How to tune a Carburetor

in this article you will find a complete guide on how to tune a carburetor. you can apply this method on any honda bike with throttle wire actuated slide. such as honda cd70 , pridor and honda cg125.

Let's start from the basics

a carburetor is based on the venturi principle. when the engine is at its intake stroke there is a venturi in the carburetor from which air passes. and that creates a low-pressure area and in that venturi, a fuel jet is placed so that when the air is passed through that venturi the fuel draws through the fuel jet because of the low pressure . and then fuel mixes with air and atomized for combustion. there are three air-fuel circuits in the carburetor from which air-fuel mixture is supplied.

1: idle jet

2: needle jet

3: main jet

1:idle jet: idle jet is responsible for idle and it affects idle to 25% throttle position.

2: needle jet there is a needle in the carburetor that is attached to a slide or piston inside the carburetor.

that is responsible for fuel supply from 25% to 75% throttle position.

3: Main Jet: it supplies fuel from 75% to full throttle position.

and each circuit can be tuned for desired performance.

Front or engine side

Adjustment Screw Side

Air Filter Box Side

Tuning Idel Mixture

every carburetor has an idle adjustment screw on it to adjust the idle mixture. it is often located on the side of the carburetor or at the bottom. it can be either an air screw or a fuel screw. the fuel screw has a pointy tip and the air screw has a round end .both do the same thing and that is, adjust the idle air mixture. but adjusting the mixture with the fuel screw is a little difficult but precise. but adjusting the mixture with an airscrew is easy but won't be as perfect as a fuel screw.

1:adjusting fuel screw: with a fuel screw there is a constant supply of air in the idle circuit in which you add fuel to adjust the mixture. if you turn it right it will supply less fuel to the air. and you can turn it left to increase the fuel supply so it will add more fuel to the air at idle.

turn it right until the rpm starts to drop then turn it left until the rpm drops again. now you know the high and the low points. where the engine starts to struggle. now turn it right quarter turn at a time and wait for 5sec for the engine to adjust. again find the point where engine rpm first reduces and slightly turn it left and you will find the highest rpm point.

then slightly give 50% throttle. and notice when the rpm drops there is no backfire or engine struggles or increased rpm.

if you twist the throttle and release it, the rpm rise and hangs and drops slowly then there is less fuel in the mixture meaning the air-fuel ratio is lean.it can be adjusted with air screw however if it is not adjusting with the mixture screw then you will need to increase the piolet jet size.

and if when you give throttle the rpm raise but the engine makes a sputtering sound while you star revving it to 25%.then there is more fuel in the mixture. you need to turn the screw to the right to find the point where the rpm is the highest and the throttle is smooth.if it is not adjusting with mixture screw then reduce the piolet jet size by one size.

Idle Mixture Adjustment Screw in this case ( Air Screw)

Idle Adjustment Screw

Before Adjustment:

Warm up the engine: before adjustment ride the motorcycle for 20 minutes to fully warm up the engine.

Adjust the RPM: adjust the rpm to the recommended Idle RPM Suggested in the Owner's Manual. if your motorcycle engine rpm is too high you will skip the idle circuit completely .then it won't adjust properly.

Turn Left to decrease the Engine Rpm

Turn Right to Raise the Engine Rpm

adjusting air screw: in case of the airscrew. you adjust the air supplied at the idle. and fuel is supplied to it also increases accordingly. after certain turns, the air supply increases to a point where the engine starts to struggle because of the lean mixture. because the fuel jet reaches its limit, the fuel stops increasing beyond that point.

if you will keep turning the screw to the left you will add more air.

how to find the highest rpm: some times after a certain point if you will keep turning the screw it won't affect rpm. in that case turn it right again and notice the point when it starts to decrease rpm from that point turn it back and you will get the highest rpm.

after achieving, the highest rpm give a little throttle and check for the response again if the rpm hangs and drops slowly then it means the mixture is lean.

and if the engine makes sputtering sound while you give throttle that means the idle mixture is rich.

Idle Mixture Adjustment Screw in this case ( Air Screw)

Turn the air screw to the left to increase

the amount of air supplied to it

Turn the air screw to the right to decrease

the amount of air supplied to it

adjusting c.v carburetor fuel screw: usually, the C.V carburetor has an idle mixture screw is at its bottom. and it has a fuel screw so the adjustment is the same .but to access the screw at its bottom, for adjustment is tricky. for that loosen the clamp and turn it to the side from which the fuel screw is easily accessible. if your carburetor has a solid mount then the only solution is to use a bit screwdriver to access it. wrap a piece of tape for easy grip.

piolet jet: piolet jet supplies fuel at idle and if the idle air-fuel mixture is not adjusting right then change the piolet jet .when changing the piolet jet move one jet size at a time for example if you want to up jet from 32jet size then use 33 size jet and then gradually go higher.

Piolet Jet Alternate view

Piolet Jet

Piolet Jet Alternate view

Piolet Jet Circuite

Piolet Jet Circuite supplying fuel at idel

Needle Jet adjustment

it has four parts. jet needle, needle jet, circlip and slide. needle jet that is either controlled by a throttle cable in case of slide pin or downdraft carburetors or diaphragm for supplying fuel from 25% of throttle position to 75%.

access the needle jet

1: Slide pin carburetor: with this type of carburetor adjustments are easy you just need to remove the slide pin cap from the top of the carburetor and remove the throttle cable that is held in place by the retainer spring.

2: C.V carburetor: in the case of a C.V carburetor you will need to remove the top cover of the carburetor to access the diaphragm and then carefully take out the diaphragm and remove the lock to free the needle. make sure that you drain all the fuel from the float bowl. don't clean the diaphragm with any kind of carb cleaner or fuel. it is supposed to be dry. if you will use any kind of carb cleaner then the diaphragm will swell up and won't fit into its grooves and later crack or brittle.

Adjusting the needle height: on the needle jet, you will notice that there is a part that is tapped and the other side has 3 to 5 grooves in it. if you adjust the circlip to the top groove then it will make the air-fuel ratio lean and if you move the circlip down that will make the air-fuel ratio rich.

Needle Jet with needle

Needle Jet view from the engine side

Needle Jet view from the top

Needle Jet Working

Needle Jet in Closed position

Air Rushing in the venturi as the throttle is opening and lifting the piston and needle jet

Fuel is Flowing from the jet as the needle jet is raise

Main jet Adjustment

the main jet affects the fuel ratio from 75% to full throttle.

access the needle jet

to access the main jet you will need to open the carburetor bowl some carburetors have a bottom bolt. if you want to change main jet . you will only need to remove that in order to do that . but for most carburetors, you need to remove the bowl completely.

there are few rules to adjust the main jet

Rule 1: if you are building a carburetor for an engine that you don't know the jetting. then start by putting the biggest main jet you think is suitable for the engine according to the air then gradually decrease the jet size. this way there will be fewer chances of hydrolock. remember if you put to big jet then the engine will flood and hydro lock this will bend the connecting rod.

and if you put it too small jet knocking will occur.

read the spark plug carefully to avoid any engine damage.

Rule 2: when you up jetting or down jeting move two number at a time

for example if you were previously using 72 size jet and you want to upjet.then move to 74 size jet. similarly if you want to down jet and then move 2 steps down for down jetting e.g 72 to 70 size.

but in some cases jets are available in this order 72 to 75 and 78 and so on. it doesn't matter.

what matters is follow the next jet no that is in order and keep increasing or decreasing until you get the desired results.