need some help on why something aint right. got a miss in one cylinder. Have new ignition (HEI cap, reluctor, module, rotor) and wires, timed correctly, new plugs and properly gapped, proper valve lash, miss is at all rpms, compression holds at 120 without dropping. There were no problems until I went to take it out for a run and it wasnt hitting on all cylinders..pulled the plug wires from the cap during running and isolated it to one cylinder..I pulled the plug on that cylinder and I found a fouled plug with a carbon chunk in the plug electrode. I looked into the cylinder and it looked carboned up some, so put some seafoam down the intake and also directly into the cylinder, let it soak overnight, put the plug back in, started it up and put the rpms up and let the smoke role away, then took it out and tried to drive it aggressively to knock some of that stuff loose, but nothing changed. I pulled the valve cover and ran the engine and didnt see any difference in rocker arm workings for that cylinder or any discernable difference in valve motion from that cylinder compared to its neighbor...push rods appeared to be turning and oiled and not bent or rubbing onto its hole edge. Plug pulled again, no fouling this time. WTF? This occurred all of a sudden and not gradually. Your thoughts? Thanks.
Buy yourself one of those $5 spark testers that go in between the wire and the plug to show if it is firing. I have one and it has saved me a bunch of times, even on my daily driver when one of the coil packs went out. If it flashes you have spark getting to the plug. Next I would put a brand new plug in that hole. It may look good but you said it had some carbon on it and SeaFoam will kick a bunch of that crud loose, maybe fouling the plug. Use that spark tester on every cylinder because it might not be the one you suspect. Harbor Freight carries them, that is where I got mine. Don
Have you tried swapping plugs with another cylinder to see if the miss moves with it?? Inlet manifold loose??? Just a thought..
THANKS DON...pretty sure its the one that dont make any difference in the motor running when I pull the wire off the cap. When I pull all the other wires, there's a really noticeable difference in running of the motor. And in my haste to post this ****er, I failed to mention that I put in all new plugs AFTER I took out the one with the carbon...and AFTER I cleared the seafoam out of that cylinder....still perplexed. I'm thinking I've either got a bad exhaust valve or a bad hydraulic lifter or a collapsed valve spring....either way motor has to come apart a little..
The cylinder that makes no difference when you pull the wire off, is the exhaust HOT on the header for that cylinder? Try swapping a different plug wire - I know you said they were new.
Nope, I did NOT check to see if the exhaust for that cylinder is hot...or hotter that the others...I will try that and swapping out a different known good wire tonight after work...but you would think that new plug wires would...well... work, like these apparently all do...but what the heck....stranger things have been known to occur in the occult world of hotrods, eh?
I've had a new wire that didn't have the core stripped and laid over before the terminal was crimped on before. New doesn't always mean that it is done right or works right. The easy way would be to swap wires to see if the miss moves. The other way is to check the continuity on that wire and maybe compare it to a wire on a cylinder that is working. The other thing to think about is a vacuum leak that is on the other end of a hose attached to the intake runner feeding that cylinder. Not likely if the cylinder is carboning up though.
I have seen it more than once.It will seal enough to make cranking compression but running compression will be almost nothing.The combustion pressure is alot higher than 120.
If after you have tried everything try putting a vac*** gauge on an intake vac*** port. If it bounces you have a valve train problem(valve,valve spring, cam).Hope it helps.
If the wire turns out to be good I'd look at the cap next to see if I'm getting spark out of that terminal. -Pat
BROKEN VALVE SPRING {JEST BET JA} SPRAY WINDEX OR WATER ON YOUR HEADERS, THE ONE THAT DIN'T FRY IS YOUR BAD ONE ,
I sincerely doubt with 120 compression that you have a burned or sticking valve. Do the easy things first, get that $5 spark checker and see if you really do have juice going down there. Why tear down the motor when it might be something external, like cracked cap, bad contact, etc. I had a stuck valve in one engine and I got 180 compression in the first 7 cylinders, then I got zero in the last one. That is what I would expect if you had that problem. Don
I'm new to the forum but I have been fooling with small block Chevy's a long time and if thats what you have may I suggest one thing that I found many times over. Check your firing order 18436572 some times putting wires on people including myself have swapped 5 and 7. They are next to each other physically and in firing order sequence. If swapped this creates a slight mis that you can't tune out at idle but revs up ok and that header pipe is cold, my thoughts, good luck...joe
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