Main Jets for 1981 XS1100H

[mmatches1]

For some reason my the carb for my 81 XS1100H had 110 main jets installed on all four carbs and the service manual calls for 115 on cylinders 1 & 4 and 120 on cylinders 2 & 3. I purchased the correct jet sizes and was going to install them but wanted to throw it out there before I did. Any one else face this? Maybe I have a different model of carb? The only identifying mark is the number 3U7 00 B081. The manual calls for the Mikuni BS34-111 3H5-01.

[skids]

110's in all 4 are stock for the H. At least it was for mine and I know that no PO had opened the carbies.

[Gallaecus]

i think they put 120's on the outside cuz 1 and 4 ran cooler and 115's on 2 and 3 cuz they were hotter. i think they stopped doing that. 110's or 120's i dont think it matters much. as long as the end result is the correct mixture.

[conquest87tsi]

I'm pretty sure you have it twisted. They ran the larger jets on 2&3 as a richer mixture results in slightly cooler operating temperature. If I remember right most people just run the richer jets on all four. I think I ran 115 mains on my spare carbs, havent opened them up for a while.

[crazy steve]

The FSM calls for 110's across on all '81 models. The 115/120 jetting was for the G model (theory was the extra weight/reduced air flow if a fairing was fitted required the richer jetting; proved not to be the case), but it appears there wasn't a clear 'line' as to when they made the change so there's been some confusion.

Keep in mind the 110's were fitted mostly for emission purposes, increasing to 115 or even 120 across all four carbs generally will work, although you may take a small hit on mileage. Richer means the motor will run cooler, but every bike is different so YMMV....

[natemoen]

i think they put 120's on the outside cuz 1 and 4 ran cooler and 115's on 2 and 3 cuz they were hotter. i think they stopped doing that. 110's or 120's i dont think it matters much. as long as the end result is the correct mixture.

Your thinking is backwards. 120's would run cooler because they are richer and the 115's would be hotter because they are leaner.

[mmatches1]

Maybe I will try using a 110 and 115 combination. Strange that the service manual specs would be different than the production specs.

Great info. Thanks.

[motoman]

110's in all 4 are stock for the H. At least it was for mine and I know that no PO had opened the carbies.

Correction Skids, 110 mains were stock in the 80-81Specials ONLY! The H(Venturer had 120 mains outer two cyl. and 115mains inner two cyls......from the factory, period. The 80-81 Specials and 80-81Standards also had different length/taper needles between the two models. The 110's were fitted in the Specials, NOT for emmission reasons, but because the needles were shorter with a quicker taper. Made that naked bike quicker on the bottom-end and lean and mean on the top-end. Made for a good bar-hopper, as it was designed to be.

[motoman]

For some reason my the carb for my 81 XS1100H had 110 main jets installed on all four carbs and the service manual calls for 115 on cylinders 1 & 4 and 120 on cylinders 2 & 3. I purchased the correct jet sizes and was going to install them but wanted to throw it out there before I did. Any one else face this? Maybe I have a different model of carb? The only identifying mark is the number 3U7 00 B081. The manual calls for the Mikuni BS34-111 3H5-01.

Easiest way to resolve that is pull the needles out, get ya a magnifying glass to read the letter/nubering at the top on the side of needle. 5GL16 for 80-81specials. Can't remember needle numbering on Standard 80-81 carbs as I'm running the Special carbs, which BTW have the stock 110's across. Also, if those main jets or any of the other jetting pilots, air etc. don't have the Mikuni 'swirly' logo, they are NOT Genuine Mikuni. Any jets other than Genuine Mikuni installed, you just opened up a tuning nightmare. Aftermarket jets may look, but are NOT the same. Hole sizing is different, even tho numbering may be the same. Also, back-side where needle drops to has a different degree tapering. Whether anyone knows it or not, Mikuni has ongoing patents, so none of these replacement pieces can be copied exactly without copyright infringements. FWIW, RD aftermarket jets come the closest to a match, but still not quite correct. K&L's are so far off that not even in the same book, let alone the same page. For example, a K&L 105 main is 'kinda' close size wise to a 112.5 Mikuni!......but backside taper is still so far off you still won't get good running results trying to play a cross reference game.

[motoman]

110's in all 4 are stock for the H. At least it was for mine and I know that no PO had opened the carbies.

FWIW Skids, 80-81 Specials factory mains were 110's across all four. 80-81Standards factory mains were 120's center two cyl........115's outer two. 80-81 Specials have different configuration needles than the 80-81Standards.

[crazy steve]

Jetting as per the Factory manual.....



110s in all four carbs....

[skids]

Correction Skids, 110 mains were stock in the 80-81Specials ONLY! The H(Venturer had 120 mains outer two cyl. and 115mains inner two cyls......from the factory, period. The 80-81 Specials and 80-81Standards also had different length/taper needles between the two models. The 110's were fitted in the Specials, NOT for emmission reasons, but because the needles were shorter with a quicker taper. Made that naked bike quicker on the bottom-end and lean and mean on the top-end. Made for a good bar-hopper, as it was designed to be.

I am convinced that what I found was factory. 110's on the 81h.

[motoman]

Jetting as per the Factory manual.....



110s in all four carbs....

Nice..........and no reference to the Standard 51 Z7 needles and 115/120main jets. Looks to be imcomplete without that reference. I can tell you the two needles ARE different, and the bike responds differently having had both the original set of Standard carbs that were on it from new to running the Special carbs that I've had on it for years. The Standard longer and longer taper needles allowed to to act like the lumbering tourer it is. The Special shorter, quicker taper needles allow ANY unintentional throttle movement to be felt. Initially was an annoyance out touring, but made it 'feel' more nimble than it really was at lower speeds. The different needle configurations played a part with the different main jetting differences of the two models. This all came to me, not from some FSM, but from the tech who put my bike together new and was specificly factory trained on XS11's. But, who knows, by then with the demise of the XS series on the horizon, and these bikes, specially new in 81 ones were getting invetory thrown on them that were not necessarily model specific. My Venturer is a perfect example of this being a 4RO system, but having slotted adjustable timing and gold-top carbs.......go figure. MMatches carb designation could be key to stock jetting, but IMO, I;d still pull the metering rods and confirm which ones are in the carbs.........way too many years and too much time not to know whether curious fingers were busy or not..........unless you bought bike new, have no way of knowing ABSOLUTELY for sure, no matter what you may have been told, or think.

[crazy steve]

Nope, not incomplete, just different. Here's the '80 specs...



The 'full' '78-81 manual just has supplements for the later models, basically only showing the differences from the earlier models. What it looks like is Yamaha simply went to the same carb set-up as the Specials on all models for the last year of XS production. Could yours have come with the '80 jetting? Why not; you've stated that yours has the slotted timing plate, could have gotten a 'leftover' set of '80 carbs too. What this shows is Yamaha was pretty sloppy about noting production changes, which could be an issue unless you carefully check.

Best bet is to identify which jet needles you have, then you'll know which set-up you actually have....

[mmatches1]

I have the manual with the supplementary section - 80-81.

It sounds like the stock 110 jets are the best option. However, I may check the needles anyway.