As you guys may know, for the GY6 platform, there are two carburetor types: The 18.5mm carburetor for the 139qmb (49cc), and the 24mm carburetor for the 157qmj (150cc).

These carburetors mostly function the same, apart from a few crucial differences. One of these differences is the atomizer, which is a long, flute-looking brass piece which holds the main jet. If you've ever swapped jets on a chinese carb, then you'll know that the main jet itself screws into the atomizer which screws into the carburetor. Behind the atomizer is a small brass needle guide.

Now, these atomizers are different because the atomizer for the 18.5mm carburetor only accepts up to a 98 main jet, while the atomizer for the 24mm carb only accepts 100 (lowest value) until 135.

What happened to me personally was that with a big bore kit, a pod filter and a very unrestricted exhaust, I was running an 18.5mm carb with a 98 main jet and I was still leaning out at open throttle. I didn't want to switch to a 24mm carb as that completely disintegrates your fuel efficiency (although it does give better performance). So what to do?

Well, I noticed that the atomizer for the 24mm carburetor actually DOES screw into the atomizer slot in the 18.5mm carburetor, but since it's considerably longer, it gets blocked by the needle guide. If you remove the needle guide, the bike won't run at all as the needle can't find its way into the atomizer.

Therefore, I realized that I could try shortening the 24mm atomizer down to the same size as the 18.5mm atomizer. I began by chopping off the end of the atomizer with pliers and then used a metal file to hone down the surface until it was about the same size as the 18.5mm carb atomizer.

The polished surface of the shortened atomizer didn't mate perfectly with the needle guide, so I wondered if there was going to be a problem. I tightened the atomizer in there (it didn't seat flush because there's some extra thread), and then tightened a 105 main jet in there. Reassembled, and....

The thing WORKS! It actually works beautifully, and acts exactly like a 105 main jet should. I can't detect any fuel leaks or issues at any point, and I stress-tested it for about 45 minutes by driving around and up various hills. Temperatures were much lower due to the bigger jet, and my bike wasn't overheating.

TL;DR, you can file down an atomizer from a 24mm carburetor to be roughly the same length as the stock 18.5mm atomizer if you want to use main jets higher than size 98 in your 18.5mm carburetor