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I pulled off my points plate and the spark advancer was stuck open (springs stretched). It didn't want to easily come off so I brushed on some WD40 and worked them free. When I got them to move freely, they wouldn't "snap" back, as one of the springs was stretched and pretty loose. I carefully shortened it by about 3/4 ring and made a new bend without breaking it. It really seems to work nicely now, but time for bed and didn't have time to reassemble and try to run the bike. Tomorrow.

I guess I really don't know what the advancer does, and what would be the symptoms caused by a stuck one?
 

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The spark advancer basically uses centrifugal force to advance the timing at higher RPM's.

If it's stuck, it wont run for **** off idle. Make sure your springs are in tact and the whole assembly moves freely.

-MK
 

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Well, when the engine is idling, the spark should be occurring just before top dead center. This is the timing you set when the engine is off and you're doing the dance with points gap, advancer plate, test light and tick marks on the alternator rotor.

The spark doesn't occur exactly AT TDC because if it did, by the time the fuel-air mixture burned to the point of doing work on the piston, the piston would be already partway down the cylinder, negating some of the work done by the expanding gases.

As the engine rpm increases, the spark needs to occur earlier and earlier before top dead center for the reason as described above. However, if the spark occurs TOO early, the expanding, burning gases conflict with the still-rising piston on the compression stroke. This conflict is engine knocking.

The mechanical advance uses centrifugal force to slightly 'advance' the points cam as rpm increases. This causes the points to open earlier and earlier as rpm increases. The amount of this advance follows a 'curve' dictated by the amount those weights move outward. Eventually, the weights are fully outward (against the stops on the advancer mechanism), and the spark is advanced as much as it's going to. The advance is "all in". The starting rpm of weight movement and the final "all in" rpm are determined by the strength of the springs. Stronger springs will delay the advance movement and require more rpm to fully advance. Weaker springs have the opposite effect - the advance will be all in at too low an rpm.

If the spark is not advanced enough, the engine doesn't produce as much power as it might (because the combustion isn't occurring soon enough). If the spark is too advanced, you get the engine knock (which can be destructive to pistons, rods, etc).

I think that about sums it up...
 

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Excellent explaination Kirk!...

My attempt (at an explaination) would have read:
When the plug fires, it takes a microsecond of time for the fuel/air charge to ignite and burn/explode.(this is the flame propogation time)....Advancing the spark timing adjusts this ignition point RELATIVE to piston speed (revs) so the maximum amount of force from those explosions are applied just as the piston crests TDC. This is why it is advanced more with higher revs.
Too much advance and it completes the explosion too soon (while the piston is still rising)and the engine "Knocks"...Too little and it wastes force (HP).... Steve
 
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