Long time listener, first time caller. Carb size question.

[noreaster]

Hi Everyone,

Firstly want to say thanks for all the wonderful posts and pictures. Think I have managed to make my way through a relatively tailor fit Primavera without bugging anyone with a question. I had a Rally 200 when I was younger that I sadly got rid of but remembered a lot from working on it. Build is listed below, I have ordered a Polini PWK 28mm and wondering if I went too large and would be better off with a 26mm. Thanks for the help in advance.

'70s ish Primavera (51mm Cruciform)

Polini 130 - Matched, ported and polished
Mazz Racecut ETS - Conversion Bearings and Seals
20mm Vape AC Static
Polini Banana
Polini One-Piece Manifold
27/69 Primary

As posted, wondering if I went to big on the carb. Reading other posts I saw 26, but also saw 30, figured I would split the difference.

This bike was purchased from a friend and sold as a "runner", after the discovery of a busted woodruff key on the primary gear, it turned into a might as well. The intention was a solid, mild tune, get around town with some highway for easy trips to the city. The banana and cylinder were already on the bike.

Thoughts? Suggestions?

[movin64]

I run a 26 on my 135 kitted v90 and it goes very well. Can't be more help than that, I'm afraid. Also run a 26 on my 187 bgm powered px and that also goes very well

[noreaster]

Jan 13, 2021 9:30:30 GMT movin64 said:

I run a 26 on my 135 kitted v90 and it goes very well. Can't be more help than that, I'm afraid. Also run a 26 on my 187 bgm powered px and that also goes very well

Thanks movin, my gut is certainly suggesting the 26. I imagine the effect of blowing through a straw compared to a piece of pipe, create far more current through that of a straw opposed to a larger diameter pipe. Should be able to pick up slack in either direction with jetting as well.

[noreaster]

Actually, what is your jetting and general elevation? I'll throw on main and slow jets to cut down on shipping.

[movin64]

I can't answer those questions, had the carbs set up on the local dyno, and I'm at work, so will get back with the answers later

[movin64]

First part of question, my elevation is 80 meters above sea level

[harleyracer59]

from my experience, a smaller diameter carb will give you better throttle response which can give you better off idle performance but has the tendency to give you a lower max rpm and the revs in the upper half of rpm arent as fast. the larger will sometimes feel lazier/ act slower to throttle changes but will flow more which can give you a higher max rpm and gets snappier in the upper half. so it sounds like you went down the middle road.
preference usually depends on riding style and usage. light to light a 26 should leave the line better. on a longer trip, the bigger carb should let you have a higher cruising speed or max speed with better rolling pick up at highway speeds.
i tend to like that middle road, same as you. my parmakit cylinder recommends a 34-40. i went with a 38. it still feels snappy off ide and should get a good max rpm when broken in.
while on these shifties, max rpm might not be as important as on my cvt transmission jap scooters, its still nice to have when its needed. when cruising at say 40-50mph, if i want to get around a car in traffic, when i twist the throttle, i want my scooter to accelerate quickly, not creep slowly to a higher speed.
but this is all just my view on the debate. im sure others will have different ideas on this. but i think thats why these cylinder manufacturers have a recommended carb size range and not just a recommended carb size...

[noreaster]

Thanks Harley, that is exactly where my I'm at with this. Ended up going with 26mm as this will be mostly used for light to light around town and short trips. A year or so from now if necessary, I can have a backup "touring" carb on the shelf should I plan a long trip.

Found an old Polini chart someone had posted on these forums a while back. Carb recommended was a 24, so still in the upper end of range. Seems the PWK has a good rev range as well, my ego was saying bigger carb, my gut was saying smaller. Plus, still running a virgin rotary pad, but will blend and polish the intake.

Using this calculator I found 4secondsflat.com/Carb_CFM_Calculator.html, the results were dead nuts 19-22mm required, (good job Piaggio!). But this doesn't take into account our modifications. Now onto jetting...

Nice Lectron!