Any idea what would cause this?

[barelycompetent]

Im sure this has been discussed before but its just come up with me.  My 1,2 and 3 exhaust headers become hot when the bike warms up, but the #4 is just warm and can still be touched.  I pulled the spark plugs, I have iridium plugs in my bike, a 1981 cb750k, and 1,2 and 3 were good, you could still see the white iridium part, on #4 it was black and had oil on it.  In the pic the #4 plug is on the left and the #1 is on the right.  Is it possible I have three cylinders running lean and one rich?  The bike starts and seems to run just fine, other then my high idle lol.  Any input on what to check, or how to remedy this situation is greatly appreciated.  Thanks in advance.

[shinyribs]

Is it black from oil or fuel? Could just be a case of weak spark. Maybe a loose connection or a bad wire on that one cylinder?

[barelycompetent]

Thanks shiney, there was oil on the plug, now whether or not that caused it to turn black I dont know.  I ran it past a buddy of mine locally who seems to think all the carb work I did on the bike could have fouled out that plug.  I put the new plugs in while working on the bike over the winter, then Ive had the carbs off and on 4 times.  He seems to think I should just replace the plug and not worry about it.  Im just wondering what could cause that and is there more going on that I should be concerned with then just carb work fouling out a plug.  Why would it only foul out that particular one?  Thanks for the response.

[cdaiscool]

If its oil on the plug, then I would suspect valve seals letting oil past. If its carbon, I'd suspect a bad tune in the carbs.

I personally would be getting new, cheap plugs (why waste money on iridium for testing? Get them once all is sorted out), run them, check for carbon and tune issues, tune as needed, verify your spark is good, then a final step of putting new iridium in there.

If you've got oil for sure messing it up (wet vs dry black) then you may want to pull the head and check out your top end. See if the valves are adjusted, if the seal is visibly compromised, and check that out. The oil could also be causing the cylinder to have a bad combustion event, and only fire sporadically, causing the exhaust to be cool.

[seestheday]

If it is for sure oil then I agree with cdaiscool.  I bet your valve seals are leaky.  The stock ones are notorious for failing badly.  There are only two paths for oil to get into the combustion chamber that I can think of (someone please correct me if I'm wrong).  Through a failing valve stem seal or through a badly failing ring.  I think that if you had a badly failing ring you would have other issues as well.

When I replaced mine my old ones all tore or broke apart.  I also had visible signs of leakage on pretty much every one.  Jebbysan (who just joined this site) sells upgraded ones that are apparently much better beefier viton ones that won't leak.

Unfortunately you have to pull the engine and head to replace them.  I believe it is possible to replace the valve stems seals in the 1100's and possibly the 900's without removing the head, but I don't think it can be done on our 750's.

While you're there you might as well lap the valves, clean up any carbon build up etc...

I just went through all of this.

[barelycompetent]

Thanks for the replies, now that Im thinking about it, I think the plug was black from carbon because it didnt wipe off, but there was a slight amount of oil around the threads of the plug.  Starting with the basics before I  go into pulling the head, replacing seals and lapping valves, the bike starts and sounds like it is running normally.  So if its a bad tune and the plug that is fouled is the number 4, does that mean that the number four carb is off, or could it be anyone of the carbs causing it?  Im still curious as to why its only the one cylinder, and I just noticed it yesterday, last thursdays bike night it ran great, until I ran out of gas on the highway and had to push it off the exit to a gas station.  Could that have had anything to do with fouling out the plug?  I appreciate the responses.

[cdaiscool]

Oil on the threads could even be the valve cover gasket leaking. Check the hole to see if there's oil there. Take the plug out and run your finger down the hole to the thread. You probably should do a valve adjustment anyways. It's very easy and takes all of 30 - 60 minutes. Plus you'll get a new cover!

Yes, running out of gas could be a reason it fouled out, possibly. Went too lean, stayed like that for a bit while the other cylinders fired long enough to get you further from the gas station, and then those died, causing you to no longer move.

Also, read this. It shows more possible reasons, as well as has pics for what to look for regarding carbon vs. oil build-up on the plug: http://www.gnttype.org/techarea/engine/plugs.html

[TOOLS1]

Swap the plug with one of the ones from another cylinder, and run it for awhile. Then check them again. That will tell you if the plug it's self is bad, or it it is something else. Could be a bad plug wire, or carb out of adjustment.
TOOLS

[barelycompetent]

Im gonna pull the plug again when I get home, looking at that chart it looks like both oil, and carbon, except when I wiped them off the black didnt wipe off, then again it also looked like splash fouling too lol.  I recently changed the oil in the bike and ran sea foam thru it before I drained it. Hmmm so many things to check with these old bikes lol, oh and check it out 100th post!