Can I upgrade my Mikuni BS32SS (32mm) carbs to BS34 carbs?

[Anonymous]

This is for a 1980 GS850L.

Could I replace the stock Mikuni BS32SS carbs with Mikuni BS34 carbs?

When I was on a Yamaha 650 list, it seemed like someone was swapping out the stock carbs for something bigger. Can that be done, and if so, can the above proposed swap be done? And lastly, if so, what other mods would need to be done?

I assume I'll need new carb mounts. Also, will I need to change the jets and/or other internals in the BS34?

Thanks for the replies in advance.

[earlfor]

Re: Can I upgrade my Mikuni BS32SS (32mm) carbs to BS34 carb

The airbox/filter, carbs and exhaust header/mufflers are a matched system. Unless you intend to replace the airbox with filters with a greater air intake capacity and then replace the exhaust system with something with less backpressure, increasing carb size will accomplish nothing except to decrease gas milage and reduce performance. I dont see any point to doing this. With the air intake and exhaust system you have, the manifold vacuum/throughflow is correct with the carb cross section of the smaller carb. Bigger carb means you need more volume to maintain the same pressure and velocity. Pressure and velocity work together with jet diameter to provide the correct fuel air mixture. You will be much better off not going there. :-)

Earl

This is for a 1980 GS850L.

Could I replace the stock Mikuni BS32SS carbs with Mikuni BS34 carbs?

When I was on a Yamaha 650 list, it seemed like someone was swapping out the stock carbs for something bigger. Can that be done, and if so, can the above proposed swap be done? And lastly, if so, what other mods would need to be done?

I assume I'll need new carb mounts. Also, will I need to change the jets and/or other internals in the BS34?

Thanks for the replies in advance.

[Billy Ricks]

I doubt airbox volume would be a problem since displacement isn't changing. But as Earl pointed out you are going to lose mixture velocity. That's more important than volume.

[Billy Ricks]

I've given this a bit more thought. As long as the intake ports aren't enlarged intake velocity should stay about the same regardless of carb size. That said, I doubt you will notice much difference in performance by going to BS 34's vs. BS 32's. You could however pick up a set of Slingshot carbs from the air cooled GSXR's. They have flatslides rather than round slides. People have gotten good results using them.

[lhanscom]

Wouldn't going bigger with the carbs, but keeping the same size ports increase the venturi effect and increase velocity? If not, when do you increase carb size? Billy, you're running RS34's, with huge performance increases, right? You currently don't have anything too drastic done to the engine, just pods and a pipe, right? So wouldn't increasing the carb size to BS34's be an increase in performance if combined with pods, and a pipe, at least in the higher rpms. Granted this would take away from the low end. Also, would the CV's work right, or would they be all screwed up, I would think the effect on the CV's would be the same flat or round slide.

[Billy Ricks]

The flatslides result in a shorter intake tract and less turbulence. Running Slingshots has been done by quite a few people. Scott Horner from Heads Up Performance has long recommended this upgrade. Scott used to post here years ago when GSR was in its first iteration. I just don't feel going up 2mm with the same model carb is worth the time or effort. If he's going to bother Slingshots are the way to go unless he wants to spring for RS flatslides and a push/pull throttle.

I had the ports matched to the manifolds and some mild porting done when I had the head done. Bigger carbs do kill the bottom end a bit, but the topend rush is worth it. His money would be better spent on something other than BS series carbs.

[lhanscom]

I agree, you can get the cv flatslides for about the same money, so you might as do that if you're going to go thru all the trouble jetting them.

Thanks Billy for the info.

[earlfor]

If no changes are made to the bike other than the addition of larger carbs, then the intake volume is unchanged and the cylinders will process the same amount of air. The intake tract as it enters the head will be the same size, so velocity at that point will be the same. This is downstream of the carb and venturi effect. A larger carb has a larger cross section and area. Velocity through the carb must decrease. Once the mixture is past the carb, velocity must increase do to a decreasing intake diameter. The biggest drawback is the disruption of a uniform intake air flow and a decrease in fuel drawing ability. Resultantly, I would expect decreased gas milage, poorer performance and a rougher running engine.

Earl

I've given this a bit more thought. As long as the intake ports aren't enlarged intake velocity sghould stay about the same regardless of carb size. That said, I doubt you will notice much difference in performance by going to BS 34's ve. BS 32's. You could however pick up a set of Slingshot carbs from the air cooled GSXR's. They have flatslides rather than round slides. People have gotten good results using them.

[Billy Ricks]

Once the mixture is past the carb, velocity must increase do to a decreasing intake diameter. The biggest drawback is the disruption of a uniform intake air flow and a decrease in fuel drawing ability. Resultantly, I would expect decreased gas milage, poorer performance and a rougher running engine.

Possibly, but drag race tuners build up the floors of the intake tract with epoxy to squeeze the mixture for even more velocity. You definitely lose bottom end with bigger carbs. Just depends on where you want to make power.

[earlfor]

I absolutely agree with that Billy. :-) My statements only apply if we are adhering to the original qualifications. Bigger carbs and NO other modifications.

Earl

Possibly, but drag race tuners build up the floors of the intake tract with epoxy to squeeze the mixture for even more velocity. You definitely lose bottom end with bigger carbs. Just depends on where you want to make power.