jet size help

[Tomek]

I have 1981 Yamaha XS1100 SH, with 4-1 black pipes. Everything else is stock......what jets should I use?....

[TopCatGr58]

What Jets.

Hey there Tomek,

I've got the same bike/engine, but I'm also running Indy filters. The jetting guide says 2 sizes up for both Indy and another 2 for 4-1 pipes, but subtract 1 with both mods, so I went 3 from 110 to 117.5.

So, 2 sizes should put you at 115's. But I ran mine with 4-1 pipes for many years with stock 110 jets with no ill effects and no loss of power or burning up the pistons!!!! But that was before I found this site and resource of info, didn't check my plugs for running condition. Are your plugs pristine white!!! If so, then you may actually be running lean, but if they have some tan color to them, then you may be alright!!!

I was also running mostly at sea level where the air is denser, and in fact I should have needed the higher jets. If you are at a considerable higher altitude, ie. 6000 feet, then your air is thinner, and your jets are possibly rich enough! HTH!

[graybird78]

jets

Hello. Tomek was over to see my bike. He has some definite issues as far as carbs go .carbs have been tinkered with before. mixture screws have been desealed.we did take a look to see what was up.his bike will hardly exelerate from idle to about 2500 rpm.then it takes off. the same when bike is in neutral. if you give it full choke will excel much better. also sometimes idle wiil hang high. His choke won't work unless you hold it out manually . He does have a 4-1 exhaust, with no other mods evident. Told him to check main jet size. If stock would reccomend going up 2 sizes, cuz of header. not a carb guru on bikes but it acts like he has bad exelerator pump.you have to feather throttle until 2500 rpms. forgot his octopus is gone. his bike is a 81 sh.

[TopCatGr58]

Hey there Graybird and Tomek,

First of all, these carbs don't have an "Accelerator" pump!!! They are vacuum fed and controlled by the action of the vacuum, the diaphragm slides, and the jets. It first sounds like he needs to pull the carbs and give them a good cleaning, especially the idle circuit, as well as the slides. Make sure the idle screw tips aren't broken off, and reset them at least to the stock 1+1/4 turns out, but probably more like 2 to 2+1/2. At least do a decent eyeball/breadbag wire tie synch. With it not kicking in till 2500rpm, could be many things, sticking slides, gummed up idle circuit, clogged air filter, sticking advance plate!?

He will also need to look at his pickup coil wires, the vacuum advance unit, ensure it moves smoothly, vacuum unit works when vacuum applied, hose with no holes in it, and attached to the carb tube, not the intake boot! etc.!! His exhaust pipes should be the least of his worries or concerns. Get the carbs and ignition right, and he probably won't have any trouble with the pipes!
T.C.

[graybird78]

tomek

t.c.
Ididn't mean it had accelerator pumps , but that is the symptom he is having. vac adv hose is hooked up correctly.vac advance unit is stiff ,but does work when vac applied to it. pickup coil wires still o.e never ben fixed.did discuss with him all the fixes he needs to pay att to .also told him web site is full of same problems ,just needs to do searches. sorry for the acellerator pump misunderstanding .just trying to describe the symptom .