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Old 10-01-2011, 12:45 PM
crazy steve crazy steve is offline
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Interchange Guide: Mechanical Advances

This comes up from time to time, so here's a 'illustrated' identifier...

[IMG][/IMG]

From left to right, you have 78, 79, and 80 advance units. At a glance, they look the same, but there's important differences. Some parts are the same though, and I'll point those out. Each one has a different 'curve', so there's a very real performance difference and this may be important depending on the motor you install it on and what initial timing number you use. Ok, the differences....

The 78 unit offers 26 degrees of advance, starting at 2150 rpm and it's all in at 4750 rpm, for a 'range' of 2600 rpm. The slots in the reluctor are .515"/.530" long; this is what determines total advance. The springs have 13 coils, and a wire diameter of .033". Remember that the 78 uses 10 degrees of initial timing for a total of 36 degrees @ 4750 rpm.

The 79 unit has 31 degrees, starting at 1800 rpm and it's all in at 5400 rpm, for a range of 3600 rpm. The slots in the reluctor are .565"/.585", and the springs have 15 coils with the same wire diameter as the 78 bits. Initial timing is retarded to 5 degrees, but you have the same total of 36 degrees, although at a higher rpm (5400).

The 80 unit also has 31 degrees (OK, the manual says 30), also starting at 1800 rpm but this is all in by 3900 rpm, for a 2100 rpm range. The slots on the reluctor are the same as the 79 unit, but the springs are lighter; 14 coils with a wire diameter of .028". Again, the same initial 5 degrees and total timing of 36 degrees, but all in at only 3900 rpm.

As far as I can tell, the weights used are all the same (as are the 'bases'), with the differences being in the slots/springs. While the bases appear the same, they do have different numbers stamped into them...

[IMG][/IMG]

It may be hard to see, but the 78 has 2H7-10/AD126-01 on it, the 79 has 2H7-11/AD131-01, the 80 has 3H5-10/131-02.

If you have or are planning on a motor swap, this is something you might want to check to make sure your timing is set correctly. I'll also note that the vacuum advance units are also different from year to year, but that seems to be a bit less important.

So, what else does all this mean? Well, from my hot rod days, it was desirable to get your mechanical advance in as quickly as possible for the best power; remember the 'recurve kits' for distributors? But you had to be careful not to have too much advance or you risked detonation. The more initial advance you could run also helped off-idle power. So it appears that Yamaha put a fairly aggressive curve in the 78 models, then detuned the 79s a bit. They further detuned the motors in 80 (slightly less compression, milder cams) but tried to restore some of the lost power with a more aggressive curve. So, which one do you use if you're running a non-stock combo or are looking for more power? Well, if you're running the early cams, the 78 unit will give the best results, mainly because you can use 10 degrees initial timing. The 80 unit has possibilities too, particularly if you use a 78 reluctor or shorten the slots to match the early part. It may be too aggressive though, so be sure and watch/listen for pinging. Another thing to remember is these motors were designed to run on 'regular' grade fuel, so if you step up to premium you should be able to run more timing.

Anyway, something to think about for the more adventurous among us...
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