Good compression, smokes like crazy.

[drewfunk]

I just purchased my first XS650, a '79 Special. Only 8k, but hadnt been running in a while. I replaced the plugs and battery, through some gas in it, and it started second kick! No filters on it, open pipes, dirty carbs, and god knows how old the oil is, but it runs great all things considered. But it's smoking like crazy! Pulled the plugs and they are pretty black. Did a compression test and both cylinders had around 125psi cold, seems good. What could be causing so much smoking if compression seems pretty good?

 

[5twins]

Valve guide seals could be bad. They wouldn't affect compression but would let oil get sucked in from the head.

 

[drewfunk]

How hard is it to access the valves on these engines? I dont have the shop manual for it yet but will shortly.

Also, could running sea foam through it help?

 

[JP50515]

There is probably a lot of build up in there. I'd try running some seafoam and riding it for a while first. You may just have buildup that can be burned out. Give it a red line for a few seconds, sometimes that can clear the valves and burn up some of the carbon on em.

In regards to the valves, I've heard its possible to replace the seals without pulling the engine but I pulled mine to get to em.

 

[drewfunk]

Well i put some seafoam in the tank and drove it around a bit, still smoking. There is decently loud chatter from the top end, are these engines known to be noisy or do i have a valve issue? There is also oil on the outside of the engine, could a bad gasket be letting oil into the cylinder?

The bike only has 8K on it, and starts first kick every time and runs pretty damn smoothly besides being lean.

 

[5twins]

Yes, oil can leak from the center cam chain tunnel through the gasket and into the combustion chambers. Bad head gaskets are common on these because the cylinder nuts on top of the head loosen up. You can try a re-torque, sometimes that fixes it. Use about 30 ft/lbs on the 8 large acorn nuts instead of Yamaha's spec of about 26. Use Yamaha's specs for all the other smaller bolts.

Yamaha used rubber/metal washers under the 4 outside acorn nuts. These cause those 4 nuts to constantly come loose. Replace those 4 washers with copper or brass ones. MikesXS sells them. Then the head bolts will stay tight better for you.

 

[drewfunk]

I put a bit of seafoam in the cylinders overnight, and that seemed to bring up the compression to about 130psi in both cylinders. I then did a re-torque of the head. I've only put abut 5 or 10 miles on it since then, but its still smoking like crazy. Ungodly ammounts of stinky-ass smoke. From both pipes.

Now, i am only running 10-40 with MMO in it, so the oil is fairly thin and probably exaggerates the smoking. Does this sound more like valve seals or a head gasket? Both cylinder seem to have an equal ammount of smoke. Am i right to rule out rings? Both cylinders are equal at 130 psi cold, and the engine only has 8k on it.