This guide will help you in tuning your motorbike for optimal power delivery and best fuel economy. The basic goal is to tune the fuel/air screw to give you an optimal air-fuel mixture. An optimal mixture gets best performance from your machine.
Every bike has a engine and a carburetor. Assuming that your engine is in perfect condition (no oil leak , damaged cylinder heads, etc). The next assumption is that your engine is not DEAD. You know where your engine is and where your carburetor is and you have a screw driver with you. Your machine must have finished atleast three services ~2000 Km or more on the odometer. 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. As of now, the carburetor is getting eliminated with modern bikes such as the new Kawasaki Ninja 400. However certain cruisers still posses carburetor due to its simplicity and reliability. Tachometer is very handy and a must for a newbie or the tuning process will be very slow or painful or imperfect. But nowadays bikes come fitted with one normally.
Disclaimer: Though whatever I have written here wont do any damage to your machine, Do it at your risk.
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. Ride the bike. Now that the engine is adequately heated up, you are ready to begin with the process.
Locating the fuel/air screw:
The above mentioned 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. 2 stroke 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 carburetor away from the engine. [ Engine – Carburetor – Air screw] Fuel screw is located on the carburetor but its near the engine. [Engine – Fuel screw – Carburetor]. If the make of your carburetor is Mikuni (Pulsar, Yamaha, Hero Honda) the fuel/air screw probably will be of brass (golden color). 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. 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 mixture means more air, less fuel. Rich mixture means more fuel, less air.
Tuning the Engine for better efficiency:
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. As you make the mixture leaner, the engine RPM decreases simultaneously. 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.
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 rotations, 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.
Once the engine runs smoothly, decrease the idle setting screw so to get the engine running at 1000 RPM. 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-start 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 to 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.
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 40kmph. You have tuned on the higher/richer side.