Bob Rooks

What was the clutch hydraulic pressure before you disassembled it?

What does the clutch input shaft spline look like?

Do you know the operating principles of the hydraulic shuttle clutch? Like what causes it to engage and disengage?

There is a remote possibility that the issue lies within the bevel gear.

What type of oil has been used in the bevel gear compartment and in the shuttle clutch?

Best bet for those parts are Circle-G and Northern Tool. You can also order directly from Yuchai.

For some reason or another I cannot post links. They don't come through complete. Anyway, just Google “manifold heater starting aid” and you'll see.

You might also check the wire size to the heater Chinese tractors are notorious for under-engineering their electrical circuits and components.

Hard to tell from the picture if it's assembled incorrectly, but it should show some resistance on an ohm meter instead of a dead short. What amperage rating is the fuse/circuit breaker that it's connected to?

Excuse my ignorance, but what is a “B nut”?

What those are supposed to do is build a little fire in the intake manifold, and that provides hot air for a starting aid. There should be an igniter (similar to a glow plug) and a small reservoir for fuel. I have seen and heard of these plumbed in various ways. Sounds like you have a defective igniter. If you can't locate an OEM replacement (if you want to), you cal always go the aftermarket route.