1981 CB900f cylinder overheating and maybe burning oil

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1981 CB900f cylinder overheating and maybe burning oil

kdub921
Hey guys, Honda noob here.

I just picked up a 1981 Honda CB900F in pretty solid condition. It originally had a problem charging the battery so I replaced the alternator rotor and the reg/rec and resolved that issue. I went for the first ride and after a few minutes, I noticed some smoke from the headers and a burning smell - seemingly oil.

I don't see any leaks or pudding on any exterior components. I also don't see any smoke coming out of the exhaust. What I do see is smoke coming from the furthest right (when sitting on the bike) header and from the heat wrap. I did some research and the culprit seems to be either a worn or cracked valve stem seal or a piston ring. The motor is air cooled and carbureted so maybe it just needs to get up to speed? Here is a link to a video I recorded showing the problem - YouTube link

I'm no professional mechanic (obviously) but with my intermediate level knowledge, I think I could diagnose the problem if pointed in the right direction.

Thanks in advance for any input!

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Re: 1981 CB900f cylinder overheating and maybe burning oil

TOOLS1
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It looks like you have oil or water on your heat wrap. I doubt you have an internal engine problem. If you did, it would be smoking out the end of the exhaust pipe or parts would be coming out of the engine.
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Life is not about the number of breaths, you take, but the moments that take your breath away.
I don't have an anger problem. I have an idiot problem. Hank Hill
Never confuse education for intelligence.
Happiness is a belt fed weapon.
I just can't imagine what could go wrong.
No fire? No explosions? So whats the point of your story?
Those who are too smart to engage in politics are punished by being governed by those who are dumber. ~Plato
It couldn't be done, but the darn fool didn't know it, and did it anyway.
We all got problems. Ksharp
I like vintage bikes because they take me away from the clutter of technology that I work with everyday and back to a simpler time of mechanical elegance and simplicity.. "ninadm"
Darkwing Duck: The worst part of public transportation is the Public.
"That is awesome shit there" Re-Run
"Fear nothing, attack everything" Eric Berry
" Oh, you read that on the internet? Clearly it IS a massive problem. Of course it CAN’t be normal operation."

1976 CB 750-A X 2
1977 CB 750-A X 4
1977 CB 750-K
1976 CB 750 F
1981 CB 750
1966 Kawasaki SG 250
1981 KZ 750 LTD
1973 CB 350
1979 CM 185 Twinstar
1982 Honda XL 80
South of Eden (Kansas City MO)
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Re: 1981 CB900f cylinder overheating and maybe burning oil

kdub921
I know it's not water because it would have evaporated already and I don't know how oil would have gotten on it. I removed the spark plug for that cylinder and it is a dry soot which indicates that cylinder is running at a rich air/fuel mixture. Do you know much about adjusting the carburetors?
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Re: 1981 CB900f cylinder overheating and maybe burning oil

TOOLS1
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There is a thread on here with very detailed instructions on how to rebuild and adjust the CV carbs that your bike is equipped with. A quick search should find it. However again, if it was something coming from inside the combustion chamber it would be coming out the end of the exhaust pipe, not through the wall of the steel pipe. Look at the front of the cylinder where the cam chain adjuster is, there is a little rubber plug in one of the holes, that plug gets hard or falls out and oil leaks onto the exhaust pipe there.
TOOLS
Life is not about the number of breaths, you take, but the moments that take your breath away.
I don't have an anger problem. I have an idiot problem. Hank Hill
Never confuse education for intelligence.
Happiness is a belt fed weapon.
I just can't imagine what could go wrong.
No fire? No explosions? So whats the point of your story?
Those who are too smart to engage in politics are punished by being governed by those who are dumber. ~Plato
It couldn't be done, but the darn fool didn't know it, and did it anyway.
We all got problems. Ksharp
I like vintage bikes because they take me away from the clutter of technology that I work with everyday and back to a simpler time of mechanical elegance and simplicity.. "ninadm"
Darkwing Duck: The worst part of public transportation is the Public.
"That is awesome shit there" Re-Run
"Fear nothing, attack everything" Eric Berry
" Oh, you read that on the internet? Clearly it IS a massive problem. Of course it CAN’t be normal operation."

1976 CB 750-A X 2
1977 CB 750-A X 4
1977 CB 750-K
1976 CB 750 F
1981 CB 750
1966 Kawasaki SG 250
1981 KZ 750 LTD
1973 CB 350
1979 CM 185 Twinstar
1982 Honda XL 80
South of Eden (Kansas City MO)
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Re: 1981 CB900f cylinder overheating and maybe burning oil

kdub921
Well first, we took the carbs apart and the dumbass who worked on this bike last, did not screw in the pilot jet for the #4 carb and was literally just laying in the bowl, not in the thread. We were hoping that resolved the issue, but it seems that it caused a more serious issue. We removed the valve cover and found the cyclider 4 cam lobes were damaged and the clearances for the cylinder 4 exhaust valves were inconsistent so we think it's broken valve springs. So now we're going to have to take off the header and see the damage :-(