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1990 Yamaha xt600, lot of start up smoke

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Posted by: skiboyracing---------------------

I have a 1990 yamaha xt600 I bought and it started to smoke really bad on cold start ups after it's been sitting overnight. Once it burns the oil off it runs clean and doesn't smoke at all. I initially thought it was the valve seals but I am hearing of a lot of people having this issue and after a rebuild it still does it so with only 7000 miles on the bike, I am thinking it's something else. It really took a turn for the worse quickly so something went wrong. I have heard there is an oil valve in the right hand cover that stops the flow of oil like a one way check valve but I don't know enough about these engines to really get a feel for it. I do know checking the oil seems to be a PITA as I run the bike, let it idle, check and it won't even be on the dip stick, idle some more let it sit for a minute, then there's too much. So I'm wondering if that has anything to do with the oil burning.

Here is a video of a cold start. Notice it runs clean for a bit then the smoke comes on, I would think if the valve seals were gone it would smoke right away and probably not last as long. It will smoke for 2-3 minutes while warming up. Again it just started this and only has 7k miles. Any experience with these that you could share would be great.

http://www.youtube.com/v/JGtAQLGjsPM


Posted by: Ol'89r---------------------

It most likely is the guide seals. The mileage doesn't matter when the bike is that old. The seals will harden and crack with age and allow oil to leak past into the combustion chamber. When you start the bike it burns the excess oil off.

That's the first place I would look.


Posted by: skiboyracing---------------------

yeah that was my first thought too.. I'm being told the only way to change them is to pull the whole motor and remove the head. I was hoping you could pressurize the cylinder to hold the valves up, then remove the locks and springs from the top side so I didn't have to tear it all down.


Posted by: Ol'89r---------------------

Quote:
Originally Posted by skiboyracing
I was hoping you could pressurize the cylinder to hold the valves up, then remove the locks and springs from the top side so I didn't have to tear it all down.


That can be done but it is actually easier to remove the head. You would have to build a tool to lever the valve springs down and you will be working in very cramped quarters. You would still have to remove the top cover and camshaft and the only thing left would be the head bolts carb and pipe. Plus if your piston does not stay at tdc, you could drop a valve. Then your screwed.

When the head is off, you can inspect the valves and reseat them if necessary. Also, check the guides for wear. It would also be a good time to install new rings. If the bike has been sitting for any length of time, you could have a rust issue. It's best to take the time to do it right.

If you don't already have one, buy a manual. It will take the guesswork out of your project.



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