I think you can make any carb work with enough fanagling. But here is my thoughts ... Think extremes.
A HUGE carb on a small piston. At idle you can make it run perfectly. Once you turn the throttle, you almost immediately move completely through the sweet spot mix of gas/air, and now you're going to be giving way to much of both to the piston.
A tiny carb on a HUGE piston. At idle the piston is not getting enough. As you give it throttle you move toward the sweet spot for idleing, and little more. You never even reach the sweet spot for acceleration / performance.
Lesson: You need to have the carb and piston balanced. Seems logical, right? Now where is it? You will get MANY people telling you it's from 32 to 40mm. Bing, Mikuni, Delorto ... Jet sizes, idle, main, needle ... the skys the limit. Seems very daunting. Don't let it get to you. Ask yourself this question:
1. What do I want out of the engine? Performance, Racing, Reliability, Gas milage? What?
For performance top end, or low grunt? Racing distance or sprint? Reliability?
I have a R100 engine that has huge low end. Mikuni 32's, 18" rear wheel, 37/11 drive. I had an R100 that had a better top end, but the balance of the engine wasn't as good ... 32/11 final drive, 18" rear wheel, and Mikuni 34's. For milage obviously a miserly approach is best ... but where's the fun in that?
For lower end city riding, I would go with the 32's. For top end freeway I would go with the 34's or 36's. But in the big picture, I really don't think it matters. We're talking about smaller differences between incredibly variable attributes. Why, the whole formula goes out the window if you go from sea level into the mountains.
For me, the 34's are a bit more rare in the world of motorcycles. The 32's were on MANY of the Japanese bikes, therefore you can get cheaper. WFO on the Dobber will lift the front wheel and have you praying for a better grip on the handlbars. I can cruise at 80mph ... and there is plenty more umph there ... but that's enough for me.
In any case ... get what you want, and we'll titrate it work great. Think of it this way. BMW put the 32's stock on R75s through R100's. Not that they did everything right, they were just able to titrate the jets to make it work well. We can do the same with yours.
Originally Posted by blaine.hale
That brings up a good thought. Am I lazy enough to buy the pre-jetted 34 kit from this Amazon purchase or should hunt down some 32mm kit somewhere. Do you think it would be a big noticeable difference on the r90? I've seen a lot of people run the 34's on r75s.