Carburettor Tuning Guide

[Runna]

Came accross this on the net and thought id post it on here as it is good advice imo.

Setting up a carb on a pitbike can be quite tricky if your not sure what your doing, most of us have come across “problem carbs” before that for whatever reason just will not run right. Hopefully this guide will help you to dial in your carburettor for maximum performance and smooth running throughout the rev range. Badly setup carbs can also cause harmful damage if you go wrong such as overheating if the engine is running lean, backfiring with an over rich mixture etc. Setting up the carb properly makes a big performance increase as well so next time your thinking of buying a performance mod make sure your carb is set up properly first otherwise it could be sucking all your power.

With a huge variety of Chinese/Japanese engines on the mini scene and hundreds of possible different carb/exhaust setups it is impossible to give any exact sizes or details of the correct jets and settings you need. Take into account however that the more suited size carb you are running the easier the bike is to tune the carb. A rough guide to engine/carb size is
50cc – 18mm carb size
88cc – 20mm carb size
110cc – 22mm carb size
125cc – 22/24mm carb size
140cc – 24/26mm carb size

If you are happy with the way your bike is running, but want to increase performance by buying a bigger carb then simply go to the next size up on the list (E.g. 125cc engine would be upgraded to a 24mm carb) generally speaking if you are using a bigger carb for the engine it will be harder to dial in. If you are not familiar with which jets are which and where the needle jet is etc then see Johnr6's how-to guide “How to service the carb and change the jets”. Before attempting to tune your carb you should always clean your carb, air filter, add fresh fuel and buy a jet kit (It is recommended by some companys to use genuine brand jets even if you are using copy carbs as the quality of the jets is very important and universal sizes are needed)

Diagnosing the problem
Find which bit of the throttle the bike is not running properly at, this could be anything from dying when you first open the throttle to slightly bogging at 3/4 revs. The best way I have seen to do this is to make a mark on your throttle housing and a small mark on your grip level with it, open the throttle fully and make another mark on the housing, now split the space in between the marks into four equal sections with two more marks. Now you can see which part of the throttle you are at (by looking at the mark on the grip against the throttle) when the problems occur. Here are the corresponding throttle marks to the relevant parts of the carb.

Throttle Ranges:
Pilot Jet/air screw: idle - 1/4.
Needle Jet: 1/4 - 3/4
Main Jet: 3/4 - Fully open

Fixing the problem

Pilot Jet
Using the air screw you can determine if you have the correct pilot jet, screw in the air screw fully then out roughly one and a half turns to start. Start the engine, speed up the tick over until you get a slightly fast idle speed. Turn the air screw slowly in, and then out, until you find the point where the idle is fastest, stop there. If your air screw is less than one turn from fully closed you need a larger pilot jet, if it is more than 2.5 you need a smaller pilot. Once you have tried different jets (if applicable) and found the correct size then it time to set the mixture. Reset the tick over to a normal speed, ride around using only the first quarter throttle adjusting the mixture screw a small amount at a time either way until you get it running sweet.

Needle Jet
This is the most important setting at it is what you will be using most of the time. Ride the bike between the two middle marks on the throttle housing, if it bogs before responding to the throttle lower the clip on the carb slider needle a notch at a time until it picks up smoothly. If the bike splutters or sounds rough when picking up the revs raise the clip until it runs cleanly. You should be able to find a mid-point notch where the throttle responds nicely (if you cannot then complete the pilot jet setup even if you think the lower portion throttle is ok because this effects the needle jet).

Main Jet
This is where a plug chop is required. Install a new spark plug, find a long straight and warm up the bike. Set off and full throttle through the gears, when you reach top gear get to full throttle and hold it for 3-4 seconds before flicking the kill switch and pull in the clutch to roll to a stop. Now remove the spark plug and compare it to the “reading spark plugs” how-to guide. If you have a a lean mixture going up on the main jet size will fix it. A light brown or brown color indicates a perfect tune. If the mixture is rich a smaller main jet, clip adjustments, and air screw adjustments are needed. A little trick to double check your plug colour diagnosis is to turn on the choke slightly when the engine is running, if it runs better then you are to lean and if it runs worse then you either have the correct jet or a over size one.

You will notice that if you go through all of these steps and your main jet mixture is rich you are back to square one again! Therefore unless you have a carb that runs good apart from a particular part of the throttle range (so you can target that section to troubleshoot) you should follow the above guidelines in this order otherwise you can go round in circles.

main jet first
needle second
air and pilot last

Following the above simple steps should have your carb fully dialed in and result in a sweet running bike! However if you are still having trouble getting the carb setup after trying this guide then go down to the stock size carburettor and if you already have the stock size carb on but still cannot get it running properly then try cleaning out your carburettor and air blasting the jets to make sure they are not fully/partially blocked.