Well, my top end engine rebuild and weber restoring has worked and my bike has rolled on the road for a short time today!!!!!

I've replaced the clutch piston for a custom made item and it seems to work but the gears are hard to engage and I need some throttle to increase revs and engage gears. With neutral it's even worse.
I imagine it's a problem of clutch. The discs seem to be in good condition but I d'ont know if the piston runs enough to disengage it. The gearbox worked fine the last time I used the bike.
I found filling the circuit with brake fluid really hard and getting out the bubbles was a real pain but it's seems to work fine now...

Any ideas? It might be the clutch? Discs?

Thanl you very much, your last advice with the Weber was the right one!!!!

I think your clutch isn't disengaging fully. Can you wheel it in gear with the clutch in, if its dragging then clutch line has still got air in it or your new custom slave piston is not correct, check you're not losing fluid. Be careful you dont damage the dogs on your gearbox by crashing it into gear if you are riding it. The early gearboxes are easy to wreck, I know!

Ditto on the air, get yourself setup to reverse bleed it from the lowest point to the top,only way to do it

Yes, I thought that was the problem and I've it solved now. I proceed with reverse bleed and the gearbox runs like silk (or almost) now.

Thank you for your kind help.