Re-Attaching Vacuum Advance

[crazy steve]

I'll add that which vacuum advance unit you have is much less critical than which mechanical advance you have. There will be performance differences, but only at part-throttle openings and particularly during the transition from part throttle to larger throttle openings. At WOT the vacuum advance is not operating.

[DGXSER]

The question I have on all these somewhat small changes, is this. Would the average daily cruiser type of rider even notice if you swapped out the 78 advance for the 79 advance. Or even the 78 or 79 for the 80 advance.

At the performance levels that most of us use these bikes, I have always felt such things would be insignificant to the rider.

[IanDMacDonald]

Well, my interest in all of this is purely to understand my bike. Right now with the way Scott altered the advance, my plate will never rest on "F" at idle, and that bothers me. I'd rather go stock with a '78 advance and figure things out with a baseline.

[3Phase]

Well, you'd notice the '78E advance.

You'd have to advance the static ignition timing and the 'F' mark on the timing wheel to 10 degrees BTDC instead of 5 degrees BTDC.


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[IanDMacDonald]

Scott- The PO of my bike modified the timing to mimic an advaced curve. I just may not notice much difference, as I don't run my bike like a crotch rocket.

[MPittma100]

Timing/Advance

I would venture a guess that all of the different timing advances chosen by Yamaha were based on US emission requirements that were "imposed" upon them back in the day. It would be quite difficult to notice a difference in your performance unless you seriouly modified the timing curve. And by doing so, you may do more harm than good. Normally, timing curve changes go along with other changes such as more fuel, hotter spark, camshaft duration, and more compression. Putting the timing curve where Yamaha designed it for each particular year model is the best thing to do.

MP

[3Phase]

Scott- The PO of my bike modified the timing to mimic an advaced curve. I just may not notice much difference, as I don't run my bike like a crotch rocket.

I've been following along, Ian, and you don't need to ride like a SQUID to notice 5 degrees of missing static base ignition timing.

You have a '79F (Standard) and its '79F timing plate along with a customized '79F centrifugal advance and a locked-out vacuum advance. Now you want to restore the vacuum advance and drop in a '78E centrifugal advance to replace the custom '79F centrifugal advance.

That's great! but the '79F timing plate 'F' mark is for 5 degrees BTDC @ idle and the centrifugal advance mark is for 31 degrees BTDC @ 5400 RPM. Both are measured without the vacuum advance connected.

The '78E timing plate 'F' mark is for 10 degrees BTDC @ idle and its centrifugal advance mark is for 36 degrees BTDC @ 5200 RPM. Again, both are measured without the vacuum advance connected.

You can change out the modified '79F mechanical advance and drop in a '78E mechanical advance but you are going to confuse yourself and anyone else that works on that engine if you don't at least update the marks on the '79F timing plate for the '78E timing or put on a '78E timing plate. To keep everything copacetic a '78E vacuum advance would be nice but it's not necessary.

You could also write down the changes somewhere like one of the blank pages in the back of the fine factory service manual like I so I don't get lost when I go back in the engine later but that's strictly up to you.

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[IanDMacDonald]

It's funny, but everything said the second time always clears things up for me. Okey dokey, I'll be looking for a '78 plate and advance unit. Thanks Scott.

[3Phase]

You're welcome, Ian!

A '78 plate is the best idea but for now I'd just use a fine tipped laundry pen to put the '78 marks the '79 timing plate and write "78 timing!" on it too until I was sure it was actually going to work.

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[IanDMacDonald]

Ehh, I'm not keen enough to know how to mark it. As of now, the bike idles well ahead of the "F" mark, and @ 3k, she's @ 35° Seems to ride well. gas mileage is still bad.

[crazy steve]

It doesn't matter which timing plate you have; each mark clockwise from the 'T' mark is five degrees, so the first mark is 5 and the second mark is 10 (the parts fiche shows the '78 timing plate for both!?).

In either case, you're looking for 36 degrees total advance, no matter which mechanical advance you use. For the '78 advance, you need to spin the motor to at least 4800 rpm, 5400 for the '79.

[IanDMacDonald]

Am I still at a "true" 36° at 5400 rpm if my idle is not resting on "F"?

[crazy steve]

The reason Yamaha specs the timing the way they do is to verify that the mechanical advance is working right.

The '78 has 10 degrees initial advance, 26 degrees of mechanical = 36 total.

The '79 has 5 degrees initial advance, 31 degrees of mechanical = 36 total.

The important thing to note here is total advance is the same in both cases. This number is arrived at by extensive dyno testing with engineering knowledge of fuel burn rates, quench areas, and flame front propagation in the chamber (to name just a few factors). This is a bit of 'holy grail', not something you change lightly. As a general rule with gasoline-fueled motors, anything beyond 40 degrees won't gain you any additional power.

With that butchered advance you have, what you have at idle if set for total at 5200 is anybody's guess. As a SWAG, I wouldn't be surprised if you're at well over 10 degrees....

[IanDMacDonald]

That seems very clear the way you put it. Thank you for posting that.

[3Phase]

1979 Timing Plate

Ian,

Assuming that no one bent or dinked around with the Timing Pointer whilst modifying the mechanical advance and locking out the vacuum advance and that when the TDC mark on the Timing Plate is aligned with the Timing Pointer the engine is actually at TDC then the rest of the marks will be correct.

The Timing Plate is just a plate with timing marks stamped on it every 5 degrees but the 'F' mark is stamped at 5 degrees on the '79 plate and at 10 degrees on the '78.

The drawing is a little shaky but here's a picture (click to download a larger image):-


1979 Timing Plate with timing marks expanded


The stock '79 mechanical advance provides 5 more degrees of advance than the '78 mechanical advance. It doesn't matter on your engine because Scott modified it but you took it out and installed a '78 advance.

Now:

If you set the timing to the 'F' mark (5 degrees) on the '79 Timing Plate with the '78 mechanical advance installed you wind up with only 31 degrees of total advance -- not enough!

If you set the timing to the '10 degree' mark on the '79 Timing Plate with the '78 mechanical advance installed you wind up with 36 degrees of total advance -- perfect!

If you set the timing to more than 10 degrees of advance at idle -- too much! The engine will waste fuel fighting itself, ping, and overheat while it merrily beats the everlovin' scarlet begonias out of the tops of the pistons, rings, and spark plugs.


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