Scooter Carburetor Jetting Guide

Due to the amount of threads being made about "my bike bogs out at top end" or "i just got a new exhaust and its running bad!", i have decided to write a generalised guide to jetting, it is not exact, and so i can accept no responsibility for its use

MODIFICATIONS

Different modifcations need different upjets, so here is a general rule.

Note - where applicable i have stated a 50cc and a 125cc option, bore kit and carb for example.

70cc/172cc Kit - 8-12% upjet (depending on kit, some need over 12%)
Exhaust - 5-8% upjet (depends on exhaust, some have been known to need 15%!)
Open Filter - 5-10% upjet (depends on setup)
21mm/25mm carb -4-6% upjet (depends on setup)

HOW TO

How to upjet is a commonly asked question, not just how to physically change the jet, but how to go about making sure the jetting is correct.

To change the jet you need to drain the float bowl of the carb, undo the float bowl and simply swap the jet for a larger one, then refit, suck the vaccuum pipe to refil the float bowl, and kick till it starts.

To make sure the jetting is correct you need to listen to the bike (this takes some practice), if it is running lean it will make a flatter noise (barrel noise, not exhaust) if it is running rich it will make a duller noise.

Another way to check is the do a plug check or plug chop, which is where you run the bike at full throttle and quickly shut the bike off and check the colour of the spark plug, white is lean, black is rich, and brown is goooood.
note - some bikes have automatic chokes, which will need to have opened before a plug chop can be done.

WHERE TO START

A general rule of where to start on jetting is to overjet, so to make the bike run rich, and work down until its just right, running rich is much better then running lean, as a lean mixture causes more heat and so more risk of seizure.

DIAGNOSIS

As i have said before you will tell the jetting by either the sound, or the colour of the plug, but you can also tell by how the bike reacts to different levels of throttle. if you are just changing the main jet then the only level of throttle that should change is past half throttle. if it is running lean it will still rev but it wont have any power behind it, whereas if it is rich it will not rev at all and will bog down and in extreme cases will cut out.

FINAL NOTES

Some people have trouble understanding the use of percentages when explaining jetting so here is a quick example using the higher of the suggested upjet.

Standard Jet - 60
70cc Kit Added - 12%
New Jet - 67

and so on and so forth...

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People often confuse jetting issues with rollerweight issues, so remember rollers/clutch springs need to be changed. As a general rule...

exhaust - drop a gram on rollers, stiffer on springs
bigger carb - half a gram off rollers, stiffer on springs

good luck, any questions feel free to ask

 

excellent guide

 

thats a really good guide mate

 

glad someones made it easy for people to understand. Brill m8.

 

this should go in the guides section of scootershack if you ask me.

 

nice one swift, your 70cc guide was amazing and this is too. I know this will come in helpful to alot of people, including me wen i sort out the mbk stunt.
nice one! :tup:

 

nice one nicely explained

Other place to learn i bit more is in this site:
http://justkdx.dirtrider.net/printcarbtuning.html

the object of study is a Keihin PWK, but the basics is the same to all carbs


cheers everyone

 

yehh totally agree with minor, you gotta remember that this is a BASIC guide

jetting is never exact science, but hopefully this helps you understand a bit more, and then you can read the other guide on the main page and get a better understanding of what upjetting actually does

 

Hopefully you can manipulate this picture and be able to read it.