Originally Posted by Pre-Tuner
I have a 406 SBC with a misfire. I have tried everything I know how to find it and I have found next to nothing.
A little history. Its a new build with about 600 miles on it. It's a mild street engine with a street/strip 276HR hydraulic roller cam, AFR 195 heads, 9.8:1 compression, hypereutectic pistons, Holley 770 Street Avenger carb, Mallory HEI distributor. The entire engine is brand new from carb to pan with the exception of the block.
It misfires most when it's cold, then seems to almost go away within about a minute of firing it up. I have done a compression check, and all are between 155 and 170. I did a running compression check and all are 100-110 or so. I've tried a different set of wires, new set of plugs, and a new cap for the already new distributor. When I take an infrared temperature, there isn't any one cylinder that is low. For some reason, cylinders 5 and 6 are a little higher than all the rest, but I can't explain that and not sure if it matters anyway.
The ONE thing that I found when changing plugs is cylinder 3 looks a LITTLE wet. It doesn't look bad, but there is only 600 miles on the plugs. A fresh set of ACDelco plugs (gapped .040) misfire immediately also. I tried changing the plug wire, again, on that cylinder only and there was no change at all.
I took the carb to a specialist just to have it checked out and it is perfect. The exhaust smells a little rich, so I wanted to make sure there wasn't a problem with it. He bolted it on a test engine and it ran like a top.
I'm lost at this point. I have no idea what to check or even how to narrow down which damn cylinder it is.
Any help would be greatly appreciated
Does the miss ever go away completely, like when you bring the rpm up? Is it consistent (same cylinder) or does it vary?
Check for arcing plug wires w/the engine running, hood up, in the dark to see.
What ignition system are you using? Cap spotless inside?