Tunning My XS650 SH with 750 BBK

BowlingSS

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I have an 81SH with BS34's with stock jetting. I have two into one exhaust and foam filters. What jets does every use? I have only run it a few miles. It runs great after about 1/8 throttle. It is stumbling at low throttle. I turned the air screw from 3 1/2 to about 2 and it seems to be a little better. I know I need to set the main sets first. I was just trying to get a starting point. Most of the ones I work on are stock. This is my first BBK. My floats are set at 22 I think, not sure on this one. Anyone have this combo?
Thanks.
Bill
 

teamWicked

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I would look at 5twins carb chart, add a size or two to the main but get a size richer and a size leaner (they are cheap) and work from there. I always wondered about jetting, I think I should jump 1 size in one carb and 3 sizes in the other and compare plugs...
 

grizld1

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2-1 exhaust and pod filters will impact jetting more than the 100 cc. displacement increase. You can tune from the top down or from the bottom up. Here are some starting points if you want to let your motor tell you what it wants instead of looking for recipes that may or may not be optimal for your setup.

First off: Go to www.amckayltd.com/carbguide.pdf and read it, especially 5 twins' section on tuning for modifications.

Next: Don't guess or take anything for granted. Check your float level and make sure it's correct for the floats you have in place. Specs are in the above.

PJ: Excessively elevated idle when warm (more than around 250 rpm from first steady idle to full warmup), rough off-idle transition, and exhaust popping under engine braking are all lean symptoms. If these can't be cured with a mix screw setting inside 3-1/2 turns (3 turns for BS38), increase PJ size by 1 step and fine tune with the pilot air jet in the intake bell (larger=leaner). 1 step is usually all it takes, but never assume the usual. MJ: In 2nd gear, get rolling till the engine turns at least 4K rpm (to be sure it's in the range where slide lift occurs) and open the throttle all the way--roll it open quickly, but don't snap it open. If the motor misfires, increase MJ size1 step and go again. For fine tuning, go to the steepest grade you can find and get the motor to 5K in 5th with the throttle wide open. Roll off quickly to 3/4 throttle. If the motor gains power, it's a lean symptom. If it blubbers softly, it's a rich symptom. Midrange: Increasing MJ size can result in a rich midrange mixture. Refer to 5twins' fixes for this condition.

The biggest challenge in tuning older carbs is poor isolation of component effects. Everything affects everything else, sometimes very strongly. If you have to install an adjustable needle and needle jet kit, be aware that a single change in needle position will force a change in mixture screw setting and can often force a change in PJ selection. You may find yourself running in circles of correction and compensation. That's normal; the circles will tighten.
 
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5twins

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The BS34's also have an air jet associated with the pilot circuit that you can change. It can be used to fine tune that circuit as it doesn't have as big an effect as changing the actual pilot jet does. In the States, on our E.P.A. mandated BS34's, we got 135 air jets. The rest of the world got 130's. So, reducing the air jet a size or two will richen the idle circuit a little, but not as much as going up another size on the pilots. On my '83 with the typical "pods and pipes" mods, I'm one up on the pilots, two up on the mains, and one down on the air jets. On a stock bike with the BS34 carbs, the mix screws like 3 to 3.5 turns out. With my richer pilot circuit (larger pilot jet, smaller air jet), I'm able to set them at about 2.5 turns out.

And speaking of mains, with your mods I think another size up might be better (to 137.5's). Have you tried it?
 

BowlingSS

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The BS34's also have an air jet associated with the pilot circuit that you can change. It can be used to fine tune that circuit as it doesn't have as big an effect as changing the actual pilot jet does. In the States, on our E.P.A. mandated BS34's, we got 135 air jets. The rest of the world got 130's. So, reducing the air jet a size or two will richen the idle circuit a little, but not as much as going up another size on the pilots. On my '83 with the typical "pods and pipes" mods, I'm one up on the pilots, two up on the mains, and one down on the air jets. On a stock bike with the BS34 carbs, the mix screws like 3 to 3.5 turns out. With my richer pilot circuit (larger pilot jet, smaller air jet), I'm able to set them at about 2.5 turns out.

And speaking of mains, with your mods I think another size up might be better (to 137.5's). Have you tried it?
I was wondering about the air Jets. I think I will change them to 130's. I have not tried the 137.5's yet. I am still fine tuning the carbs.
Bill
 

5twins

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I tried 130 air jets on mine and they made things a bit too rich (the plugs got black). The air jet is a Mikuni small round main, just used to flow air instead of fuel here. It's the same main that's used in the BS38 carbs so you probably have some laying around.
 
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