What is the best carburetor for a Lambretta?

OK, you changed the exhaust on your stock Lambretta, or maybe not, but in either case are thinking of a better carb. What are your options?

There are three basic types. From worst to best: old-tech original style, roundsldes, and flatslides/D-slides:

Old-school

Jetex or Spaco 22mm Indian copy of the original design from 300 years ago. Variable quality on some, so open and check before installation as often the jets are falling out, or their bases cracked. Fairly cheap, though not really considering what it is, but if it works fine, it'll be better than what you have. There is also a 26mm.

But if you are buying a new carb, you might as well update to a better design. As one person wrote, “Do yourself a HUGE favor. Dump the Jetex and get a Mikuni or Keihin. Those Jetex carbs will give you nothing but trouble. I fought with mine for close to a year before I got a Keihin pattern from scootRS and the difference is amazing. It's soooo much smoother, idles like a dream, never a hiccup.”

Roundslides

Mikuni and Keihin (Japan), or Dell'Orto (Italy) up to 28mm. Roundslides are a step up in technology from the original-style carbs, giving better throttle response and tuning capability (i.e., more things to tinker with if you want it just perfect). OK option. Japanese carbs have the best reputation but, depending on your location, people might have more experience with Dell'Orto, especially in the UK, though dirtbike shops anywhere are a good source for parts and tuning of Japanese carbs. We have a pattern Mikuni roundslide but nonetheless recommend...

Flatslides and D-slides

Again, all three companies offer carbs ranging from 24 to 35mm and up. Flatslides give better throttle response than roundslides because of the design of the slide, as well as incorporating other new design features. D-slides improve upon flatslides, reducing air turbulence in the carb, and are the best. Keihin PWK D-slides are great carbs, better than overpriced, inferior Dell'Orto VHSA/B, and, some say easier to tune than Mikuni, though others like the TMX system. (Read the Scootering showdown for the lowdown on PWK vs. Dell'Orto.)

These carbs are quite expensive, or we at scootRS have pattern PWK cheap. We'd recommend the 24mm on 150/175, and the 28/30mm ovalbore PWK-type flatslide on a 200cc and most setups with a tuned exhaust. It's a very good setup.

Some feedback on our PWK:

  • I run the 24mm scootRS [Keihin PWK] copy and am very pleased with it. The pick up on them is instant and it was very easy to set up. Would certainly recomend them to anyone.
  • I love my PWK 28/30 from scootRS. It works awesome on my Lambretta with GP200 engine.
  • I've built 2 TS1s. The first with a Mikuni - it took a while to set up but then it was golden. The 2nd time I used a scootRS Keihin - much cheaper and easier to set up. The next Lammy will get a Keihin as well.
  • Do yourself a HUGE favor. Those Jetex carbs will give you nothing but trouble. I fought with mine for close to a year before I got a Keihin pattern from scootRS and the difference is amazing. It's soooo much smoother, idles like a dream, never a hiccup.
  • While in Australia this year I saw a back to back dyno test comparing Milkuni and Keihin 35mm carbs on a 250cc two stroke cart engine. The difference was 2hp, not a lot but enough to knock half a second off a lap. The most amazing thing was the way you could slow down the cart into a corner using engine braking and then floor the throttle with out the usual Amal/Dellorto gurgling. This is purely down to better air flow through the venturi.

Extras

You need a new manifold with all of the above carbs, so factor each manifold's cost into the price. (We sell the 22mm manifolds for Jetex/Spaco types, and include manifolds in our carb kits; not sure who sells Dell'Orto ones, or what all mixes and matches.)

You also need to remove the baffle in the air scoop under the seat to allow more airflow, and should cut the internal baffle out of the airbox entirely via the neck, and even drill 8 or large holes in the side of the airbox. Or just use a our foam filter on the end of a GP 22mm carb rubber. For a TS1 with a panel hole cut, you can attach the filter directly.

Have fun!