Re: Engine Misfire & Stumble
 

If your exhaust pipes are quiet enough so you can hear the knocking, give it a couple more degrees of spark advance and see if that clears up the misfire. More spark timing can help mask a lean condition, but the risk is engine damaging knock. If more spark eliminated the bog, then you could go after the carb and get to the root of it.

That plug reminds me of a problem I once had on a Chevy 3.1 Lumina on the dyno. It idled great and everything was perfect at the low end. Once I went to wide open for power testing, I had one completely dead cylinder. Checked everything. Finally changed the plug and the WOT problem went away. If I wiggled the plug just right, anyone could see the insulator rocking back and forth inside its shell. Turns out that the crack in the porcelain was just below the shell so it couldn't be seen. But at high cylinder pressures, the spark jumped there instead of through the super insulating high pressure gas inside the cylinder.

Those skinny little insulators on modern plugs are fragile and if we're not super careful they are easy to crack.

Re: Engine Misfire & Stumble
 

So after much grueling and testing I finally got down to some nitty gritty fingers. Last night I started playing with the timing. Set my initial to 0, vac advance to 16 and took it for a spin. Started it sputtin and poppin around 2000 rpm. I knew the next morning it was time for a compression test.

Fast forward 9:30 am, got it done. I was confident it was not a mechanical issue with the following numbers

160 #1
175 #3
180 #5
170 #7

170 #2
180 #4
180 #6
175 #8

Not perfect by anymeans, and more or less why I am looking at a rebuild. I refocused my attention to the distributor. Accel Super Coils like a tight tolerance according to their techs, 8500 +/- 1-2 percent. I tested the coil according to general GM specs of 6-10k ohms and came in around 9650.

New cap, rotor, matching ignition module and coil and away I went. Thanks to all whom have chimed in, It has definitively put me back in time and tune.:chevy:

Re: Engine Misfire & Stumble
 

So it was a bad Accel coil? Your compression is very good or your gauge is pretty generous.

Re: Engine Misfire & Stumble
 

Yes, the coil was bad according to Accel specs, still would run and fire easily. The gauge was a screw in type and very new, I trust it. Ran the test on a cool morning with a cold engine.

Hot compression most likely would show even better.:chevy:

Re: Engine Misfire & Stumble
 

Initial timing at zero?
Really!
Why?

Re: Engine Misfire & Stumble
 

Used it as a base line to figure out exactly how much vacuum advance timing I was getting, then set back up to 14.

I do like those adjustable vacuum advance units, makes dialing an HEI a bit more precise.