timer cam lobes

[Dave Beck]

I have a 62 pan that is almost finished. It has an auto advance aftermarket single point distributor, as I was going through the motions of timing the engine, I was reading about the front and back lobes being differnt and sure enough the back lobe is much bigger. Is there a reason why? The old gal starts right up and seems to run fine, just waiting on a nice day to take it out on the madien voyage.

Just wanted to get the correct reason from the experts here as why the back lobe is much wider.

Dave Beck

[Cotten]

Dave!

It is not a distributor.

The firing of the cylinders is not evenly spaced through the motor's revolution cycle, so your timer has a wider lobe for the rear cylinder than the front to provide sufficent dwell to recharge the ignition for the next spark.

...Cotten

[Dave Beck]

Cotten,
Thanks for the explaination.
Dave

[steinauge]

As Cotten says.Also our engines fire 315-405 degrees .Adds up to 720 degrees-a hard # to get away from in a 4 cycle engine!

[tomfiii]

Hi, I really don't care if someone calls it a distributor. As stated the difference is the time it will break the points,I suspect the rear lobe is wider due to it being easier to manufacture or to keep the dwell the same for both cylinders.

[partshunt]

The reason the breaker cam is made that way is for to time the points open and coinside with the uneven fireing of a v-twin. As Stienauge mention, 315 and 405 degrees on the firing position so the breaker cam needs to break the points open at the same piston BTC position on each of those cylinders. And as Coton sez, to create the same dwell time for the next spark occurence. Has nothing to do with machining short cuts or even being a four cycle. A 2-cycle V-Twin (if there are any) would need the same breaker cam grind or configuration to match or time piston position before top centre. No other reason.. Uneven fireing, uneven breaker cam. We all love the V-Twins but they are born with a single crankpin connected to two cylinders in a vee configuration to create an uneven fireing order, vibrateing and uneven power pulse that even robs it of power. Most of the British and other V-Twin engines went out the window back in the thirtys for that reason.....Joe.

[kitabel]

This is called a "circuit breaker" because it interrupts primary voltage to the coil.
A "distributor", or Kettering system, interrupts the secondary voltage from the coil.
H-D used both on the JD engines back when.

[Cotten]

This is called a "circuit breaker" because it interrupts primary voltage to the coil.
A "distributor", or Kettering system, interrupts the secondary voltage from the coil.
H-D used both on the JD engines back when.

Thanks Kitabel,
I didn't know either of those things.

But I knew Indian ran Autolite distributors (except when a mag was chosen).
I can't find any advantage at all.

....Cotten

[tomfiii]

Hi all, when the points are closed is when dwell is created. The rear lobe does not have to be as wide as it is it only is wider to change the degree position it opens and it could be just as narrow as the small lobe , they just would not line up 180 degrees accross from each other.