Cold engine smokes, what possible causes

[NWetRider]

My 94 xr600 was smoking bad on start up. I replaced the valve seals, started it up and let it idle for 5 minutes then rode a little and all seemed well. Anyways, I go to trade it in today and the dealer starts it cold and revs it up repeatedly and it was smoking. I let it warm up easy so I didn't know it would do this. I don't want to tear into this pig again if I don't have a good idea of a fix. I am thinking it needs rings. I want to sell it so I don't want to do any extra work.

I didn't do the ring when I was in there because I have the service records from the previous owner. It had a bore and new piston late in 02. I looked at the records closer tonight and saw that 3 months later the cam chain was replaced beacuse of "overheating". If the bike got hot could that have toasted the rings?

[Wayward Son]

worn out piston,bore and rings.

[reknelb]

Couold also be the valve guides. Too much clearance. The symptoms sound about right. once the motor is warmed up some of the clearance gets taken up by the warm valve stem and less oil gets by into the combustion chamber. If it got better when you put new seals on then I would guess valve guides. Do a compression check and that will pretty much tell you if the rings are worn out.

[Wayward Son]

compression check on a motor with auto decomp doesnt tell you anything.

a leak down test is a better test.

ive seen guides that are really worn and the motor not smoke.its not a car.

[reknelb]

a leak down test is a better test.

ive seen guides that are really worn and the motor not smoke.its not a car.

Yes A leak down would be better. You could always un-hook the auto decomp if its the kind that runs from the kicker to the exhaust valve via a cable. (like on the xt600 yammy's) The valves are about the size of a v8's, so is the piston. ?

[BWB63]

The auto decompression is on the cam. It can't be disengaged without work (removed).

[NWetRider]

New thought. When I had the motor apart I scraped some of the carbon off of where it builds up at the top of the cylider. After I did this I noticed in the manual where it says in bold letter not to do this because it can cause oil burning.

I rode the bike today. I could smell it was burning just a little oil when warm. It had never done that before. Afer a couple hours I didn't notice it anymore. Maybe I built the carbon back? I'll start it tomorrow to check.

I think the guides are OK. I tried to feel for play when I put the valves in and it felt tight.

[truck6driver]

I have a XR650L that did the same thing. I had the engine built. 3 months later smoking when cold that goes away after 5-10 mins. I thought I had not seated or incorrectly seated the rings. It turns out that the piston is indexed with an IN stamped in the top of the piston stating correction to the intake side of the cylinder. The ring groves have more oil control holes there for added lubrication on that side of the piston. When it was installed 180 degrees out it allowed more oil to enter the combustion chamber when cold. When the engine warmed up the rings sealed better and oil loss stopped. The excess oil cleaned the carbon off the head and pitted the exhaust valves. The engine builder owned up to the mistake and corrected the problem no charge. This builder has been in business for 25 years. This shows that anyone can make a mistake. This could be a problem if the engine was just rebuilt no long ago. Something to look at.

Ray H.

[NWetRider]

Well I started it up again today. The first time I reved it there was a very small abount of blue smoke. None after that. I'm wondering if it was the carbon at the top of the cylinder? I really don't understand how it would matter though, I meen if a motor was new there would be no carbon at top either.