What's the difference?

[SanDiegoMike]

I'v never fully understood what the difference between vacuum and centrifugal advance is and how they work together. Anyone got the Reader's Digest version that would help me out? I really want to figure out if this may be what is causing my bike to run so poorly. Thanks folks.

[TopCatGr58]

Hey Mike,

Check out this recent thread on the Igniton module , this link starts on page 2, Jerry and I and a few others get into some good discussions about whether an XJ could use an 81's TCI, etc., has diagrams of timing curves and such!
T.C.

[SiP]

As an explanantion of principal:
Vacuum advance is the natural vacuum cretaed by the engine processes that becomes stronger as the engine revs higher usually working against a diaphragm of osme kind or in some cases a sensing circuit.
Centrifugal advance is a pre-determined set of weights that are 'thrown' or forced outward against spring pressure the faster the shaft on which they are mounted turns.

Is that what you wanted or did oyu know that and it was something else?

[skids]

Re: What's the difference?

Basically, I think the centrifugal advance adjusts electrical firing according to the rpms. The weights spread out against the springs forces and cause the firing to be sooner in the cycle. The vacuum "advance" is actually a mechanism that retards the timing of the spark (so that it isn't as advanced by rpms alone. At least that is what I think is happening.

[John]

Actually the vacuum side of the equation serves two functions...At lower RPM the throttle plates aren't open very far and the vacuum applied to the diaphram in the advance mechanism is high and that advances the timing. When you open the throttle, the vacuum drops, and the diaphram returns to a more retarded state for better combustion.

[TADracer]

Both act independently but in layers. If you do the tuneup check then you disconnect the vacuum tube and run the revs up to around 5000, you should get 31 deg for a special and 36 degrees for a standard. If you do the same with the vacuum tube connected you will get up around 52 or 53 degrees of total advance. The vacuum unit keeps advancing after the centrifugal mechanism stops but when you close the throttle the vacuum dies out instantly and the timing retards quickly to react to the instant lean condition.

[trbig]

???

I'm with John on this one... I've always been taught that the vacuum DROPS on WOT and high on idle??


Tod

[randy]

Here's an excerpt from the '78 Cycle road test, I hope it helps explain...

[SanDiegoMike]

Thanks for all the replies! I guess what I'm trying to determine with all of this info is this:

Do you think that it is possible that the portion of the timing that is controlled solely by the movement of the advance plate because of the vacuum canister would cause a "surging" or "flat" spot when trying to accelerate if the bearings that the advance plate rotate on had some flat spots that prevented smooth movement? It seems that if I apply throttle VERY slowly, I can accelerate on a level surface without hitting the "bad" spot, and shift up through the gears. As soon as there is any demand on the system, (going up a hill, merging into traffic, etc.) I get that problem. If I really crank on the throttle, it'll be a slight hiccup and then hard acceleration until I let off (or the clutch starts slipping...another issue to address later ).

DiverRay shared his opinion of this with me...I'm asking for more thoughts from all of you Gurus and Super Wrenches out there (this means you too, Prom ). Thanks a lot!

[prometheus578]

"I live my life in a vacuum... and it sucks!"

Sadly, Mike,
It's too early in the morning for me to comment on your problem.... whether you're advanced or retarded.
As for degrees... the ones I have aren't in the appropriate fields of study to make this diagnosis.
I shall dwell on this matter no further.

[randy]

If your advance plate does not move freely, that is a cause for concern. That plate can be lubed, off the bike.

[AlanB]

Hi Mike,

If you want to find whether your problems are due to the Vac advance.... Disconnect it!... Just make sure that the spigot on the boot is sealed up, and try it!

Almost NO bikes apart from the XS11 (and RD200.. go figure!) use vac advance, so without it, it will run like other 1100s.

Vac advance makes NO difference at WOT, or anywhere near it. It should help economy on the highway, and help to prevent backfires on the overrun... It is all to do with smoothness and "refinement", which is why cars use it.

So to trace problems, disconnect and go!

AlanB

[TADracer]

Originally posted by randy
[B]Here's an excerpt from the '78 Cycle road test, I hope it helps explain...

Well I stand corrected....at least I was close

[planedick]

Right TCI

Do you think that maybe a PO has replaced the TCI with the wrong one? Maybe it has the one that has advance programed in besides the vac advance. You might want to check the numbers on it just to see.

[fredintoon]

Was it always like that?

Hi Mike,
did your machine always run poorly so you can blame it on the PO or did it just go from good to bad during your ownership. Or worse, did it go bad after you adjusted something? Anyhow, centrifugal advance has little weights that fly out as the engine revs faster and mechanically advance the spark. Vacuum advance (although according to previous posts in this string it's more properly vacuum retardation) varies the spark according to the negative pressure in the induction tract. The vacuum is greatest when the throttle is closed. Folks as old as me may remember when cars had vacuum operated windshield wipers. Stuck behind a truck in the rain and your wipers are beating like hummingbird wings as you cruise along on 1/8th throttle. Floor it to pass the truck and those bastard wipers just about stop dead. Options? Go ahead blind or lift off the gas to see if anything is coming.