Vacuum Advancing at Idle ?

[Chadkid]

Just got my 79F running today but I cant get it to idle. It will idle around 1100 for about 5 seconds and then mysteriuously it will pull the vaccuum advance which raises the RPM to 5000. The butterfly valves are remaining closed during this positive feedback situation which makes me wonder why its revving so high. Is there a spring or something that I may be missing on my vaccum advance ? Could it be running rich? I am baffled.

[79XS11F]

tell me you have the vac advance hose attached to the #2 CARB body and NOT the #2 carb BOOT ...right???
Rob

[John]

There should be no vacuum at the advance at idle, as the vacuum port in the carb is coverd by the throttle plates, until you open the throttle. Your carbs may be out of sync, allowing #2 throttle plate to be open more than the others.

[Chadkid]

No, my advance tube is on the #2 Carb.

[Crazcnuk]

What happens if you unplug the vacuum hose?

Chances are the engine is reving first, thus moving the advance, not the other way around.

[79XS11F]

Originally posted by Chadkid
No, my advance tube is on the #2 Carb.

OK the vac tube is hooked up correctly .. then check what happens with that tube disconnected or even easier just pinch it off. If no change do as John suggested and sync the carbs... they should be synced in any case before trying to address other issues.
Once they are synced set the idle with that line still pinched off and see what you got. Readjust as necessary after un-pinching the vac tube.
Rob

[Chadkid]

Ok, I disconnected the vacuum tube and it doesnt rev out of control. Now should I even bother adjusting the carbs with this unhooked?

[Crazcnuk]

It never hurts to check the synch on the carbs unless they were done fairly recently.

However, if the engine doesn't rev with the hose off I would say it looks like you have a vacuum leak in the advance assy itself. There are ways to test this, if you have access to a leak down tester, or someone else may have another idea.

[Chadkid]

Well I recently rebuilt the carbs- along with a complete engine overhaul so I will indefinately need to synch the carbs and adjust the mixture. I just got the bike running for the first time in years last weekend and have no experience with tuning everything but I'm getting a feel for it by trial and error as well as reading the posts in this Forum.

What I will try is to synch the carbs with the advance hose unplugged and capped (at the timing end). Hopefully that will allow the bike to idle just enough so synching is possible. Once the carbs are synched and the mixture is tweaked, using the RPMs as a gauge, I will reattach the advance hose and pray.

Sound about right?

[DiverRay]

See if you can find a hand "Vac Pump". Use it to test the advance unit. Pump it to about 15, and see if it will hold. If not, the unit is bad, and you need to try and find a used one. They are NO LONGER available from anyplace.
If needed, do a search, as I know there was a thread with info on the advance units.

[79XS11F]

Well usually if the diaphragm in the advance is holed or damaged the vac will fail to work at all as compared to advancing too soon. Is the control spring still on the shaft? Also check to see how much vacuum you have at that hose. Perhaps the hole in the carb body has been enlarged by some one in the past and is allowing
for too much vacuum at the carb port.
Rob

[Chadkid]

Control Spring? Would that be inside the vacuum advance cylinder or external to it??

[79XS11F]

NO sorry .. got my wires fully crossed there the springs are on the bob weights and not involved here Daaaaaaa
Rob