Tuning Your Carburetor For Optimal Performance And Fuel Efficiency

Dear Biker,

• Are you tired of cold starts? Need to raise the throttle everyday to get the cold engine running?
• Does someone have the same bike as yours and are you tired of losing to him in drag races?
• Suddenly, the fuel efficiency has reduced after a service?
• Has the mechanic has screwed up your tuning?
• Engine gives hiccups when you open the throttle?
• Or your bike is just missing that smooth acceleration whenever you want?
• Or your engine overheats even if you did not race too much.

May be the following will help you in tuning your mean machine for optimal power delivery and best fuel economy. This guide aims at tuning the fuel/air screw to give you an optimal air-fuel mixture. An optimal mixture gets maximum performance from your machine.

Getting The Basics

I am assuming that you know driving very well and have completed at least 1000kms on road. If not, then you are too amateur to read this guide.

Every bike has a engine (motor/mill/muscle) and a carburetor (breathing system/heart)  – Yeah I know that. My point is, I assume that your engine is in perfect condition. That is, there is no oil leaking from it etc etc.. The next assumption is that your engine is not DEAD. You know where your engine is and where your carb is  and you have a screw driver/blunt knife with you. Your machine must have finished at least one service ~500kms or more. Also make sure you do not change the fuel type while tuning i.e. Power, Speed, Extra premium, normal unleaded etc.

This Guide Applies To

All the single cylinder bikes, 2 stroke or 4 stroke having a carburetor. Tachometer is very handy and a must for a newbie or the tuning process will be very slow or painful or imperfect.

Disclaimer: Though whatever I have written here wont do any damage to your machine, finally you must not blame me for whatever happens. I am here to help if anything goes wrong.

So Lets Get Started

Warm the engine nicely. Go get a ride around the town. Speed nicely at least upto half the top speed of the bike. Drive at least for 10-15 minutes so the engine system warms up nicely or you’ll get a bad tune. Do not just leave the engine idle and start with process. I mean it. Ride the bike.

Now that the engine is adequately heated up, you are ready to begin with the process. You can select a place far away from the city or your residential area so that you do not disturb the tired, sick, disgusted, old people or babes (babies) living in your colony. You are likely to be shooed away from them when you are at a critical point. You may choose a place near your girl friend’s house just to impress her! Put the bike in main stand and get hold of your screw driver. That is all you need, together with your eyes, brains and ears.

Searching the fuel/air screw… The above mentioned screws are responsible to adjust the air-fuel ratio which forms ‘food substance’ for your engine. You can relate it to our human body, where proteins, carbs, fats, water etc all are required in appropriate proportions with respect to each other for good strong body.

There is another screw which sets the idle speed of the machine. This screw is not related to pickup or mileage. It just sets the engine rpm at ’’idle’’ run. 2stroke machines have an Air screw and 4 stroke machines have a Fuel screw. I hope you note this VERY well. Air screw is located on the carb away from the engine. [ Engine - Carburettor - Air screw]

Fuel screw is located on the carb but its near the engine. [Engine - Fuel screw - Carburetor]. If the make of your carb is Mikuni (Pulsar, Yamaha, Fiero) the fuel/air screw probably will be of brass (golden colour). If you fully unscrew this screw and take it on your hand, you will see a needle like tip.

Idle screw is closely linked with the throttle cable. Idle screw can be turned by the hand. Screw driver is not essential for it. Finding these screws are very simple. If you still cannot find the air/fuel screw, ask your mechanic or I can help.

Ok now what? Apart from the above difference I gave between Air and Fuel screw, there is one more major difference. Fuel screw turned in (clockwise) gives a lean mixture and turned out (anti-clockwise) gives a rich mixture. Air screw turned in gives a rich mixture and turned out (anti-clockwise) gives a lean mixture. Lean means more air, less fuel. Rich means more fuel, less air. This ends the basics.

Getting Things In Action

Turn the idle settings screw so that rpm reaches about 3000 rpm. Now tune the air/fuel screw to make the mixture leanest as possible. Please refer to the above whether you must turn the screw clockwise or anti clockwise.

As you make the mixture leaner, slowly the engine RPM decreases… Go on doing this until you have put the fuel screw to the leanest possible point. At the same time, ensure that the engine does not stall, by turning the idle-screw.

Tough huh? You’ll get used to it 

OK the engine is running and the mixture is lean. If you notice from the engine sound OR if you see the tachometer, the engine RPM will not be steady at this point. Now very slowly start turning the fuel screw anti-clockwise, quarter to 1/8th turn at a time. You will notice that the RPM increases slowly and steadily. Again, do this very very slowly. Also count the total number of turns as you wind out the screw.

You’ll notice that when you have turned it to about 3-4 full revolutions, the engine RPM slowly becomes constant. It is this point that you must stop screwing more. This probably is the optimal setting for your engine.

Further on, try turning the screw even more and more to 5-7 revolutions and you’ll notice that the engine RPM will slowly decrease. When this happens, you are just putting in a too rich mixture in your engine. At this point the engine loses all the fuel efficiency and the mixture is not optimal. Repeat the process about 2-3 times. Count the revolutions each time and get the setting which you feel is correct.

OK the engine is humming perfectly, now what? Decrease the idle setting screw to about 1000rpm in the tachometer. When the engine slows down, just twist the throttle. The response should be crisp and quick. It should not give any hiccups! Try shutting off the engine and restarting. The engine MUST start in a single kick or self with out giving throttle. If this happens, the setting is ok. Now get a ride and you’ll notice the difference for good or bad 

You’ll immediately notice change in the engine sound and the throttle response. Your engine can become more smooth or harsh. Another important point is, Ride and Feel. Always take a ride and get the feel of the bike in each gears, check the response and the engine sound. You’ll quickly come to know once you get the feel of the bike that you want to make the mixture rich or lean.

It may take a few iterations before you fix a setting as permanent. Try calculating the mileage per liter and tally it with your setting and the feeling you get.

Troubleshooting

“Ahwini, I messed up my tuning because of you!”

Here are some symptoms and their quick solutions:

• Engine dies while tuning.
  Try the process all over again. This time set the idle screw higher/faster.
• Engine gives hiccups while driving, specially while in higher gears.
  May be the mixture is too lean. Try again.
• Engine heats up.
  The mixture is too lean or too rich. Try again.
• Whenever I race up the engine, the rpm increases fast but very slowly comes down to idle.
  The mixture is not optimal, probably towards leaner side. Try again.
• Too much low end torque and the engine sound is very beaty/thumpy.
  You’ll face a low mileage surely, when you drive below 40kmphr. You have tuned on the higher/richer side.
• I have doubts or I have other problems.
  Why don’t you put in a comment?

I hope this helps! Take care. Any comments are invited.