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Discussion Starter · #1 ·
I was checking out my 72 cb350k the other day post ride and noticed a black rubber tube about 1/2 - 3/4 in diameter connecting to the top of the cylinder head and traveling to about mid-bike and ending underneath the bike. There was some smoke flowing from this tube and I am trying to figure out what this is and is some smoke out the tube normal??

Thanks
 

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Sensei
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It is the crankcase "breather" tube.......Shouldn't be smoking...... Whats the compression PSI's of the bike?... Also, smell the dipstick for a fuel (gasoline) smell in the oil....
 

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Discussion Starter · #3 ·
Ok, that sounds like I may have a problem...
I will try to check the compression this weekend and will smell the dipstick tonight and report back.
Any idea why it might be smoking? or do I need to do the above suggested to get an accurate estimate?

Thanks for the help
 

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cycle_enthusiast said:
Ok, that sounds like I may have a problem...
I will try to check the compression this weekend and will smell the dipstick tonight and report back.
Any idea why it might be smoking? or do I need to do the above suggested to get an accurate estimate?

Thanks for the help
Many times they smoke due to excess pressure and blow-by from the combustion process. My experience indicates it's usually a head gasket or broken ring and both of these issues would show up in a compression test.
 

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the tube is a vent for your crankcase, as inevitably you have a bit of pressure getting by the rings of the pistons as the engine runs. When the amount getting by your rings and pistons (blow-by) gets excessive, you will get pressure and smoke into the crankcase, and then out of that tube.

the compression test is a good test of general engine health, and in this case will tell you how much pressure is getting by your rings.

this is somewhat simplified, and there are other reasons you can have low compression, but this is what pertains to this discussion.

when doing a compression test, the bike needs to be all warmed up, take both plugs out, make sure choke is not on, and open the throttle all the way. put the gauge into one sparkplug hole and crank the engine over with the starter ( i have found that using the starter provides much better numbers than using the kickstart.) until the pressure stabilizes (around 5 times) do the same with the other. they should be close to each other and above 160 or so. you will probably find that the numbers are much lower.... if you then put a tablespoon or two of oil in the sparkplug hole and repeat, if the pressure number jumps up, then you know the rings have gone. (the excessive oil temporarily creates a better seal around the rings).

rings and cylinder honing time.
 

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couldn't it smoke a bit if you've over filled the crank case oil ?
 

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Sensei
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To have smoke, you must have fire (flame front reaching the oil supply) could be bad rings, aligned ring gaps, or a gasket leak into the camchain galley... Liquid oil or oil mist is another/different story ....
 

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Sensei
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Condensation (water) produces steam when heated.... Smoke is burning oil and should NOT occur.... If your engine is smoking out the breather, it's usually time to re-ring..... However, fuel thinned oil can/will burn off at a much lower temperature and can cause smoking just from "normal' engine temperatures.... But it is still a problem that must be corrected......ANY smoke out the breather is indicative of SOME problem....
 

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Discussion Starter · #11 ·
Ok guys, sorry it took me so long to reply but I finally checked the compression.

without putting oil on the piston the right side came out to 165 with oil it is 200psi
the left side was 190 with oil

how are these numbers?? they seem ok to me and if so, what else could be the problem.
I'm thinking of doing an oil change tomorrow with Motul 5100 syn blend 10w40. Anyone ever used this in their bike?

Thanks
 

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well, you know that you are getting blow-by as the oil makes the compression jump up significantly, not sure on your right cylinder, as no pre-oil pressure given.

the rebuild limit for the engine is somewhere around the number you gave of 165, i cannot remember what it is exactly, but that number may be falsely high as you probably have quite a bit of carbon buildup in there, which makes your chamber smaller and bumps the compression ratio up.

you can either run it the way it is and deal with the blow-by and smoke, and pull the engine apart later, or your pull the engine apart now and hone the cylinders and buy the proper pistons/rings for the final cylinder size. if you are lucky, you will still be within spec for stock, then you just need new standard rings.
 
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