes blue, it has for years.
But I can't tolerate the idle miss anymore so I checked the rockers for proper adjustment, they were fine. I replaced the intake gasket, no difference.

Today I started checking compression, and number four plug was black and only 155 lbs whereas 3 others all have 190psi which is what all the cylinders had a few years ago. I figured I found the culprit and the other four plugs are hard to get at so I thought I'd throw this out.

Would a lack of 35 psi on one cylinder cause a total misfire or do I possibly have a bad spark plug wire and maybe that cylinder hasn't run proper for a while and caused the compression to drop?

So then it is either a valve or rings, short of that I am stumped.

Oh, I have MSD 6AL plus 3 step Retarder.

Originally Posted by DennyW

Get your water spray bottle out, and mist the wires at dusk, and see if there are any fireworks display. Check cap and rotor for any cracks. Check plug porcelain for any cracks. Replace the black plug, because it won't fire now that it is coated black.

If that cylinder is washed down, the compression would be less.

Thanks Denny. I did replace the plugs a month ago and I thought it made a
difference for several miles but now I can't say for sure.

I'll check the cap and rotor and replace that plug.

So are you saying that 155 psi wouldn't cause a total misfire? My thinking is that it wouldn't!

Originally Posted by DennyW

Replace the black plug, because it won't fire now that it is coated black.

You ARE the MAN. haha, I replaced that plug, and she ran on all 8. Thanks DennyW, stop by for a couple of beers, eh!

I'll check the compression after I set the timing and retune the carb, I had played with it to try and compensate, oh well.

Wow, it seems slow in this section!
Anyways, I drove it for half an hour last night and it runs great and then I ran it for a few minutes today before checking the compression. #4 was at 170 and the rest around 190-200. I have no idea what's #4's problem, but I'm not tearing it apart for 20-30 psi. Maybe it's just something stuck in the valve seat, or maybe it's a broken ring, but until it gets worse, she's on the road. .
So how come so quiet in here?