Hi Tom. I have had brain freeze on a few occasions when starting my bike after a rebuild and thought that the problem was X when it was Y. To my unlearned eye it was also hard to know which timing mark works for which lead and whether pushing the timing lever away from the bike was full advance or not. They seem simple things now but as a modern bike person it was new to me.
I set my timing to 5/16 BTDC and usually there is a mark on the flywheel there from previous owners. The mag should be on full advance when you set the timing, if the points end of the mag is on the left side of the bike, and full advance is when the mag is the furthest point clockwise. On my bike I pull the lever towards me and the mag arm is on the left to set it.
I have taken the transfer ports off before and poured quite a bit of oil into the crankcase. As far as I understand this provides the seal to get the suction from the crankcase to pull the fuel in.
Air leaks are from where the carb bolts on to the engine as the ears tend to bend if done up too tight (I had to grind mine flat), the transfer port doors and the crankcase doors which I remove, put a smidge of fine grinding paste on and fit them that way with a rag stuffed inside to capture any rogue paste. And a final gasket. If you undo the bolt and remove the crankcase door strap and the door falls off on its own accord it is not sealing. It should take a kick or two to blow the doors off.
Fuel height, don’t know for your bike but if your plug is wet then I would say that there is enough fuel for starting and that I am inclined towards saying that fuel height affects running more than starting as long as you actually have fuel. You could set you float bowl in its highest position by rotating it backwards to ensure there is indeed enough fuel if you like, but as a last resort. What about sticking the plugs in the flame of the gas cooker and then trying it then. Does pouring a capful of petrol down the plughole do anything? Do you have a sodding great hill nearby?
Air screw 1 turn out and see how it runs once started and make sure that the slide in the carb is fully down to the screw as a stuck open slide will mean there is too much air for it to start. I had this on my Yam at the weekend on one side and the cable had come out of its locating sleeve because it needed lubricating.
Saying all that, it is likely to be none of those. If you remove the plugs and earth them on the frame does it spark?
Apologies if all that sounds like I am insulting your intelligence and knowledge Tom and I hope that what I have written is right in the first place! These are the things that have flummoxed me before. Sounds like you have good compression anyway.
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