How big dilemma

[jetsteve2011]

I’m currently running a vertical Minarelli with 70cc, big intake, 21mm polini cp, stage6 R1400, and Malossi Overrange etc..
I’ve purchased the largest cylinder as far as I can find for the vertical a Parmakit 50mm bore and 43mm stroke. Resulting in an 84.43cc. This zuma is going to be just a “race” / for fun vehicle. How large should I go on carb? Would the 21mm be ok or should I get a 28-32mm.

In hindsight I kinda wish I’d just gone to a horizontal conversion and had even bigger options but oh well.

[repherence2]

Size of carb will depend on what kind of performance you want. I have used a Polini CP24, OKO clone 26 with power jet, and an OKO 28mm with a venturi diffuser.

CP24 was able to run fairly well. It's pretty good on gas efficiency. Good in the low-mid range.

OKO 26 power jet was a headache to learn and understand. However, once I got it tuned, it worked for me. The power jet kicks in just before full throttle. I normally tune for WOT on non power jet carbs. Your main on a power jet carb is going to be a smaller # than on one without power jet because the power jet kicks in near full throttle. Some people block off the power jet feed line with a small BB to disable it.

OKO 28 works well on my 54mm horizontal motor. Wide open throttle pulled harder than the cp24 and the 26 power jet. It sucks a lot of gas though. Low-mid was a pain to tune to get it to run good on low throttle settings. I made my own venturi diffuser (copied Rich's Taylor's Porting) and installed it. It actually helped low-mid throttle performance.

Carb choice depends on how you intend to ride. I like to go ripping WOT a lot so I prefer the 28mm carb.