Hi everyone!

[cruncherkid]

Thing is cam speed is 1/2 crank speed so 180* on the cam is 360 on the crank so the timing is exactly as it should be.

IE, piston TDC inlet valve just opening, turn cam 180* piston TDC both valves shut points just opened. Turn crank 360* on this new position and you get piston TDC inlet valve just opening. So 180* isn't actually a miss time it is exactly as it should be, it's a common misconception just about everyone makes.

However when your points are on the camshaft as they are on a 6V 90, it is truly possible to get the ignition timing out by 180 on the early ones even though the valve timing is correct....

Cruncher

[ll81]

Of course, sorry was thinking only about valve timing. Yes that point cam isn't actually part of the valves cam. I though I may well have been missing something but couldn't see what.

I stand corrected.

[Newtsalad]

Thing is cam speed is 1/2 crank speed so 180* on the cam is 360 on the crank so the timing is exactly as it should be.

IE, piston TDC inlet valve just opening, turn cam 180* piston TDC both valves shut points just opened. Turn crank 360* on this new position and you get piston TDC inlet valve just opening. So 180* isn't actually a miss time it is exactly as it should be, it's a common misconception just about everyone makes.

[ll81]

Think of it as holding the cam still and turning the crank 360*. The cam sprocket is now 180* in orientation to the cam (we undid it from the cam and left the chain on) but the cam and piston are still in the same place in relation to each other. It's impossible to have the cam 180* out in relation to itself, you can get a crank 180* out in relation to the cam but that would involve putting the chain on with parts in the worng position.

Also been a while since I had a ZZ engine in front of me. Is there not a key way in the camshaft that locates the points cam?

[Typhoon]

There is a dowel on the camshaft that aligns the centrifugal advance mechanism onto the camshaft. Ignition timing is fine on my bike, however, I checked valve clearance whilst the carb is off soaking in lacquer thinners. The clearances were incredibly tight, so all that extra duration wouldn't have been helping cold starting!
Ignition system has a nice healthy, well timed spark, cam and valves are right so once the carb is rebuilt I think I should see some action!

[ll81]

The tappet clearances are tight on the cub, there only 0.002 in (0.05 mm) which is a super flexible feeler guage.

Carb is the next logical step, keep it up and it'll soon be purring like a kitten, or rattling like a cement mixer.

There is a dowel on the camshaft that aligns the centrifugal advance mechanism onto the camshaft.

I may still well be missing something but would that not make getting the ignition timing 180* out also impossible.

[Typhoon]

The tappet clearances are tight on the cub, there only 0.002 in (0.05 mm) which is a super flexible feeler guage.

Yes, teh clearances on this bike were down to around .02mm, which would make it harder to start, reduced lash means more cam duration. I'd be surprised if it wasn't running badly when hot previously too. I had a Z 650 that wouldn't idle when it got hot as the valve clearances were too tight (like previous owners who failed to have lash done) and teh valves would unseat on the base of the cam!

[ll81]

I acquired a C70 like that, exhaust was slack and inlet was so tight was breathing like mad out the carb. A quick adjustment and was fine.

[Newtsalad]

I'll ask ag what he did. He'd had the head and barrel off, I'm fairly certain that putting it back together isn't foolproof, the marks do matter.

I've experienced difficult starting with tight valve clearances myself. It make a lot of difference.

I did a piece on here some time ago experimenting with the valve clearances. It seems the tighter the better it goes. Curiously, on the 85cc 12v lump I experimented on, the clearances increased when it got hot. I would have expected the reverse. It reassured me that if I can start it, it's prob ok to ride it.

Also, if we went to another race event, it could be good to thrash the bike up to temp, and shut the clearances down to .002". It might make it difficult or impossible to start if it got cold though. Another performance experiment I'd try if I had bags of time!