Vacuum advance behavior?

[Bob S.]

I recently purchased my first 1100 (79 SF). After cleaning the carbs, lightly grinding the seats for the needle valves, doing the black & decker fix, installing new petcock innards, and a few other jobs, I now have a running and driving bike (the power is a hoot!)

The vacuum advance hose was not connected to a vacuum source when I got the bike. Today I connected the hose to the no.1 intake boot and noticed a marked improvement in the idle.
However, with the timing cover off, I noticed that the advance mechanism jerks rapidly (too fast to watch) back and forth and generates a clicking noise. I seems that this is is caused by the vacuum pulses in the intake boot. Is this normal?

Thanks, Bob

[xssiveone]

Vac. advance should be connected to the brass nipple on the front of #2 carb, unless they changed from '78. The advance requires a metered vacuum source, straight to the intake boot is too strong for it to work properly.

[Bob S.]

Brian:

Thanks for the quick help. I guess my five bikes address a mid-life crisis too!

Bob

[skids]

Truth is, there is a tiny hole to the throat of the No 2 carb beneath that nipple. The hole helps dampen the pulses to the vacuum hose. Yeah, you will wear out the pick-up coil wires very quickly if you hook the hose to the intake manifold boot nipple!

Originally posted by xssiveone
Vac. advance should be connected to the brass nipple on the front of #2 carb, unless they changed from '78. The advance requires a metered vacuum source, straight to the intake boot is too strong for it to work properly.

[MartyA]

Hi Bob S.

One other thing comes to mind. Replace the vacuum hose. I was having some problems, while tracking down the things that can go wrong on one occasion I noticed the vacuum advance pulsing. After searching, I found a small crack, not seen in two other inspections. It would stay closed, then as the vacuum increased it would open, then as the vacuum dropped off, the crack would close, and so on. All this happening about 3 times per second. I had enough hose to cut the end off below the tiny crack, and re-attach the hose. The pulsing continued, but much less drastic. I considered the hose itself as too flexible (old), slightly collapsing as the vacuum increased, thus not transmitting the vacuum to the advance plate. I replaced the hose and the pulsing went away.