Yamaha XT 600 (start-up smoke) *video*
 

Hello. I have a Yamaha XT 600(DJ02) model. Recently i rebuild my engine:rebored, new valve seals, new ProX piston and rings,new .

In the morning,or after a few hours when i start-up i have for about 1minute a white/blue smoke and then after warm-up,disappear. Compression about 140+psi.

https://www.youtube.com/watch?v=Oa7wVmz0zMI

First i would check the plug and oil level, maybe overfill? New seals maybe damage on the valves or oil get past the rings when cold and seal up when hot. What exactly do you have on hot engine compression, 156 is the number you shuold have reach with all new.

Like I said on another thread, my 84 did just what your is doing and seemed to clear up when warm, however, on deceleration it would smoke and it consumed too much oil. Again 140 pounds compression on a new engine is very suspect. Was the cylinder honed to a good crosshatch pattern. This is very necessary to allow the rings to wear in, seat, break in, what ever you may call it.

Valve guide seals. Either new ones are the wrong spec? (some cheapo Chinese copy?) or installed incorrectly?

Could also be rings aren't yet fully seated or hone job not done well. New Piston and rings will take about 2000 miles to fully bed in and achieve max compression.
DO NOT use synthetic oil until fully broken in.

But semi-synthetic it's ok? (10w40)

in deceleration not smoke. no oil consumption,i mean not exaggerated (i have almoust 800 miles done,basically i'm in rode period)

Now i have 20w50 Cepsa,this week i will change with 10w40 Procycle. Some guys told me they have the same problem,it was bag choice of oil,after change it,everything was ok and the smoke disappear.

YES! Semi-Synthetic is fine for break in.

As mentioned earlier, make sure the bike isnt overfilled.

Oil level it's ok. Remains to change the oil these days with some 10w40 Semi-Synthetic.

most likely dirt in / bad check valve letting the cases fill with oil even after a few hours.
if anything else was suspect, there's no reason the engine would stop smoking after a while.

easy to check:
check your oil level after a long ride.
check again after an overnight, if level dropped considerably, then it's a bad check valve

Sidenote to the above... Oil expands when hot. I think in our bike with a good amount of oil, the difference could be 1-2dl in volume from very cold vs very hot. (I think i calculated the difference once, but cant recall the exact number)

Take notice of how much you drain :)

Smoke on startup but not when normal running is valve seals , oil seeps past and settles on the piston when it's sitting . I polish valve ends to keep from tearing new seals.

The rings are 95% fully seated within the first 15-20min running . I tested one new motor and it reach 90% of its final compression in 5min, then took another 10 minutes to get the rest . After the 30min mark it was done (208psi ). Never, ever, ever, use synthetic or semi synthetic(not as bad), they prevent seating .

JJ, what is your suggestions for breaking in a newly bored cylinder, new OEM piston and rings? I am a few days away from starting it.

I had noted it in another thread (and got bashed for itdoh) , just start it up letting it warm for a minute or 2 but be ready to ride it . take it through the first three gears no more than 2/3 throttle , hold for a bit then release the throttle and let the motor slow the bike down . Do that for the first 10-15 min ( reasonably high rpms, then use the motor to slow down). this forces the rings against the hatch marks in both acceleration and deccel). Change oil & filter . It's now mostly done but I don't run full rpms for 100 miles at least , always varying the throttle and will heat it up good for 10 min then shut down to cool till it's past the 1/2hr run time. No sustained high rpms for 250-500miles.(Change oil ?? or not , what ever a person wants here)

The one I tested throughout break-in for compression had been done in my garage using the rear brake for loading . Shutting down every couple minutes to check compression.