1974 XS/TX650 rebuild

[th62]

Deleted. Moving on.

[tonyfoale]

Is the compression loss equal on both cylinders?

[th62]

Yep, within two or three psi.

[olderdan]

If the piston ring gap is a little too small it can cause the ring to distort if the ends meet as it gets hot. Nothing to lose by increasing it slightly, as the rings wear it gets larger anyway. You probably gapped them to spec but the problem you have exists so worth a try?.

[th62]

The cylinder won't hold pressure so I should increase the ring gap. Yeah, that makes sense, now why didn't I think of that?

[th62]

Valve grinding.
I ground the valves in but they still leaked. So I pulled them and found the seats way to wide, they won't seal if too wide. Yamaha says, 1.3mm. So I mounted the valves on the lathe and using a drum sander in a rotary tool ground the back of the valve down so the seat was 1.2mm wide. The edges are around .5mm or better.
I sent off for a 30 degree cutter head a while back, but didn't have a handle for it, so turned up a handle from stainleas with a 10/11mm taper to suit the cutting head and an 8mm shaft to fit the valve guide and cut behind the seats in the head until the seats were also 1.2mm.
Once all that was done, I lappd the valves with fine paste, then assembled with springs and collets and poured fuel into the port to test for leaks.
The last pic shows a piece of dry, clean paper towel held around the edge of the valve by small magnets. The paper towel shows up any leaks.

Now I know all the armchair experts will say wrong, wrong, wrong, should have used a collet instead of a three jaw, should have done this, that and the other. It'll probably surprise you armchair experts but I do things my way, the way I know works and how I've been doing it for the last 50+ years. I really couldn't give a hoot how you do it.