Timing

[Oggers]

Somebody wake my brain up please! On the 250RS and doubtless many others, I believe valve timing is basically achieved by lining up the marks on the cam sprocket and the head with lobes at 8 and 4 o clock when crank is at the T mark.

However, there could be 2 feasible positions for the crank at the T for this eventuality. Does it matter? Ignition Timing could be 180 deg out - or could it? If not, why not?

[K2-K6]

It's one of those things that you can go round and round until you doubt yourself 

With cam chain connected and cam in place the T position of crankshaft (piston always has only one T on a single) it will give you cam lobes both up or both down on sucessive 360 degree rotation.  That's on exhaust stroke of compression stroke.

When you assemble it, crank only has one T until you put the cam in, that defines which stroke is which. If ignition is on crank then it'll fire every revolution (doesn't matter as waste spark happens on exhaust stroke as well as compression).
If ignition is on camshaft,  it'll follow the compression stroke only as it's linked to the lobes, so wherever you set the cam the ignition should be in the right place on a single like this.

Well I think that's right 

[Oggers]

Yes - thanks for that. I was forgetting the wasted spark CDI system employed. Sensor is on the ATU on crank - I think - so I guess it does fire twice per cycle. Brain fade by me. More used to old Brit bikes with distributors, points and magnetos. 

[K2-K6]

You can sit there trying to think it through sometimes and get to a conclusion, then think, have I got that right 

Interesting that the Honda bikes used this wasted spark in the sixties, now most car engines have gone the same way, eventually. Means theres no reason to make distributor, gears, etc in design, just a set of crankshaft sensors and with the rest electronic either paired coils as Honda did, or coil on plug system.