Jetting question

[soulrydemike]

i bought a 79 xs11 special and had to replace the carbs. the stock carbs i replaced has 137.5 main jets and 42.5 pilots i believe. the replacement ones i bought ended up being from an 80 i believe due to the 110 mains in them. the pilots are supposed to be the same as the 79 set. i have straight pipes with custom super traps and pod filters, so i thought that could be the main reason i was running lean. so i put in 120 mains to see what happened and they performed grat. a few guys i was riding with said my exhaust smelled a bit rich and suggested toning down the jets a few notches. but toight i read that the stock jets on my bike were 137.5s. much higher than what i have in there now.

im not sure which way to head. because the stock ones were so much larger , should i try larger, or is each carb rack so unique that i should stick to tuning off the 110s. is so, after reading the rejetting thread, i'd have have to jump to 125s and my pilots should go up 3.

please help with any thoughts or suggestions.

[mro]

Significant differences between 79 and 80 carbs.
Float level affects all RPM ranges, 80's set to 23 mm.
If that's good then it's trial and error when you have pods and after market XShast.
Tech tips are a good starting point tho.


mro

[DGXSER]

Yep, they made a change between the 79 carbs and the 80 (later 80) models like yours. If you look at the 79 carbs, the pilot jet had a screw over the opening to get to it, correct? That screw is there because the design has a cross over tunnel to let fuel flow through the main jet to the pilot jet. Thus the large mains as it carries both jets fuel. On the later model carbs they did away with that crossover and let the pilot pull from the bowl. So the main size went down.

Any year carb will work on the bike, but you have to jet the carb based upon the carb and not the bike year. Then adjust for your mods.

[GARTHXS]

i agree

Hey mike he's right we have to find out what year those carbs are first then we will jet them let me know

[wingnut]

Like DGXER said its the difference between the 2 models of carbs. If your new carbs came with 110s my bet would be they are the latter version without the cross over tube. That being said with your mods the the 120s are probably in the ball park. Do a plug chop and see what that tells you. If rich you may have to drop down a size on the mains. You can't play with the needle position to go leaner as the new slides are not clipped, they are can only be shimmed up to go further rich. And no you can't put the old style slides in the later style bodies. Different size

[soulrydemike]

great. thanks for the good info. i just got 117.5s for the inners and 112.5s for the outers. hopefully this fixes the richness and i may shim the needle a bit if need be. thanks for the float level as well. ill make sure i dial that while i have the rack off as well. thanks.

[TopCatGr58]

Hey Mike,

You said you just GOT the 117.5 and 112.5 jets, so I'm assuming that you went to a local bike shop and picked some up? OR...did you just ORDER them? Wish you had come back and asked about the staggered jetting before getting/ordering them. The staggered was attempted by Yamaha early on, but found that it really wasn't necessary even though the books show them in the later years. Many have reported finding 110's straight across, and no changes in plug color/performance noted. The 117.5's should be about right.
I'm running them all across on my Indy Pods, 4-1 pipes, with 42.5 pilots.
T.C.

[Crazcnuk]

If these are 80-81 carbs the 120's should be fine, even a bit small with your mods.

Easiest way to tell, is: are the idles screws (top, engine side of the carb) sticking out like your 79 carb? If they are, instead, down in a 'well' then they are 80-81. (assuming they aren't off a Suzuki or Kawasaki! DAMHIK)

I would double check the pilot jets, I have found different sizes in there. They are supposed to be 42.5 for stock. I've found 40's and 45's in different sets of carbs. If you have 45's it will smell very rich at anything but wide open throttle.

I would suggest you check the needles and tubes as well. Both to make sure they are correct (type and setting) and to see if there is any wear. Wear on these will cause richness in the midrange.

Finally, check your float levels. They can be out a lot. As MRO said the stock setting is 23mm, and you can safely go 2mm lower to lean the entire range.

The mains are only fully open at wide open throttle, so changing them will not affect your cruise range much.

It's best to get them back to stock (with your 120s) then adjust from there.