Considering Distributor “ Upgrade “

It would however have a big impact on the emissions measures that need to be applied.

 

High compression ratio coupled with some static timing advance would make cranking hard. Add the possible manifold vacuum pulses applied to the vac adv during cranking and the possibility for kickback increases.

 

Well, this was a fun discussion!
What I am taking away from this is that, most likely, vac advance was used when the vehicle was more likely to be used on the highway, at least some of the time. It should provide a somewhat smoother idle, and less backfire when letting the throttle off suddenly. In other words, it makes the engine just a bit more civilized. Like a level just above a tractor. Efficiency would have been a minor bragging right.
My best guess is that cjs didn’t get this feature to keep them as simple as possible.
Just that much simpler to repair or service. That’s why you can remove the distributor while leaving the timing bracket locked tight. This feature eliminates the need for a timing light in the field.
I’m applying my experience with vintage Honda motorbikes to this.
Virtually all trail only models have no form of spark advancement.
Virtually all street models do have spark advancement.
The difference is immense. Trailbikes backfire and surge on the street. The spark is fixed at a compromised setting between good power and tolerable starting and idle.
Streetbikes are rather sluggish at low rpms and take longer to rev. The spark is not fixed and the time it takes to change is very noticeable.
Sooooooo….. I am going to try the vac advance distributor. We will see if the extra advance is worth the trouble or not.
I’m going to check it for runout very carefully as this affects spark quality a great deal.
I can always reverse this easily if it doesn’t work out. Unfortunately it will probably be spring before I can test this.

 

What models have no centrifugal advance? CRs?

 

Granted my engine has been "souped up" I have 21 more HP then stock measured on a dyno at the rear wheels. BUT part of my engine improvements is the IAT-4405 Vacuum Advance Distributor and I am sure that contributes to the 17.2+ MPG I see on the Hwy at 60mph I have run my jeep a little over 8000 miles and the Mileage has constantly shown 17.2+ on LONG 60-60+ MPH runs some as long as a whole tank in one run.

My 48 CJ-2A has 5.38 gears a 30% OD and 28.5" tall tires at 60 MPH the engine is turning 2665rpm. For hills I will shift into Direct and run as high as 3500rpm.

 

Since I am old as a fossil, think late 60s/ early seventies.
Z 50, Ct 70, trail 90, 110, etc.
Old 4 stroke singles. Been riding them since 1972 or thereabouts.

 

What are you using for initial timing?
Sounds like you are already there where I’m trying to go.

 

I'm running 87 octane E-10 and 12° BTDC initial timing. Never had a ping.