Topic: 1974 CB350F Cyl #1 burning oil

[CAKELIE]

Hey everyone,

Recently picked up a CB350F and am getting acquainted with it.

It has the original 4-4 exhaust and I have noticed Cyl #1 occasionally smoking what looks and smells like oil. It's not consistent, but every once in a while it will smoke a good bit when above 7,000rpm. I can go for an entire ride without seeing anything, and other times it will leave a big cloud of smoke after I wring it out.

Performed a leak down test and all cylinders were at 1-2% however, I noticed that if I performed multiple tests on Cyl #1 (releasing pressure, spinning the engine over until it is back on compression stroke, then repressurizing) about 1/3 to 1/2 the time, I would get a lot of air coming out of the breather hose on top of the engine until the input psi was raised to ~30-45 psi. At which point I heard something in the engine thunk, and the air immediately stopped leaking and the other gauge equalized. I tested this multiple times. The crank is not moving at all from TDC, and I put my hands on the backs of the intake and exhaust valves (which are within spec) and could not feel them move at all.

I removed the breather assy from the top of the engine to inspect. Couldn't find anything wrong with it, I'm guessing there is no need for a check valve, since the breather doesn't recirculate into the intake?

Even if it was a breather issue, I'm not sure why it would only be Cyl#1 that would have this problem. So I'm guessing it is something else, but I can't think of what could be causing this.

Cyl#1 plug looks maybe a tad rich, but no deposits or signs of oil burning.

Hoping someone can chime in with some ideas or things to try. Thanks!

[markreimer]

Maybe you have a valve hanging up? Stuck partially open then thunking closed while you apply pressure?

What pressure did you do the leak down test at out of curiosity? 1-2% is really really good, which is why I’m asking.


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[CAKELIE]

I would have thought a sticking valve would either have air coming out the exhaust or intake. Though I suppose if an intake valve is sticking, it could be letting air back into the head and out the vent?

Test was done at 90psi. If I went to 100, maybe the result would be a little less impressive. But not bad still.

[markreimer]

Re read your description. You’re right, valve doesn’t make a ton of sense relative to your testing. I was thinking a valve hanging might cause your oil burning. But now I think this is probably something piston related. Leaky valves shouldn’t have any way to direct pressure into the head and out the breather unless there was a crack, which would be constant. However if pressure escapes past the pistons it will definitely blow into the cases, up the cam chain tunnel and out the breather.

I can’t explain what might be happening in that cylinder or what the sound is, but if it were me my next step would be a bore scope with the piston at the bottom to check for signs of damage. Strange problem!


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[scottly]

There should not be any air flowing from the combustion chamber to the breather in any condition: if a valve was hanging open, it would leak out through the intake or exhaust, rather than the breather. The only way this could possibly happen would be if there was a hole in the piston, but then the leak-down test would have failed and the motor wouldn't be firing on that cylinder. I think you are worrying too much.    

[Tracksnblades1]

I’ve seen similar on a SBC when the top ring is sticking. In this case it was a Keith Black piston with a high 1st ring land which had been damaged by lightly contacting the cylinder head... 😜

[CAKELIE]

Thanks for the responses. A borescope is not a bad idea.

Scottly, I think I am worrying a healthy amount considering the amount of smoke that comes out the back of the bike

Tracksnblades, that sounds plausible for sure. As far as I know, the pistons are original in this bike.

[Tracksnblades1]

Thanks for the responses. A borescope is not a bad idea.

Scottly, I think I am worrying a healthy amount considering the amount of smoke that comes out the back of the bike

Tracksnblades, that sounds plausible for sure. As far as I know, the pistons are original in this bike.

Might check with HondaMan on this forum.  I remember him posting very tight piston cylinder wall tolerances for the 350.

[scottly]

I can go for an entire ride without seeing anything, and other times it will leave a big cloud of smoke after I wring it out.

Sounds like a valve guide/seal issue to me. My K1 750, which doesn't have seals on the exhaust guides also smokes occasionally, and always has.  Does the 350 have seals on the exhaust guides?

[Kevnz]

If a valve is closed and seated properly, how does a valve guide seal allow leak down?

[scottly]

When the exhaust valve opens, the hot exhaust blows the oil off the stem, into the hot exhaust pipe. Also, when the engine isn't running, if an exhaust valve is open, oil can seep down the stem and drip onto the top of the piston.

[CAKELIE]

To my knowledge, the 350F does not have exhaust seals. I wouldn't be worried if a bit of smoke came out every now and then. It just seems excessive when it does happen. That being said, I haven't had an issue for the last 3 rides... Inconsistent problems are the worst kind of problems.