Tuning a Carburettor for Optimal Performance

Are you tired of cold starts? Need to raise the throttle everyday to get the cold engine running?

Does someone have the same bike with same setup 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 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.


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 won't do any damage to your machine. finally if you end up doing anything wrong, you must not blame me for whatever happens.


SO LETS GET STARTED

Warm the engine nicely. Go get a ride around the town. 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 searching the fuel/air screw… These screws are responsible to adjust the air-fuel ratio which forms ‘food substance’ for your engine. 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].

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, 1/8th turn at a time. You will notice that the RPM increases slowly and steadily. Again, do this very very slowly.

You’ll notice that when you have turned it to about 3-5 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.

Anyways, repeat the process about 2-3 times and get the setting which you feel is correct.

OK the engine is humming perfectly, now what? Decrease the idle setting screw to about 1100rpm 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


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.