Help me decide

DoubleDub wrote: Yes it does - and it would be cool if someone flow tested all of these and then provided the info...guess we better pool our funds for a flowbench... :whistle:

Can Larry C. do this on his head flowbench? If so, that would be amazing to do a heads up comparison between the different types of carbs!

I'm trying to wrap my head around this running CV's across the flowbench. The slide works in relationship to engine vacuum and a flow bench measures using the same vacuum numbers for each measurement soooooooooo, where's Larry to explain how he would do it without changing vacuum #'s on the bench.
I'd be interested in knowing myself. I bought an 81 KZ750 with CV's, replaced them with 28's and pumps, big improvement. Tried 26's and they seem to be fairly equal to the CV's in overall performance.

Les

Ok here is a bit of info as well the early 750's used Kehien CV's which were 34mm. These have a choke butterfly in the inlet as well as a throttle butterfly. The later 750's used Mikuni BS34's also 34mm carbs, but these use a fuel enrinchening curcuit as oposed to a choke butterfly.

Drastic diference in performance between them, the Mikunis provide much better flow/response.

I'd say the BS's would be in the same leauge as the 28mm pumpers that Less used on his. And better than the 26's.

29's are what I would like weather VM29's or CR29's....although the CRs are my preference.

BS's are what were on it and the 28's were better. May have just been what the bike liked. I know of the kehien carbs but have never had them.

Les

810cc's good flowing head. meant to rev.
My 1st choice on a budget. 28mm with pumps
1st choice no limit cr29.
best compromise. RS34's. There r afew guys here running them with 810cc bikes.

why did manual throttle control get put to pasture in favor of vacuum operated throttles slides??

the reason is because of the lag that is created when we open the throttle to fast. and the larger the carb the larger the lag, or bog. by using vacuum to operate the throttle vale, the motor will never bog. but it will never have great performance either. u can get this lag/bog with vm29 too, but it is more evident with vm33's. i have used both,and now use RS34s, on my 1200cc motor.


u would be surprised how well the flat slide perform. they were built to improve low end response.
as carbs get larger in diameter, low slide openings provide less vacuum signal than a small bore carb .
so they came up with the flat slide to help. the thickness of the slide is considerably less and this helps increase vacuum levels across the needle jet.the fuel is pulled up higher into the airstream and atomizes better.
i have some dyno and flowbench #s from my Mikuni tuning book. the #s are a comparison between 38mm round vs 38mm TM flat slides. these test were done on a Kawi 440cc Invader engine.
the book says the flat slides perform at least 10% more from .25% to 50% throttle on the flow bench. then taper off at the top. this leveling at the top is because both slides are out of the way and have little influence eat this point. (the newer TMX carbs boast a 40% improvement over the TM's.)
now the dyno #s.
there was a gain throughout the power band with a 5% max gain at higher rpm's with the flat slides. the gain at the top end is due to the improved atomization and the fact that a flat slide carb has a shoter body that a round slide carb. a shorter distance from fuel jet to intake valve, aids top end, and a longer distance favors bottom end performance.

mark, that is precisely why FCRs are so freakin' responsive. Can you say "light switch" ? :evil:

les holt wrote: I'm trying to wrap my head around this running CV's across the flowbench. The slide works in relationship to engine vacuum and a flow bench measures using the same vacuum numbers for each measurement soooooooooo, where's Larry to explain how he would do it without changing vacuum #'s on the bench.
I'd be interested in knowing myself. I bought an 81 KZ750 with CV's, replaced them with 28's and pumps, big improvement. Tried 26's and they seem to be fairly equal to the CV's in overall performance.

Les

I think I know what you're getting at. A basic W/O flow test with the CV slides held open manually(from the cap side) compared to a manual slide VM carb is what I was thinking.

That would be interesting to know how they flow with the slide held open but I'm like banshee, I'm not sure how often they are open all the way if ever. I woke up thinking about this.

Les

BlackZ1R wrote: mark, that is precisely why FCRs are so freakin' responsive. Can you say "light switch" ? :evil:

i have been looking at the fcr's in the past, but gave up on them , when i read they have no chokes.


The problem with cv carbs at wide open is the butterfly disturbs the flow, so u need a bigger carb to get the same flow as a smaller smooth-bore. there just doesnt seem to be anything good about vacuum operated carbs, form a performance standpoint .

Stock 26's will work. Not the ideal carburetor for maximum performance.

Stock 28's will work. Later model pumpers would be best.

CRS29's or VM29's will work and if jetted correctly will make it really pull with good street manners. Need to know how to tune them.

RS34's are entirely too big for the 650 port runners.

If looking at new carbs, call Sudco and price CRS29 to FCR29. Some flat slide FCR29's would be nice if the price is comparable.

Mikuni TMR carbs are interesting. I'm curious as to how a set of TMR32's would work on a 750. Setup with an 810 kit, mild port clean up, set of mild cams ( say GPz750 unitrack cams ) 4-1 etc.

This set is spaced at 70-90-70, part # TMR32-D6

I know CR29's would be cheap compared to these TMR's but if your going to spend around $800 for a set of carbs a few $$$ more for the TMR's dont seem out of line.

I just don't understand why the Keihins are $100 more than the Mikunis.