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Points cam lobes

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Richie
Posts: 21
Joined: Mon Jul 16, 2007 5:18 pm

Points cam lobes

#1

Post by Richie » Mon Jan 24, 2005 4:21 am

Why is one lobe on the points cam in the circuit breaker larger than the other. This has the effect of keeping the points open longer than the other lobe. Why is that?



kell
Posts: 404
Joined: Sat Jun 19, 2004 2:58 am

Re: Points cam lobes

#2

Post by kell » Tue Jan 25, 2005 2:43 am

The lobe keeps the points closed. The points are actually open the same amount of time, because the flats cause the points to open and the flats are the same size. The oddball shape is just because the cylinders are timed to fire 315 degrees (crank) apart which translates to 157.5 degrees of breaker rotation (the breaker rotates once for every two rotations of the crank). So the flats are not 180 degrees directly opposite each other on the shaft. They are 157.5 degrees apart.

Cotten
Posts: 6911
Joined: Mon Aug 26, 2002 9:09 am
Location: Central Illinois

Re: Points cam lobes

#3

Post by Cotten » Tue Jan 25, 2005 1:51 pm

The lobes on all of mine open the points, thus letting the coil discharge.
Because of the asymmetric timing of the two cylinders, the rear requires less dwell time to recharge the coil, thus it keeps the points open longer.
Or so I was taught. I just accept elecktricity without understanding it.
I do know if you leave the ignition on too long while the points are closed, it can let all of the smoke out of the coil and it won't work any more.

kell
Posts: 404
Joined: Sat Jun 19, 2004 2:58 am

Re: Points cam lobes

#4

Post by kell » Wed Jan 26, 2005 3:58 am

Cotten
Right, I stand corrected. The points are open on the lobes. My mistake. Ahh, the subtle aroma of Crow Pie.
However! "The rear cylinder requires less time to recharge the coil, thus it keeps the points open longer" is a confuddling statement... if recharge means "build up and store energy for the next spark," that happens during dwell -- same for front and rear.
Richie
The asymmetry of the points cam is for mechanical reasons. The timing has to be offset because the cylinders need to be fired at different moments, being at two locations 45 degrees apart from each other. 360 degrees minus 45 degress on the crank translates to 180 degrees minus 22.5 degrees on the breaker shaft.

Billy
Posts: 781
Joined: Tue Mar 23, 2004 6:57 am

Re: Points cam lobes

#5

Post by Billy » Wed Jan 26, 2005 7:36 am

Cotten-

I wonder how they get ALL that smoke 'in the coils' ???

Richie
Posts: 21
Joined: Mon Jul 16, 2007 5:18 pm

Re: Points cam lobes

#6

Post by Richie » Wed Jan 26, 2005 7:52 am

Okay, I get it now. The coil discharges when the points open so the longer lobe just gives more time till the next points opening. The flat spots when the points are closed is the time when the coil is charging up (dwell). They are equal lengths because the coil charges up to the same level each time.

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