Posts Tagged ‘tuning’

How to Tune 4S Motorcycle Carburetor

October 27th, 2010 No comments
Tuning a motor cycle carburetor could not be as easy as it could get.  All one should know for doing the tuning oneself is to guess the speed of the engine motor (the rotations per minute).  Have you ever wondered about the sound a motor makes when it starts and stops ??  I am meaning the MMRRMMRRMmrrmrrmrrmmm….. sound 🙂  If you know about that you can tune your motorcycle yourself.

The Basics

The motorcycle carburetor is meant for atomizing (converting the liquid to gas state) the petrol and send the gaseous fuel into the engine cycle for combustion.  So, a carburetor gets the petrol fuel from the petrol tank, mixes that with atmospheric air (after filtering it using air-filter) to atomize the fuel in to a fuel charge using the venturi float setup.  The fuel charge is fed into the engine cylinder for combustion.  The quality of combustion is a function of the air-fuel mixture ratio.  The air-fuel mixture is the ratio of quantity of fuel to the quantity of air.  When there is too much air in the charge, the mixture is called “lean mixture“, and when fuel is more than air it is called “rich mixture“.  For complete combustion, there should be enough air for combustion.  Complete combustion enhances mileage.  For pickup tuned bikes, the fuel is kept little higher than air.  Also for cold starts, one needs rich mixture for quick firing up.

Modern day bikes are equipped with Constant Vacuum Carburetors which ensure best air-fuel mixture by adapting to conditions.  The carburetor adjusts the air flow automatically based on the load on the engine despite the accelerator position.  This is done by the baloon and back-pressure setup in the CV carburetors.  The CV carburetors come with two adjustments, a) Idling b) Air-Fuel mixture.  The Idling setup is trivial, it is just a offset setting of the accelerator wire.  Adjusting the idling screw (the screw with a spring) is just like you accelerating a bit.  The air-fuel mixture setting is just a screw near the idling screw, most likely the screw with have dirt on it (I mean more dirt). The air-fuel mixture screw controls the air-fuel ratio.  Full tight means low or no air flow and Full loose means more air.

The Method

  1. Set the idling screw for little more throttling, meaning the engine should rev faster than before.  Typically, 10% more than the idling speed that you are used to.
  2. Set the air-fuel mixture screw to full close (tight, don’t tight it too hard). You should feel that the engine speed has reduced a lot now.
  3. Open the air-fuel screw slowly and observe that the engine speed is increasing.
  4. There will be a point, which if you cross by opening it further, the engine speed will start to decrease.
  5. You will have to adjust the screw to find the point where your engine speed is higher. Consider the following graph for better understanding.

The graph (indicative) shows the trend of the engine speed for various air-fuel screw positions and various idling speeds.  The green line is the locus of all the peak engine speeds.  To the left of the green line, you see the enriched mixture condition and to the right of the green line you can observe lean mixture condition.


  1. Don’t tune the engine when it is cold (cold start conditions). You may rev it for a while and tune it.  Basically, the engine oil has to pass through all the chambers and gears (otherwise too much friction is offered).  The dynamics of the engine are different when it is cold and hot.  If you tune when it is cold, you may supply lean fuel  mixture when the engine gets hot. Because while cold start, the engine needs enriched fuel charge.
  2. Set the idling to a reasonable value when you tune.  Too low or high idling can get you local maxima conditions, where you would not get the best peak point (refer graph).
  3. Check whether the air-filter is in good condition.  If the air-filter is blocked, tuning will not help.  Likewise, engine oil level and viscosity should be good.

Freeflow Exhaust Silencer

April 28th, 2010 No comments

என்னுடைய Thunderbirdன் Silencer பழுதடைந்தது. மாற்ற வேண்டி சென்னை Taylor’s Roadல் உள்ள சௌந்தரராஜன் மெகானிக் கடையின் ராஜு என்ற மெகானிக்கிடம் சென்றேன். அவர் Silencerஐ சோதித்து பார்த்துவிட்டு மாற்றித்தான் ஆகவேண்டும் என்று பரிந்துரைத்தார். அப்போது, இந்த Freeflow Silencer போட்டால் என்ன என்று கேட்டேன். அதற்கு அவர் கொடுத்த பதில் இதோ:-

ஒருவன் சத்தமாக தொண்டை கிழிய அலறினால் என்ன ஆகும்? தொண்டை நிரந்திரமாக பழுதடையும்; அதேபோல Freeflow Silencer போட்டால் இஞ்சினின் செயல்பாடு பழுதடையும், ஆயுள் குறையும் என்றார்!

ஆமாம், freeflow silencer போட்டால் பவர் (BHP) கூடுவது உண்மையே. அதேபோல mileageம் குறையும். ஏனெனில் அதிகமாக எரிபொருள் இஞ்சினிற்கு உள்ளே சென்று எரிந்து வேகமாக வெளியேரும். அதனோடு சரியாக எரியாத எரிபொருளும் வெளியேரும். Freeflowவினால், முழுமையான எரிதலை உறுதிபடுத்த முடியாது. அதனால் எரிபொருள் விரையமாகும், mileage குறையும். மேலும் freeflow silencerலிருந்து வெளிப்படும் புகையானது (கண்ணுக்கு தெரியவேண்டும் என்று அவசியமில்லை) emission standardsக்கு ஒவ்வாது.

Half Clutch

July 17th, 2008 No comments

A motorcycle rider’s attitude could be easily
guessed by checking the clutch setting in his motorbike. There are
generally two modes of clutch setting, a half clutch, a full clutch.

brush up, clutch is a device that controls the amount of power
transferred from the engine to the rear wheel. Generally clutch device
is made of multiple discs called wet plates
and power is transmitted across these plates by friction. The clutch
lever on the left hand side controls the amount of friction and hence
the power transmitted. When one fully pulls the lever, the clutch
plates move away from eachother and no power transmitted. On the other
hand, when the clutch lever is released fully, maximum friction is
available amidst the plates and maximum power transfer. The magic is
how good one is, in handling the clutch to get the maximum out of the
bike, be it 100cc or 1500cc.

Half clutch is a
setting, where a full pull of the clutch lever does not pull the clutch
plates far away. So effectively, the operating range of the clutch
lever is so small. Even when the lever is released a little, one can
see the rear wheel starts rolling.

If you check
the bikes of boys vrrooomming in the signals like rockets, they would
have half clutch or even smaller. Boys with half clutch have a fast and
furious attitude. They need everything on their plate faster. These
guys cannot wait for anyone. These guys are performers given
opportunity. These boys are quick witted and even some of them are
impulsive too.

On the other hand, the full
clutch boys are patient movers. They take their time to do things.
These guys are steady and focussed. If you take a statistics across age
groups, full clutch setting would be opted by a lot of middle aged men,
who are good in decision making, logical thinking. These boys can wait
for their best catch. It is pretty difficult to make these boys angry.

respect of biking, half clutch boys outperform full clutch boys
significantly. The reason is full clutch will not let the bike to
perform to its fullest. One should not forget the half clutch ruins the
clutch plates and engine gears faster.