Re-Attaching Vacuum Advance

[crazy steve]

...the advance was altered, but not in the way I assumed. Scott elongated the advance slots making them longer. Also, unless it was from the factory, it looked like the metal behind it (weights?) were ground down a little. I'm thinking about ordering another advance unit and starting over...

The slots aren't elongated, they've been shortened, which reduces advance. Go back to the link in post eight for what it's supposed to be....

Throw that advance away, and start over....

The vacuum advance can't be checked by running the motor in neutral and watching the timing. The only way to accurately check it is to connect a vacuum pump/gauge to it and see if you get the right amount of advance for the given vacuum levels as shown in the manual.

[IanDMacDonald]

Okay, looking at your Advance thread, I know that the '78's have a single hole in the left side. I searched ebay and see mostly '79+ advances. Are '78's hard to find? Something for Andreas to find?

[crazy steve]

That's just the pic; the slots on all of them are roughly the same length, with the differences noted in the text.

[oldnortonrider]

Well, I just gotta ask this ~~ ¿Actually, he'd be "good to go" with any of the 3 years' models of the mechanical advance.................wouldn't he?

[crazy steve]

There's actually quite a bit of difference in how the bike will run with the different advance curves. Remember that the only real difference between the 78 and 79 bikes is the mechanical/vacuum advance and the 79s lost about .3 of a second in quarter-mile times.

With his 79, due to different cam timing compared to the 80/81 bikes, the best results will be with either the 78 or 79 advance. Some have tried the early cams with the 80 advance, but had issues with getting the bike to 'run right'.....

[IanDMacDonald]

Steve- I guess I do not understand what the "slot" is. Do you mind?

[crazy steve]

The slots are what the pins on the weights move in as they move outward as RPM goes up. Their length determines how much advance you get. Note that the 79-80 slots are .050" longer, which why these have 5 degrees more advance. I'll also note that the amount of advance compared where it is in the slot probably isn't linear because of the pivot points and angles involved, so when modifying these you should check at different RPMs to see what change you've made.

As to the 'hole', as you referring to the one in the back of the baseplate? On the ones I currently have, the 78 has no hole, the 79 has one, and the 80 also has one but smaller. I don't recall if others I've seen matched these in that regard, those could be balance holes....

Best way to ID them from a pic is by the number of spring coils you see; each year is different.

[MPittma100]

Vacuum Advance

The vacuum advance operates off of "ported vacuum" meaning no vacuum at idle. Starts advancing the timing as the throttle blades open. Note: as long as it is hooked up to the correct port.

MP

[IanDMacDonald]

Steve- I really believe I am confused pertaining to the term "slots". By slots, I assume you mean the two long slots, one on each side that the pins slide in. If you look at my pic, you can see they have been filed longer, thus "elongated".

http://db.tt/tC4SP9TZ

[crazy steve]

Steve- I really believe I am confused pertaining to the term "slots". By slots, I assume you mean the two long slots, one on each side that the pins slide in. If you look at my pic, you can see they have been filed longer, thus "elongated".

What he did is weld them shorter, then filed them to the length he wanted. Those still aren't as long as OEM, so that was limiting your advance.

[DGXSER]

I agree with Steve, look at the pics in the interchange guide. See how little metal is left between the slot and the end of the "arms". Now look at your pic and note how much MORE metal there is.

[oldnortonrider]

There's actually quite a bit of difference in how the bike will run with the different advance curves. Remember that the only real difference between the 78 and 79 bikes is the mechanical/vacuum advance and the 79s lost about .3 of a second in quarter-mile times.

With his 79, due to different cam timing compared to the 80/81 bikes, the best results will be with either the 78 or 79 advance. Some have tried the early cams with the 80 advance, but had issues with getting the bike to 'run right'.....

¿Are you saying, the vacuum advance mechanisms are different on the different years' (also) ?

[IanDMacDonald]

Ahhhhh . . . totally missed that. Hmm, that's interesting. Maybe soon I will at-least re-sync the carbs, and see if I notice any difference.

[crazy steve]

¿Are you saying, the vacuum advance mechanisms are different on the different years' (also) ?

They are different.... Yamaha lists a different part number for every year, and if you check the FSM you'll see that while the curves look the same, the amount of vacuum needed changes.

[oldnortonrider]

They are different.... Yamaha lists a different part number for every year, and if you check the FSM you'll see that while the curves look the same, the amount of vacuum needed changes.

Ok; I'm glad to have that information. Thank you for that.