Modern Vespa

I'm upgrading the GT200 with a mellosi kit to get her up to 225cc and the kit is almost complete.

Looking around in it though, I don't see a replacement for the carburetor or its jets. Bigger engine is going to need more gas, won't it? Should I just get a 250cc carb kit and swap the jets or is there more involved? Should I look into a complete swap? Am I wasting time and money again? YOU be the judge!

Thanks in advance for reading.

When I upped the LX150 to 198cc, I didn't touch the carb. My thinking (happy to be corrected if wrong) is that the larger volume of air being sucked in leads to a higher vacuum leading to more gas being sucked up through the main jet. So mostly self regulating! No doubt the original jets are chosen for emissions etc, so may not be absolutely optimal for performance, but if I had similarly modified my GT200 I'd start by leaving the carb alone. Once you're happy the bike's running well, then's the time to try other tinkering.

I agree with Jim..while the total volume increased, the air/fuel ratios should be the same....theres a slim chance you might have to go up one size on the main jet...but i highly doubt it...just try it as is and see how it rides...if it bogs on wide open throttle let us know......just be mindful that it does take a few miles (maybe 300) for the top end to fully seat...

There's some mixed information regarding the efficacy of upping the stock jet (92) to a 95 on a stock GT200 setup on here and while I entertained the option of doing so, I decided to leave it alone as everything was running fine.

In doing my research I did see that ScooterWest sells jets in various sizes for the stock Keihin CVK carb (though if you have the older Walbro carb these may not work): https://www.scooterwest.com/main-jet-for-keihin-carburetors-vespa-et4-lx150-s150-gt200-mjk60.html

Read this thread where the OP couldn't get his kitted GT200 to impress him until he up-jetted.

GT200 with Malossi 218, 4v, cam, lv4road. No change?

How woudl I know which one I have? Its a stock carb with the euro modification (SHHH dont tell!) on it.

on the side it will say Keihin or Walbro. If it says Walbro, it's a Walbro carb. If it doesn't, it's the other one.

In theory, a larger engine needs more air. Which actually calls for a larger carb, not just more gas. In practice, unless the carb size was marginal to begin with, it probably has enough capacity to cover. Moving to a larger carb opens a whole new can of worms. Unless you have a dyno, and, preferably, a tailpipe fuel-air ratio meter, best not to open it.

Jetting is complicated, especially if you are messing with the cam, which can change the vacuum 'signal' to the carb. A 'hotter' cam often generates less manifold vacuum, needing a much larger jet.