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Discussion Starter #1
Hey everyone,

I purchased a '71 CB350 K3 last year and got it running for the season. It was burning oil after it warmed up all last season in both cylinders (Right more the Left), but I kept ridging hoping maybe the issue would resolve it's self with some use. Not the case.

This winter I rebuilt the head with new valves and valve guides thinking that was a prime cause, and put in all new gaskets and seals throughout the engine. I have it all back together now and the same problem is occurring with the Right being more intense than the left. So, I am down to 2 ideas to resolve this:

1. See if there is some way to incorporate Valve Seals into the head to ensure oil is not getting in this way. Anyone know of anyone making some sort of aftermarket valve seal for this engine?

2. Replace the Piston rings. The compression ring is very snug in the cylinders, but the Oil rings are possibly a little loose.

Any other ideas of how Oil could be getting in?

Thanks for your time folks!
 

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Was the valve job done by a machinist?

a machinist should have measured / decked the head
and measured the piston, rings and cylinders

there are specs for all the above in the shop manual

replace anything out of spec

you don't need valve seals just an inspection by a competent machinist and follow his guidance
 

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Discussion Starter #4
The work was done at a classic bike shop my usual bike shop refers folks to for such work, so not a machine shop but I am sure he used good technique and confirmed he had it all setup within spec. I could try to find a second shop to confirm specs were done right, but it sounds like you think I can trust the setup if I trust the work done? I was just wondering if there was any way to incorporate valve seals on cb350s, as I would like to just be safe.

Assuming the valves and head are not the cause for the oil getting into the cylinders, where else could I look?
 

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17,000 on the tach, but being an new owner I can't really vouch for the mileage. I think for sure after you comment I will put in new rings as I already have it apart.

Any other items to check or replace that you can think of? I have had the dang thing together and apart 2 times now, so I am just gonna do everything I can think of this time.
 

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Is the vent tube pinched?
 

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17,000 on the tach, but being an new owner I can't really vouch for the mileage. I think for sure after you comment I will put in new rings as I already have it apart.

Any other items to check or replace that you can think of? I have had the dang thing together and apart 2 times now, so I am just gonna do everything I can think of this time.
Honestly - and I'm not trying to spend your money - but with it already apart, I'd absolutely put a new set of rings in it based on the potential for more miles than you are already aware of. Remember these things are air cooled and short-term, inexperienced owners prior to you can lose track of that and let it idle in traffic in hot weather, etc., not realizing the detrimental effect it can have, particularly on older used parts. If the pistons look good, just a light hone to break the glaze and give the new rings at least 200-300 miles of moderate break-in running and that should make a difference.
 

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Discussion Starter #8 (Edited)
Is the vent tube pinched?
Having it all apart now I can't say what the vent tube looked like when it was running. Looking at it now, it doesn't appear to show signs of being pinched. Would that cause oil to go into the engine to the point that it drips out of the tail pipe?
 

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Honestly - and I'm not trying to spend your money - but with it already apart, I'd absolutely put a new set of rings in it based on the potential for more miles than you are already aware of. Remember these things are air cooled and short-term, inexperienced owners prior to you can lose track of that and let it idle in traffic in hot weather, etc., not realizing the detrimental effect it can have, particularly on older used parts. If the pistons look good, just a light hone to break the glaze and give the new rings at least 200-300 miles of moderate break-in running and that should make a difference.
Thanks Tom, ya at this point I totally agree with you and will put in new rings plus any other suggestions. I want it done right and I don't want to do another rebuild any time soon. The prior owner only claimed they did a top end rebuild on it, so it's possible the rings are quite old.
 

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When you lightly hone cylinder walls, afterwards place a coat of oil on bores and let sit a few hours. Then wash out with kerosene or gasoline. The oil will lift minute metal bits off of the cylinder walls. Place ring gap 120 degrees from one another. I assume three piston rings.
 

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When you lightly hone cylinder walls, afterwards place a coat of oil on bores and let sit a few hours. Then wash out with kerosene or gasoline. The oil will lift minute metal bits off of the cylinder walls. Place ring gap 120 degrees from one another. I assume three piston rings.
Thanks for this. I will note that for the install.

Everything on this was real helpful.
 
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