Vacuum advance

[stereomind]

Hey folks.

How should the vacuum advance behave on an '81? Would it even twitch if I was revving the engine in neutral? Or does it need to be under load?


Thanks,

Vlad

[GLoweVA]

Hey Vlad, i ask a very similar question this after noon. Mine only moved about 3mm. While on my other bikes it moves at least twice that.
I also want to know. When i get home I'll look it up.

[stereomind]

Ah, sure enough I guess I should've looked first.

Mine doesn't move at all in neutral, which is why I was wondering if it was load-related. I replaced the vacuum line, and verified that the solenoid reacts to vacuum.

Hey Vlad, i ask a very similar question this after noon. Mine only moved about 3mm. While on my other bikes it moves at least twice that.
I also want to know. When i get home I'll look it up.

[crazy steve]

You'll see minor movement in neutral, but it really needs to be under load...

[motoman]

Hey Vlad, i ask a very similar question this after noon. Mine only moved about 3mm. While on my other bikes it moves at least twice that.
I also want to know. When i get home I'll look it up.

81's the mechanical advance is done thru the TCI. They rarely get stiff in movement since there is no springs or centrifical weights.
Not the case with the 80's or prior. They get crudded up with that brownish dust you would see internally on the distributors.
Either case, that advance assembly, with the vacuum advance removed needs to be floppy loose in movement......litterally .
On the later ones, you can get away with spraying Brakleen using the straw on end of can held against where that outer assembly rotates. Then compressor blow area dry then carefully at top of rotating assembly put a small amount of machine grade or gun oil. My Hopped gun oil worked well for that
80's and earlier, that advance assembly has to be removed and everything behind it cleaned and lubed, the weight pivot points, etc. So that assembly rotates freely.

[GLoweVA]

Here's what the manaul says.

Okay so gleaming thru the manual, it shows the vacuum advance curves.
78:

79:

80:

81:


What I don't understand is why the 81 portion of the manual says that the 81 has a mechanical curve, but we all know that the TCI inserts the curve.

What I want to know is the vacuum advance the same for all the years?

[motoman]

Okay so gleaming thru the manual, it shows the vacuum advance curves.
78:

79:

80:

81:


What I don't understand is why the 81 portion of the manual says that the 81 has a mechanical curve, but we all know that the TCI inserts the curve.

What I want to know is the vacuum advance the same for all the years?

I believe the advance electronicly done thru the TCI is the mechanical curve .

[crazy steve]

What I want to know is the vacuum advance the same for all the years?

Nope.... While the amount of advance is similar for all years (but not identical), the amount of vacuum needed to pull in the advance varies. the 78-79 bikes need roughly 150 mmHg to reach full advance, while the later ones have it all in with between about 80 to 120 mmHg.

[BA80]

I believe they are adjustable if you can carefully remove the epoxy from the top of the pot. I've never tried it though.