I've got a mystery here

[hawkdoc60]

My little 88 L "Cherry Bomb" has a cylinder issue. When I rescued it, it was running on three cylinders. I narrowed it down to the # 3 cylinder that was not firing. When I replaced all four plugs, there was baked-on carbon all over the electrodes of the # 3 plug. I changed the wires, cap and rotor. It runs pretty good, especially on the highway. However, here's the mystery:

At idle and slow speeds, it is still running on three cylinders. The number three is getting spark and the plug is still clean. However when you pull the plug wire off the # 3, there is no change to the rough idle. It sparks from the wire to anywhere I place it on the engine, so I know the wire is good. If I pull any of the other three wires off the plugs, the engine fizzles.

Anybody have any idea what's happning?

Oh, I have driven it to work and back twice(45 miles each way) and on the highway at 70+ mph with no problems. In fact, it lives for the interstate.

[jasonpaulbauer]

Check compression. Sounds like you might have a compression leak.

[hawkdoc60]

I know that there is some leakage, because she burns a little oil. Would a compression leakage cause this much problem?

[jasonpaulbauer]

yes it could.

[Festy46]

If the #3 cylinder is weak (compression wise) pulling the plug wire wouldn't be as noticeable as pulling the wire from a strong cylinder.

[25Horseplay]

How about adjusting the valves?

[Pu241]

Another indicator is MPG!
But I'm betting on compression/valve adjustment!
DOes it use any oil?

[hawkdoc60]

It does burn a little, yes. It also has the typical leaks around the valve cover.

[Pu241]

Just thinking could be valve stem seals!
But just as likely a ring issue.
Really need to do a compression check!

[hawkdoc60]

I appreciate all the input. I guess I'll be hitting Harbor Freight for a compression tester.

[jglutz]

Yep.....most likely compression related....valves or rings

jJust a thought...if prior to your tune up....the No.3 plug or wire was shot and the plug showed baked on carbon....the carbon built up may be pretty severe....like...the intake port between the manifold and the valve seat is blocked.

[hawkdoc60]

I'll have to look at the intake port when I tear it down. The idea does sound intriguing.

[ejp2fast]

on my festiva when i got it the #3 cylinder had a burned valve and a chunk was missing (whats with the #3 cylinder????) anyway, to check where the leak is take the plugs out and squirt a little oil in #3 through the plug hole. what you want is the oil to seal around the cylinder wall. this will tell you if the rings are bad. actually 1st, before you put oil in, do a compression check on all cylinders and record them down. THEN, put the oil in and re-check and record. if the compression goes UP in the #3 when the oil is in, then it may be rings. this depends how much it goes up.. I'd say it is more likely to be in the valves or valve stems than the rings.
Here was my situation. Did compression check and #3 was only at 60 psi (supposed to be 180 psi).. then i added oil and it went to 120psi. so i thought it was rings... I took the head off and #3 valve had a huge gouge in it, so some of that oil must have gone and did a little sealing in the valve. i ended up putting in new rings, new valves, etc while i was in there. hope this helps.

[WmWatt]

Look for carbon deposits on the underside of the valve cover, an indication of exhaust valve not closing properly. Mine was dirty over the #3. A chemical cleaner oil additive desovled the carbon and raised the compression, although some don't recommend chemical cleaners as they might dislodge crystalized carbon into the cyclinder.