carburetor size and jetting

First post. I know nothing about engines, thanks for nothin'.

Can someone give me a rundown on what the carburetor and jet sizes are all about?

I assume that larger means more flow of fuel. But what about intake/output and are there different kinds of jets? Whats a 15/12 carb all about? Whats going on with jeting up or down according to the weather?

I am looking to upgrade my 1990 tomos bullet a3n(?) and I'm not sure where to start with the carb.

While I'm at it is an a3 pretty much the same as an a35? It seems like a lot of parts are listed for as compatible for both.

Re: carburetor size and jetting

Carburetors mix the fuel with the air at a very precise ratio. If your ratio is off, your moped will run like crap and/or seize. A "rich" mixture has more fuel than it should, and a "lean" mixture has more air than it should. (Lean mixtures can lead to seizure, so it's better to be rich if you can't be dead-on).

The size of the main jet determines how much fuel is added to the fuel/air mixture at full or mostly full throttle. When it gets colder outside, (seriously, don't quote me on this - I know I've heard discussions about this, but for some reason I still feel like I'm pulling it out of my ass), the air is denser. If the air is denser, more of it can get into your carb. More air = leaner mixture = you need to put a bigger jet in your carb so you'll have more fuel and your fuel/air ratio will be happy again.

If you've got a 1990 Tomos, I'm pretty sure you've got a Dellorto SHA 14.12 carb. I don't remember exactly what dimensions the numbers refer to, but I believe there's a description in the wiki in the Dellorto break>

If you want to upgrade your moped, don't start with the carb. Get a performance exhaust like this one or this one. That should be easier to install and tune than a new carb, and it should also get you more bang for your buck. Don't forget to upjet though. (And before you ask what size to buy, here's your answer: You'll need a few sizes larger than what you have now. That means you'll have to take your carb apart and check the number stamped into the jet. If you have, say, a 54, then I'd get 55, 56, 57, and maybe 58. Start with the biggest jet and run test the moped by running plug chops and downjetting until you've determined that you have the right jet.)

Assuming I'm remembering things right (aka the A3N is the same cylinder as the A3), you've got an wee little intake port and a matching wee little intake. The bigger carb isn't going to do as much for you as you'd like unless you rectify the wee little hole situation (and I don't think you want to mess with that right now).

One major difference between the A3 and A35 is that the A35 has reed valves and the A3 is piston port. Other than that, maybe differences in .. the crankshaft? Electrical system? I dunno really. You can check the wiki, search the forums, or hope that someone who knows more than me sees this and tells you.

Re: carburetor size and jetting

I was hoping lynda could answer this one too. I have a Puch Magnum 1 speed and I want to upgrade my Carb system with a puch dellorto SHA 15mm carburetor that I found on Treats. On the treats website though it says that with magnum engines, the cylinder doesn't have an exact fit. Is this something I should avoid buying since I too am new to this. I have a couple of friends who know what they are doing who will help, but I'd like to know my self. And maybe I start with the exhaust system too? Any help would be appreciated,


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