Modern Vespa

Every time I think I've solved my high rev'ing problem on my 74 primavera it rears its ugly head.
I hate the stock collar manifold system (which I believe is the source of my air leak - yes I have done leakdown test). Is there an alternative way to secure the carb or is there a better option available to buy? The scoot of totally stock.

Polini CP21 with clamp on mount... does away with felt ring. Comes with a pull up flip choke, but you can get a cable fitting if you want to use the stock smalie cable choke. Nice carbs. sdjohn loves his. I have a CP24 on my PV.

https://www.treatland.tv/polini-CP-21mm-carburetor-with-pull-choke-p/polini-cp-201.2102.htm

Have tou cleaned the jets?
Are there ANY engine mods? Have you tried a bigger main jet?

Only mod is 12v electronic ignition. Would clogged slow and main jet cause high revs??
Thought I finally licked this problem until today going to work. Wouldn't stay running so I cleaned idle jet then on ride home revs went out of control.

Getting gas somehow apparently, but how's it getting all that extra air?

I'll assume that you know about the felt ring. There is some controversy over exactly where it goes in the stackup. I'm in the camp that says it goes on just before the carb does, NOT behind the aluminum sleeve. Where did you put yours? Like this so the carb butts tight against it?


Not a bad idea to fill the slots in both sleeve & carb w/ some silicone goop for extra insurance. Be sure you push the carb back tight before you tighten the clamp. I lever it back firmly w/ a length of wood against the choke bracket.

Try to get the original system working right first - A bigger Polini or Delly PHBL does make a nice upgrade but you give up the aluminum sleeve and rubber bellows that's designed to keep road dirt out of your carb area and air supply. That's when it's all working right - I run a Malossi intake w/ no sleeve for a PHBL & no bellows but a good air filter.

If you do go bigger carb, you MUST get the jetting right or risk possible engine damage. If you go that way, consider getting a more efficient exhaust to better use the bigger carb, but do it all at once so you don't need to do the jetting ritual twice.

I could never get that collar and carb to seal. As socal mentioned, I love my CP21.

Polini CP21 on a smallie - making a choke cable

was 130 DR loss of power - now CP21 jetting

those will keep you busy if you are interested in going for it.

Voodoo,
Yes I have the felt ring where you have it and it's good and greased up. Have also rvt'd the slits.
During my hunt for the phantom air leak I shot some carb cleaner at the collar (while it was running). Engine immediately died. Carb seems to be on there good and tight.
Anyone ever experimented using two greased up felt rings? Or smearing silicone around the collar and carb (where it meets)?

So Polini doesn't make a 19/19 carb? The only way to make it work is to go up in size? Is that the only option?

Jay, you've taken your carb on and off so many times that the manifold and/or sleeve might be slightly bent. I'd drop the engine and check to make sure that all of that is straight. Before I replaced the manifold and sleeve, I had a lot of problems getting the carb to fit correctly.

Did you happen to see this posted on Scootnet a couple of weeks back? Looks like they are made to connect directly to the existing manifold.

with the plastic bushing, it does exactly that, bolts direct to the stock primavera manifold and never leaks. It's socalguy selling that one on scoot.net. they're good carbs.