Topic: advance timing unit ?

[scunny]

probably a silly question but give an ageing man a break.
on the advance timing unit, should the cam be sprung loaded so it returns to normal advance as the motor slows down.
thoughts !!!

[HavocTurbo]

Which bike? Or all of them?

I thought there were spring loaded arms that did that... push back I mean.

[scunny]

it's a 650 but all the advance units I've seen look the same.
it has the weights and springs that throw out as the revs rise allowing the centre cam to advance but what brings the cam back to normal idling advance ?

[Bodi]

The springs hold the arms in. At higher RPMs the (imaginary) centrifugal force acting on the weighted arms moves them out against the springs. The little ears on the inner ends of the armes engage slots in the points cam and cause it to rotate "forward", making the spark happen sooner in the crank rotation - advanced. When revs go down again the springs pull the arms back in.

[HavocTurbo]

The springs hold the arms in. At higher RPMs the (imaginary) centrifugal force acting on the weighted arms moves them out against the springs. The little ears on the inner ends of the armes engage slots in the points cam and cause it to rotate "forward", making the spark happen sooner in the crank rotation - advanced. When revs go down again the springs pull the arms back in.

That would be the technical explaination of what I was trying to say.

[Hush]

Thats how mine works Scunny!

[scunny]

OK guys the plot thickens.
the advance unit I have on the 79 motor is off a 82 model and dosn't "index" properly on full advance nor at idle
have managaged to pull apart my other " very rusty spare" advance which is off a 79 and the index is positive woohoo, now to find the weerubber bit that fell off it when I was cleaning it 

[NitroHunter]

Is weerubber bit one of them there teknikkle termz? 

[scunny]

yep, thats a weerubber bit alright, and I've robbed it off one set and super glued it to the hopefully good one

[Sam Green Racing]

I think you'd be better off sticking to fishing and boozing, mechanicing is not your forte

Sam.

[scunny]

ya cheeky old coot 
after carefull inspection of the misbehaving advance it appears the PO had filed the index marks off the inside lever thingys.
this came from the 650 motor he had installed in a go cart, I wonder what he did that for ?
don't worry Sam, tis beer o'clock shortly

[Soos]

it's a 650 but all the advance units I've seen look the same.
it has the weights and springs that throw out as the revs rise allowing the centre cam to advance but what brings the cam back to normal idling advance ?

nope... the '79cb650 has a different advancer...or at least the part # stamped on it is different.
Not too sure if it advances the timing to a different degree than the '80-'82 cb650 advancers do.

My '79 advancers have #426  stamped on them, the '80, '81, and '82 advancers I have seen all have the same part #460 stamped on them.



l8r

[scunny]

correct soos, 426 and 460. the backing plates are the same but I cant compare the advance due to PO butchery. my cobbling together the good from both seems to work ok, bike starts well and can now rev past 3000 without setting my carbs on fire  but I will delve into the finer points of timing later.