Topic: Bike smoked....then it didn't. Why?

[Scott S]

 I recently purchased a '78 550K. It was ridden into a local shop by a guy that needed money. The shop owner didn't notice any smoking, but the bike was idled high...~2200 rpm. It on;ly smoke(ed) on the #1 cylinder/pipe.

 It sat in his shop for a few days and when he cranked it and lowered the idle, it smoked....badly. He felt like he had been taken and that the P.O. had raised the idle to hide something.
 It sat for another couple of days until I could see it. It wasn't as noisy or as smoky as before, but it smoked a little on start up and out of the #1 pipe on the way home. The shop owner theorized that maybe it had a stuck valve. I was thinking a broken ring or something.

 However, since it's been in my garage, it no longer smokes. I can't MAKE it smoke. I played with the idle a little bit and plan to give the bike a full tune-up and carb synch, but why would it stop smoking?

 Think maybe it had a stuck ring?

[gnarlycharlie4u]

I recently purchased a '78 550K. It was ridden into a local shop by a guy that needed money. The shop owner didn't notice any smoking, but the bike was idled high...~2200 rpm. It on;ly smoke(ed) on the #1 cylinder/pipe.

 It sat in his shop for a few days and when he cranked it and lowered the idle, it smoked....badly. He felt like he had been taken and that the P.O. had raised the idle to hide something.
 It sat for another couple of days until I could see it. It wasn't as noisy or as smoky as before, but it smoked a little on start up and out of the #1 pipe on the way home. The shop owner theorized that maybe it had a stuck valve. I was thinking a broken ring or something.

 However, since it's been in my garage, it no longer smokes. I can't MAKE it smoke. I played with the idle a little bit and plan to give the bike a full tune-up and carb synch, but why would it stop smoking?

 Think maybe it had a stuck ring?

valve seals warmed up and started doing their job?

[sick_seven_fifty]

I would run a compression test on the cylinder...if it has good compression, and is no longer smoking, I wouldn't worry about it.  Gnarly could very well be right.

I once had a guy that sold me a two year old dirt bike that he couldn't get to run...he had correctly narrowed it down the to CDI box, which for this particular bike was very expensive, and hard to get to.  I bought the bike for less than $500 and when I stripped it down to remove the CDI, I noticed that the connector felt loose.  I gave it a little push...CLICK.  Threw the gas tank on it as fast as I could and gave it a kick.  Fired right up!  Sometimes you just get lucky.

[Duke McDukiedook]

Yeah, could have been a bike that sat for a while and sealed the problem up with some riding.

[TwoTired]

I have a 78 CB550K that behaved the same way.  For a while.  Smoked from sitting.  Got better (went away) with some run time.  Then it started smoking after warm up, particularly at idle, progressively worse.  Valve guide seals were shot, and so are the exhaust valve guides.  Work in progress.

[Scott S]

 Well, even if it's valve guide seals, that's not the worst thing in the world. Better than a broken ring or scarred cylinder, right?

 Can I tell from a compression test or leak-down test if it's the rings or guides or something else?

[OldSchool_IsCool]

Do two compression tests. First one "dry" and the second "wet" with a teaspoon of oil into the plug hole first. If the numbers go up by say 10% or more with the wet test, that suggests ring/bore issues. If there is little change, then valves or possibly no troubles at all.

Compare readings to other cylinders next. If all are within 10% then you prolly have nothing to worry about.

[TwoTired]

The compression on mine is fine.  But, if the valve guides have been bad long enough, then the valve seats will leak (stem wobble prevents the valve closing properly forcing uneven wear.

A compression test can tell you about valve seating or ring seal, not specifically the guide issues.

Wet/ dry testing can tell you about rings.  A teaspoon is too much oil for these small displacement bikes.  Read the FAQ.
 You only want enough to wet the piston rings, not change the compression ratio of the engine.  Otherwise every test with a bunch of oil in the chamber is going to tell you your rings are bad.

If after using a two or three drops oil to seal rings the peak compression is unchanged, then your rings are good.
If you do a leak down test instead of a compression test, it can tell you if your valves are leaking in their seats (or past the rings).

If the bike was newly put back into service, it may take a while for the rings to reseat on the cylinder walls.  If there was bad cylinder wall pitting, it will never fully seal and just wear the rings out quickly.

[Scott S]

 I'll try all of these tests eventually, as well as a complete tune-up. This is bike #4 and is last in line right now.

 I have no idea what the previous history is. I should have written down the P.O.'s name from the title; he only lives about 5 minutes away from me.