vacuum advance question

[jvandyke]

I'm curious to understand how the standard set up is engineered. 1098, 25D, HS2's. Is the vac adv. running off ported vac or manifold when attached to the HS2 front carb's port? Is this supposed to advance timing a bit at idle, then go away as throttle is opened? I really question whether mine is working at all (and if I even care). I pulled the vac line and plugged and timing light on 800 rpm 8 BTDC (but the mark seems to migrate around a bit, could be the cheap light I guess). Reattached the line and timing did not change a bit. Then I revved to 3000 rpm and had about 22 degrees more advance over the initial 8, or 30 total which seems about right to me.
When I had the dizzy out earlier this year I sucked on the vac adv. line and it moved a bit, seemed pretty reluctant to move to me. There didn't seem to be much of any vacuum at the carb orifice at idle when I put my finger over it, which seems to indicate it is ported vacuum, carb side of the throttle plates?

I meant to go through my books last night and forgot.

 

[jvandyke]

This is interesting
other discussion

That thread seems to suggest some carbs had an orifice that was ported, some not.
Butterfly swings right over the orifice. Maybe my HS2s are from a later 1275? Although they don't have any PVC Y pipe or anything.

 

[Sarastro]

The vacuum advance mechanism does fail occasionally; nothing unusual about this. Best way to check is with a mity-vac or something similar, so you can get a measure of the advance vs. vacuum. Note that you won't get any vacuum advance until the vacuum reaches some particular level. I forget what it is with these distributors--there's some info on the web, if you search around.

Also, don't confuse centrifugal and vacuum advance. Of course, you'll get advance with no vacuum advance connected, and will get little or no vacuum advance at idle.

 

[Bayless]

Whether you get vacuum advance at idle very much depends on whether you have a manifold or ported system. If manifold then you get much vacuum advance at idle since you have near maximum vacuum. Ported gives none at idle since the port is on the atmosphere side of the butterfly therefore exposed to very little vacuum. I understand this was done to retard the spark at idle as a polution solution. Not sure how that works though.

 

[jvandyke]

I'll research it and see what I come up with. I have books that should tell me.

 

[jvandyke]

Looks like with the setup on a 1098 and 25D vac advance is ported, right by butterfly, kicks in at 4, done at 13 and yields total of 20 crank degrees (a lot more than a 1275 which is 6).
I'm looking at getting a hand vac to apply vacuum and see what it does (could use it for a bleeder anyway) and might just plumb in a vacuum gauge to monitor from the cockpit.