burnt dry piston head?

[Godric]

Hello all, been a while. I broke my arm pretty bad so I wasn't able to work on Godric but I'm back and more fervent than ever. Last I rode the old man he was blowing smoke out the right side and wouldn't start so I decided to park n fix him I didn't notice the blown fuse until after I tore it apart so that's probably why it wouldn't start. Didn't seem to have compression.

I took the motor apart and found nothing unusual except it seemed the right side wasn't getting oil as it was dry. Cylinders are good, everything looks good, any ideas as to why this happened?

 

[xjwmx]

could be as simple as a bad spark plug on the wet side. did you measure the ring gap? the best way to remove rings (and put them on) without breaking them and without special tools is grab each side of the open end with a thumb and forefinger and slowly pull the two ends apart until there's barely enough room to lift it up the piston, and off. careful, they are very brittle. then push the ring an inch or so into the bore and measure the gap. you could pour a little gas into the combustion chamber and see if it runs out, meaning leaky valve. could be a bit of something stuck under a valve. there was a thread here that mentioned a great spring compressor for these on ebay that's pretty cheap. if you get around to lapping them, go easy, the contact band spec is surprisingly narrow. it's in the book

 

[5twins]

It could very well be a valve guide seal that has gone bad. They're rubber and nearly 40 years old now after all.

Your right side doesn't look burnt, it looks about right. The left side looks like it's leaking, either past the rings or though the valve guides, maybe a bit of both. Notice the brown discoloration on the sides of your pistons. The right one is pretty clean which indicates good sealing. The left one has lots more color. That coloring is caused by burnt oil.

 

[gggGary]

 

[Jim]

...was blowing smoke out the right side and.....

I'm guessin' you meant smoke out the left?
It's apart now. Pistons look good from the pics, so I'd guess the cylinders look good too? Clean everything up, check the clearances, new rings, lap the valves and new guide seals and you're back in business. Might as well do the cam chain and front guide while you're in there. Looks to be a pretty easy top end job.

 

[5twins]

Yes, it looks like the rubber has fallen right off the front guide.

 

[Godric]

could be as simple as a bad spark plug on the wet side. did you measure the ring gap? the best way to remove rings (and put them on) without breaking them and without special tools is grab each side of the open end with a thumb and forefinger and slowly pull the two ends apart until there's barely enough room to lift it up the piston, and off. careful, they are very brittle. then push the ring an inch or so into the bore and measure the gap. you could pour a little gas into the combustion chamber and see if it runs out, meaning leaky valve. could be a bit of something stuck under a valve. there was a thread here that mentioned a great spring compressor for these on ebay that's pretty cheap. if you get around to lapping them, go easy, the contact band spec is surprisingly narrow. it's in the book

Sounds good! A few questions, what do you mean by lapping them? And what's a contact band?

 

[arcticXS]

Lapping means applying a little grinding paste on the valve/seat, and rotating valve back and forth with a suction cup on a wooden stick. Springs need to be removed to do this. ""Contact band" would be the area where the valve is actually touching the seat. If this band is too wide, the valve most likely won't seal.
Personally, if the engine has a lot of mileage, or seems to be in poor condition, I would prefer to re-cut the valve seats first, and maybe have the valve ground as well. Here are the required tools:
https://www.heidentuning.com/xs650-...valves-guides/valve-lipping-stick-detail.html
https://www.cylinderheadsupply.com/...MI1bCEqbn26AIV2KSaCh2EFg2JEAAYAiAAEgLiwfD_BwE

 

[arcticXS]

And one more thing. I would check all valve guides as well, since you have the head off anyway. After changing a valve, guide, re-cutting the seats is definitely required. New valve stem seals are definitely required in any case.

 

[cra-z1]

Find a local shop to check the top end for wear. There is a definite learning curve to cut your seats, install new guides ,resurface and lap your valves in. You may not need anything other than seals and a quick lapping of the valves. You still need the correct tools do do the job. If your not sure what lapping is I assume you do not have the tools.