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Last Updated: August 29, 2012

Tuning Shogun CARB (for starters)

Shogun FD125xrm, FL125 carburetor are not so difficult to tune for optimal mixture and for maximum performance. Newbies or those who is just beginning to ride a motorcycle of course tend to drop by at any shop they spotted just to make their bike tune in for no particular reason for a carbureted bike without altering intake and exhaust system definitely will not give any problems unless they are ridden hard, usage of poor quality gasoline, using fuel additives, and or riding in a dust prone areas to where the airbox or intake system will accumulate dust particles that will clog the paper air filter of the box.

Fine tuning of the carburetor is sometimes not nescessary if you are just changing air filter, but when it is time for carburetor maintenance, some of the problems may be noticeable as follows below


              1.       Are you tired of cold starts? Need to raise the throttle everyday to get the cold engine running?
                   2.  Does someone have the same bike as yours and are you tired of losing to him in drag races?
                   3.   Suddenly, the fuel efficiency has reduced after a service?
                   4.  Has the mechanic has screwed up your tuning?
                   5.  Engine gives hiccups when you open the throttle?
                   6.  Or your bike is just missing that smooth acceleration whenever you want?
                   7.  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.
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.

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. Clockwise is for closing the air screw (richer) and counterclockwise is for opening for more air (leaner)
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 ? 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 2-3 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 4 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.

Remember, this simple tuning process is for those who have no leaks on the intake manifold, as well as for those who have not accumulated dirt inside those carb passageways, and that the air filter is intact or still in its place..

Altering any of the mentioned intake and or exhaust system may definitely have problems such as throttle transition from close to 1/4th throttle to 3/4th up to WOT or wide open throttle.

GUNSON COLORTUNE. If you have access on this kind of REALTIME visual indication of what's happening inside your combustion chamber when you are turning that screw to indicate whether you are on the lean side or rich side part of the mixture.

I'll have my bike top overhauled soon, for will replace valve seal, valve guide, and valves, so it will be needing carb re tuning and i'll once again use this colortune for my VM-18 carb maximal performance be sure to come back here anytime for updates and to show you how it works real time.


Thibroni Misni said...

"Tachometer is very handy and a must for a newbie or the tuning process will be very slow or painful or imperfect."

Hi, very nice tips you have there. it us really useful. But i got few questions regarding tachometer.

1. What type of tachometer did you use? is it digital with sensor?

2. How you use the tachometer? Did you install the tachometer directly to your motorcycle, or you use non-contact tachometer?

I believe some mechanics have messed up my bike carb ratio. Thanks.

Nathaniel Berdan said...

hi, the tachometer i made for my small bike is a digital tachometer
see this link

it uses the pulses made by the generator, not the pick up coil, that is 1 is to 1. if the engine is rotating at 3000 rpm, the tachometer will display it.

2. The tachometer is a very handy tool, it display the actual revs of the engine, to use simply find a way to get pulses of the stator side. Tachometers fitted outside the engine like pulses from ignition coil and or contactless are in accurate, due to the EMF noises generated by the sparkplug when firing.

Nicholas Maspara said...

hi bro san mo nabili ang gunson colortune
hows the effect?

Nathaniel Berdan said...

it was bought by a friend from UK and shipped to me. It will only be used once on a motorcycle being tuned to get the maximum overall performance of the carburetor. Once the settings is known, it will be marked so that every carb cleaning and tear down it will be easier to know the right mixture settings back by that marked.