Inliners International Forums General Discussion Electrical Misfire?

I have a '69 chevy 250 and I found my number 5 plug was black while all the others were in the tan/white color range. Number 6 was darker, but within the color range of running within specs. I will be replacing the plugs, rotor, cap, and points this week to make sure everything is up to spec and hopefully solve the next problem

Next problem. My Tach will bounce around when I am holding the engine speed over 2500 with out a load and it matches the backfire(s) just about everytime. When attempting to acclerate, the truck will bog down, the tach will bounce around, and then will accelerate. I don't notice a change in engine speed during the bouncing. The bounce is between 500 and 1000 "rpm".
Any body else have any suggestions?

If #5 is a dull black it's not burning the gas, bad plug or wire. If #5 is shiney black it's oil contaminaton. Since your getting a backfire you may have a bad head gasket between 5 and 6. When you change the plugs do a compression test.
Larry

You might want to check your distributor to make sure there is not too much play in the shaft or a bad gear. Either could cause your bounce. You may also want to check your engine grounds and the tach ground.

I'd also take a few minutes to check the plug wire for #5. Resistor wires break down over time.
I can't recall the exact resistance per foot but a quick check would have it read somewhere between the resistance of #6 wire and #4 wire. When checking it work the wire around a bit with the meter leads connected. A break will sometimes show continuity and vice versa when a wire is twisted into different curves.

Could also be as simple as a bad plug too.

Thanks for the input.
The plug is dull black. The engine has about 2500 miles since the rebuild, so I hope it is not a head gasket, but I will do a compression check anyway. I'll report as things progress, or degress.

Update...Problem still there
new plugs, rotor, cap. Old cap looked as if the rotor was not centered. I will check this in a couple of days. No play in distibutor.
Compression, All cylinders were equal.
PCV was causing some of the backfire, This and adjustment on the carbs resulted in 20 in hg for the vacuum.
I took an old plug wire and tried each cylinder and there was no difference, My test meter is broken so maybe a resistence check tomorrow.