81H vs SH Main Carb Jets Question?

[Slimbob]

Hello,
I am a bit confused. Going to rebuild a set of 81 SH carbs and want to be sure.

There are 2 posts that disagree on the Main Jet sizes:

1. From Blaine's XS11 Carb FAQ
http://www.xs11.com/forum/showthread.php?t=867

'80G & '81H had larger main jets on 2-3 than 1-4 for the same reason, SG & SH did not.

2.
From Carburetor Facts under MAINTENANCE CARBERATORS
by Phil Stewart
http://www.xs11.com/forum/showthread.php?t=73

1980 SG/LG 110(1&4)
1981 SH/LH 120(2&3)

Question is why the discrepancy and if they are the same are they all 110's?

Thank you,
Slimbob

[motoman]

The SG/SH Specials had 110 main jets across all four carbs.
The staggered jetting was temporarily used by factory on the Standard models of equivelant yrs.
reason for the differences was because the Specials had different metering rods than the Standards did,also factory stamped numbered differently). Also, the staggered jetting on the Standards in Yamaha's mind produced better cooling for the inner two cyls., but was done away with in short order. The Special and Standard also had differently configured metering rods......Standard had longer, AND longer taper. Specials had shorter and quicker tapers of metering rods, which gave it its nickname as "bar-hopper" bike. The different metering rod configuration also resulted in differences of main jet jetting of the two models. With your bikes listed year, having the original carbs(important for correct main jetting), it has to have 110 mains. Important that they also beGenuine Mkuni, NOT aftermarket RD's or K&L main jets. Sizing of Genuine Mikuni jets is done by some flow rating scenario they use.....not true for ANY aftermarket main jets.
Aftermarket main jets are metered substantially larger(example: 110 aftermarket main jets run equivelant in metered size to approx. 120 metered size of Genuine Mikuni jets. Holds true for pilot jets also.

BTW, if yours still have the Special carbs, numbering of metering rods(on side at top portion of rod) will be 5GL16 stamped into metal,(may need magnifying glass to read).

[skids]

Good explanation, Brant!

[TopCatGr58]

Hey Slim,

I've had my 81SH since '83, and it was bone stock when I got it. When I finally got around to tearing into my carbs, I found 110's across all 4.

Keep in mind that Yamaha detuned the engines by leaning out the fuel/air mixtures for EPA requirements, which are not necessarily the best for performance vs. mileage.

I did run mine with the stock jetting for decade (+), even with 4-1 aftermarket Mac Pipes which were supposed to lean things out . The Intake was the OEM airbox, and it ran okay. Once I rebuilt the engine in 2000, I put on Pod filters along with the 1179 big bore kit, and was worried about it being too lean and burning holes in my pistons, so I bumped the jets up to 115's. Later I did a dyno run, and found that they could actually go up to 117.5's, which I finally did a few years ago, along with upping the pilots to 45.0. I'm also running hi output DynaTek Coils, and it runs great, has decent color of the plugs without blackening.

SO...moral of the story, you could put 110's in there, and it would probably perform okay, but you'll want to check your spark plugs to see how they color with a Throttle Chop check and adjust accordingly. YMMV.

T.C.

[Slimbob]

Thanks Guys. That's some good info.

But the 2 locations on the site still may steer someone else wrong.

Two (#'s 3 & 4) of my original carbs are toast as the pilot jets were locked in and the screw driver slots damaged when I open them and I snapped off a easy out in #3. I learned a lot of what not to do and some good hints on this forum on how to remove the pilots if stuck. Will have time later to play with the original carbs and recover them if possible. ~10K miles on the bike. Freebee. Want to see if it will run before investing a ton in parts.

I bought a set of 81 SH carbs from Andreas. Haven't seen them yet. Working out of town. Will check them out this W.E. Thanks Andreas for good price and prompt shipping!

Can't wait to start.

Thanks again. Will try to post some pics as the rebuild moves along.

Slimbob