2-cycle fixed ratio, why?

ever since i started getting into mopeds, i've been wondering why there is no manual mixture adjustment. why use fixed jets instead?

i can only think of three reasons:

1) keep it simple and cheap

2) keep the users' hands off as much as possible

3) prevent drifting of the mixture screw or just prevent improper mixture setting

i guess the last reason is good enough, but i just wondered if there was something more.

it's kind of annoying to have to buy new jets for different configurations. guess i'm just used to 4-cycles :o


Re: 2-cycle fixed ratio, why?

InfectedBootSector /

I think the big thing here would be expense. I'm sure someone could come up with a device such as this to go along with an oil injection system, but it would be a lot more than a 6$ jet. It would also be more apt to damage and/or repair. Neat idea though. Kinda like fuel injection for cars, but instead, oild injection for mopeds... hmmm ... I wonder if I can come up with something....

nothing but the carbs

hmmm... i forgot to mention that i was specifically talking about the carbuerator and the air/fuel mixture not an oil injection adjustment or something.

like the basic tecumseh 4-cycle carb, all it has is a screw that goes into the jet. turning it in or out adjusts the mixture. so it wouldn't be too expensive, just needs a needle screw, matching jet, and threads to attach the screw at the bottom of the bowl.


Re: nothing but the carbs

InfectedBootSector /

Oh, I see! That would be something fairly simply to implement I guess...

Re: nothing but the carbs

Steeltoad /

The amal carb that comes with most speed kits has a mixture screw.

Re: 2-cycle fixed ratio, why?

The "fixed" jet is the simplest way to provide the "fixed" top speed that most if not all states require to be a "moped"

Re: 2-cycle fixed ratio, why?

Ron Brown /

Let's try to get this right.

A larger motorcycle carb is just like a ped carb with 3 possible exceptions. The ped carb has no IDLE mixture screw, no replaceable idle jet and no metering needle on the slide.

The lack of an idle mixture screw, as near as I can tell, is to save money. It may also be to improve reliability and prevent tinkering. The idle mixture is controlled by precision drilled orifices (jets) in the carb body (except I think my Batavus carb uses a groove cast into the bottom of the slide that sucks fuel from the main jet at idle). Keep this in mind when cleaning the carb by poking wires through holes.

The lack of a metering needle on the slide is, I am sure, just a case of saving money. I have a problem getting my mind around how this works without severe mid-range mixture problems and flat spots. My tentative conclusion is that a 50cc ped is operated most of the time at close to full throttle, otherwise you are not going anywhere, so a perfect mixture at mid-range is not a problem. The changeable main jet is provided to allow for changes in operating environment like altitude and gas quality. This is typically set once by the dealer then left alone. Imagine the guarantee problems of having a screw on the carb that cause the engine to sieze up and self destruct if you set it to where the engine produced the most power. This is a feature of two strokes, that they make the most power when running just too lean to survive.

Another consideration is that a ped carb is made in large quantities for a specific application. A motorcycle carb is often used on several models with different requirements and jet, needle and slide changes allow the same carb body to be used.

Lawn mower carbs are designed to do two things, idle (so they have an idle mixture screw) and run at working rpm (so they have a high speed mixture screw). Running a mower at intermediate speeds is ineficient and not recommended by the manufacturer. This allows the same carb to be used in different applications by just turning the screws. Allthough it seems to me that the newer small engines with mostly plastic carbs are eliminating some adjustments.

I hope this helps,



thanks for the info everyone.

i figured having a mixture adjustment on a 2-cycle could be bad, but adjustments are still ooo so much fun :)


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