Oil out the exhaust pipe..bad valve seal?

10516 Views 8 Replies 3 Participants Last post by TheTruant, Dec 4, 2007

TheTruant

Discussion Starter · Dec 4, 2007

I just picked up my first bike project a few days ago. It's a '74 Honda CB750. It ran good when I went to look at it but realized it wasn't running on one of the cylinders (exhaust pipe was cool). Once I got it home I immediately found the problem, the plug wire was old and had pulled loose from the boot, dosen't get much easier than that

So I cleaned off my spark plugs (it's been sitting awhile) and looked things over again. Rolled it out side and started it up, runs for about 10 secs. and dies. Get it running again but it's running real rough, sounds like it's not hitting on one cylinder. After trying to get it to run for a few minutes I look down and see my shoe is covered in oil (shorty pipes) and oil dripping from the exhaust pipe. Shut the bike off and see that oil is also coming from the exhaust flange. Pull the pipe off and see lots of oily residue in the valve area. Remove the spark plug and it's clean as can be. Look in the cylinder with a scope and it's a little oily, but not any more than the others.

Sorry to be so long winded but wanted to give you all the best description possible to work with. So what's the culprit? A bad valve seal? Does that make any sense?

Thanks a bunch!

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Flyer

· #2 · Dec 4, 2007

That really doesn't makie much sense. A valve stem seal shouldn't leak that much, unless it is just totally non existant.

I assume the cylinder not firing is the one that has the oil coming from it?? If so, what you say doesn't explain why it wouldn't be firing. Replace the plug, and get a new end on the plug wire to be sure that's ruled out.

If you have problems with the plug wire ends, check with Napa, if they have problems, tell them to look in the Belden/Napa Wire catalogue.

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CB750F

· #3 · Dec 4, 2007

Even a non existant one wouldn't leak that bad. Check your oil level. From sitting the gas might have leaked through the carbs then through the rings making the oil thin and the level too high. CB's are a pian to fill with oil being a dry sump system. They take around 4 quarts. What you have to do is drain the oil tank and crankcase, replace the filter, throw 3 quarts of oil in it and start it for a like 15 seconds and check it. Keep putting in small amounts until its full. Everytime you check it run it before you check it. Pick yourself up a shop manual, you won't be sorry.

If your oil level is ok start checking compression.

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TheTruant

Discussion Starter · #4 · Dec 4, 2007

Flyer said:
That really doesn't makie much sense. A valve stem seal shouldn't leak that much, unless it is just totally non existant.

I assume the cylinder not firing is the one that has the oil coming from it?? If so, what you say doesn't explain why it wouldn't be firing. Replace the plug, and get a new end on the plug wire to be sure that's ruled out.

If you have problems with the plug wire ends, check with Napa, if they have problems, tell them to look in the Belden/Napa Wire catalogue.

I didn't think a valve seal would leak that much oil either, wouldn't it be more of a spray or mist at start up (leakage building up in cylinder while parked)?

But after seeing that the plug was clean couldn't understand where else the oil would come from. I'll swap plugs and change the ends and see what happens. Oh yeah, the cylinder that was not firing originally is NOT the one leaking oil. It seems to be fine since I reconnected the wire. The one leaking oil is the one that is not firing now (or so it seems)

Probably be a couple of days before I can work on it but I'll post the results.

Thanks so much Flyer!

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TheTruant

Discussion Starter · #5 · Dec 4, 2007

CB750F said:
Even a non existant one wouldn't leak that bad. Check your oil level. From sitting the gas might have leaked through the carbs then through the rings making the oil thin and the level too high. CB's are a pian to fill with oil being a dry sump system. They take around 4 quarts. What you have to do is drain the oil tank and crankcase, replace the filter, throw 3 quarts of oil in it and start it for a like 15 seconds and check it. Keep putting in small amounts until its full. Everytime you check it run it before you check it. Pick yourself up a shop manual, you won't be sorry.

If your oil level is ok start checking compression.

Ahhh, very interesting.

You know, now that I think back I'm pretty sure I left the pet**** in the "open" position the other day when I fired it up for a few secs right after getting it home. I remember him saying he always closed it when he parked it. So overnight the gas could have leaked into the oil, never would have occured to me

I'll change the oil when I change the plugs. I had planned to run a compression test as well but am not sure of the pressure. I think I read somewhere 120psi? Is that at cranking speed?

Thanks so much for the help guys!

P.S. A shop manual is on the way

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CB750F

· #6 · Dec 4, 2007

I'd have to look in my manual but that seems right. That should be over a couple revolutions.

If you do have fuel leaking into the cylinder while sitting it might be time for a carb rebuild.

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TheTruant

Discussion Starter · #7 · Dec 4, 2007

CB750F said:
I'd have to look in my manual but that seems right. That should be over a couple revolutions.

If you do have fuel leaking into the cylinder while sitting it might be time for a carb rebuild.

Alrighty, thanks again!

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· #8 · Dec 4, 2007

Bet it is coming from the left cyclinder side

TheTruant said:
I just picked up my first bike project a few days ago. It's a '74 Honda CB750. It ran good when I went to look at it but realized it wasn't running on one of the cylinders (exhaust pipe was cool). Once I got it home I immediately found the problem, the plug wire was old and had pulled loose from the boot, dosen't get much easier than that

So I cleaned off my spark plugs (it's been sitting awhile) and looked things over again. Rolled it out side and started it up, runs for about 10 secs. and dies. Get it running again but it's running real rough, sounds like it's not hitting on one cylinder. After trying to get it to run for a few minutes I look down and see my shoe is covered in oil (shorty pipes) and oil dripping from the exhaust pipe. Shut the bike off and see that oil is also coming from the exhaust flange. Pull the pipe off and see lots of oily residue in the valve area. Remove the spark plug and it's clean as can be. Look in the cylinder with a scope and it's a little oily, but not any more than the others.

Sorry to be so long winded but wanted to give you all the best description possible to work with. So what's the culprit? A bad valve seal? Does that make any sense?

Thanks a bunch!

Bet it is coming from the left cyclinder (kickstand) side? Older MC typically burn oil on the low side after sitting for loooong periods. Like others have suggested minor leaks from dried seals can cause that. If the cylinder was not firing oil & gas collecting could have contributed to your problem.

Ride safe & long,
Colorado Fats

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TheTruant

Discussion Starter · #9 · Dec 4, 2007

COFats said:
Bet it is coming from the left cyclinder (kickstand) side? Older MC typically burn oil on the low side after sitting for loooong periods. Like others have suggested minor leaks from dried seals can cause that. If the cylinder was not firing oil & gas collecting could have contributed to your problem.

Ride safe & long,
Colorado Fats

Coming from far right (as you sit on it) cylinder.

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