mechanical advance parts questions ~ 2H7 or 3H5 ?

[Bullet]

So I am going through my mech advance /p.u. coils parts to get them cleaned up and on the crank so I can get TDC setup and pointed before I slap the cyl head on my engine.

I got complete sets of timing parts/ pu coils from both a 80SG and a 79F engine.

The 80SG advance parts have a 3H5 stamped on the pu coil plate and also the inductor/mechanical advance unit.

The 79F stuff is marked 2H7.

1) Is there any difference between these 2 mechanical timing units ?

By the eye they look the same, I even measured the slots on the 80SG mechanical throw-outs and they are no longer than the 2H7 ones. They look the same.. Maybe the springs are different, it's hard to tell, but they look the same.

2) Also in the OEM manual, the only reference I can find to full mechanical advance timing @ 5400 rpm is 36 degrees, while the clymer's manual says 36 degrees for standard models and 31 degrees for "specials"

Who's right, who's wrong ?

I rather use the 2H7 stuff, as it is in better shape than the 3H5 stuff ..

thanks ..

[dbeardslee]

On the 80's the you can't mess with the timing due to the safety bolts (which can be drilled out, and then you can adjust it). Other than that it's the same stuff. The 81's are the ones you have to watch out for - with the 4R0 TCI.

[fabricgator]

The 2H-7 / 3H-5 numbers correspond to the 78/79 AND 80 cycle serial numbers accordingly...

http://www.xs11.com/forum/showthread.php?threadid=396

Cheers!

[DGXSER]

Dbeardslee is correct. The 1980 models all used the 2H7 TCI with the same ignition components as the previous models.

[Bullet]

On the 80's the you can't mess with the timing due to the safety bolts (which can be drilled out, and then you can adjust it). Other than that it's the same stuff. The 81's are the ones you have to watch out for - with the 4R0 TCI.

yeah, I am aware of that, I am into a brand new engine build here ... just wanted to know about the mech advance timing reference ( 31 vs. 36 degrees ) in the manual, and if this relates to the difference between a 2H7 and 3H5 mech advance plate , weights, springs, throwout slot position .. I am sure someone has come across this before ..

Is it Yamaha just updated the prefix code on the parts, or is there a genuine difference between them ? 78, 79, 80, special or standard engine, aren't they the same except for carb difference on the 78/79 vs 80 ?




I left some time capsule markings for the next guy who rips this engine apart after I am long gone , hahahaha ..

[dbeardslee]

The heads and the pistons are different from 78/79 to 80 also, but the ignitions are basically the same.

My Haynes manual lists the centrifugal advance as 31 degrees at 5400 rpms for SF model, and 36 degrees at 5200 rpms for all other models, and vacuum advance at 16 degrees at 1.5 cm of mercury vacuum on all. Not sure what accounts for the difference.

[81xsproject]

Believe it or not, there are actually 3 different mech advance units as there IS a difference between 78 and 79. 78 is the one to get if you can find one.

[dbeardslee]

I took a look at the fiche, and low and behold the part number for an SF Centrifugal advance is different than it is for the F model. If you look at the diagram it looks a little different too - weird little square box on it. Anyway, you might compare what you have to those drawings, and if it doesn't look like the SF you're probably in good shape. Probably in good shape anyway, even if it is an SF mech advance. My understanding is that the timing advance if for fuel economy - not performance.

[Bullet]

Sorry , I was typing away and forgot about heads, cams, pistons difference .. dohhh ..

I just checked these mech advance parts side by side again, and the springs on the 2H7 are little longer and thicker than the 3H5 springs ..

Be interesting to see how the mech advance differ's from each other and what the rpm's vs's the advance degrees are between the 2 plates .. An experiment for later I guess ..


pic attached, spring on left is from 2H7 plate, the right is 3H5 ..

thanks again

[bikerphil]

Yep, that's probably the 5º difference in the two.

[Bullet]

Believe it or not, there are actually 3 different mech advance units as there IS a difference between 78 and 79. 78 is the one to get if you can find one.

I got 2 - 2H7 units and 2 - 3H5 units, what's the prefix for the 78 one ?

I looked both these units over pretty good, and only difference I can see is the springs, which sorta makes sense, due to cam differences on ( 2H7) pre 80 engines and the 80 3H5 cams ..

I am using a 80 head ( bigger intake valved one ), 80 3H5 cams.. and late model carbs ..

Soooo, if I use he 2H7 plate assembly ,will I be looking for 36 degrees on the mechanical advance at 5400 rpm's when I get this thing running ?

I am not messing around with the cam sprockets, using stock timing on the chain and sprockets .

[Bullet]

I took a look at the fiche, and low and behold the part number for an SF Centrifugal advance is different than it is for the F model. If you look at the diagram it looks a little different too - weird little square box on it. Anyway, you might compare what you have to those drawings, and if it doesn't look like the SF you're probably in good shape. Probably in good shape anyway, even if it is an SF mech advance. My understanding is that the timing advance if for fuel economy - not performance.

Yep, 3 different models of advances used ..

78 -79 Standard - 2H7-81653-11-00

79 Special - 2H7-81653-10-00

80 Standard and Special - 3H5-81653-10-00


I got a 2H7-10 , 2H7-11 and 3H5-10 .. to choose from ..

[81xsproject]

Ray mentioned on this topic a while back, that he installed a 78 unit on a 79 and noticed a difference in performance. Anyway, if any one wants one, I have one sitting around for $25 to the door.

[cywelchjr]

Yep, 3 different models of advances used ..

78 -79 Standard - 2H7-81653-11-00

79 Special - 2H7-81653-10-00

80 Standard and Special - 3H5-81653-10-00


I got a 2H7-10 , 2H7-11 and 3H5-10 .. to choose from ..

And I believe that the 80 standard and specials originally came with the 2H7 unit but the fix for the rattle under light acceleration I believe is the newer number which advances slower than the 2H7 model. I'll have to look at both my engines that have the timing parts (only 2 of the 3) and see if they both have the same advance mechanism. I have the rattle, so it might be worth swapping them if they are different (rather than swapping the engines since this one is working so good right now.)

[tinman905]

And I believe that the 80 standard and specials originally came with the 2H7 unit but the fix for the rattle under light acceleration I believe is the newer number which advances slower than the 2H7 model. I'll have to look at both my engines that have the timing parts (only 2 of the 3) and see if they both have the same advance mechanism. I have the rattle, so it might be worth swapping them if they are different (rather than swapping the engines since this one is working so good right now.)

Mine has the newer unit and I still have the rattle but it's only in fifth when I try to pass someone without gearing down.