I'm doing a full engine rebuild on an Honda GB250 clubman, RFVC & DOHC four stroke. I would like to know how to set cam timing from scratch?
Usually there are marks on the crankshaft and camshaft(s).
The crankshaft mark will match a specific point, mark or even have a hole with a pin to locate the timing point precisely on the front cover for example and the camshaft(s) will also have a hole for a pin so they will be correctly located.
Once all are located then driving belt/chain can be fitted and tensioned.
I tend, once the locating pins have been removed, to rotate the engine two or four revolutions by hand - checking for binding etc and get back to TDC on the crank to check the cams also land in the correct places.
Some engine require the use of dial gauges on the cams to locate the precise timing points...