that's right.the extra room is for safety purpose.if you see any plastic soft drink bottle or mineral water bottle,there is always little bit space left so that in case the plastic gets pressed the liquid would move up in that extra space...which otherwise would break the cap and spill out.This is because liquid cannot be compressed.The same rule applies to fuel tank.In case there is an accident...there is possibility of fuel tank getting crumbled and compressed and fuel will spill out breaking the cap and possibility of fire due to spark as metal rubs on road surface or electrical short circuit.so the extra space will keep fuel safe against spillage during accident and internal splashing while bike is running.

few words of caution.first better read the tuning procedure available online at various forums and understand the basics of tuning.otherwise you will easily mess up the tuning making things worse.you will surely not get it right in first attempt.Twister does not have RPM meter i.e Tachometer so your tuning will be purely based on hearing the engine sound and how well you recognize the changing engine RPM while turning the fuel screw.it took me lot of time and patience to learn tuning process all of my own due to lack of Tachometer.initially its more like a trial and error method but with practice you will learn it..and then you can tune any other bike having carb. I have now successfully tuned one of my friends twister and two HH passion besides my own Twister.

After the tuning how is the performance of your twister & what about mileage

yeah ! hav to be really cautious.. did my research :P. also hav an old tach it computis spark fires. i know it wont be too accurate but still will do ..

this is what i plan to do..
1. Ride the bike for about 2kms.

2.Center stand it and panels off :P

3.Set idle screw such that engine rpm is arond 2500-3000 (depending on which is more smooth and vibration free :P )

4.Adjust the fuel screw , to the point that the rpm drops to being stable at a point , also there should be no stall or backfires. vibration also a factor to concern.

5.repeat if stall. Till steady rpm achieved.

6.whn rpm is steady at a point,thn slowly reduce the idle screw till engine rpm is around 1200 (i know factory recommend is 1400 but am just doing this to save more fuel :P.. may go lower if engine is able to sustain by itself)..

7.Most important TEST RIDE ..


So that is what am gonna be doing today ..At 9am will be judgement call :P

Will surely let know the performance.. will be tuning at 9am thn will go for a long run to calculate mileage

Mileage has increased by approximately 3-4 kmpl. I dint notice much improvement in performance...more or less same as before. but in third gear it pulls slightly better on a inclined road but all remaining gears it feels same.

first and foremost choose a totally quite place for tuning so that you can concentrate on engine sound otherwise noises such as other vehicles passing by will blend with bikes engine sound and lead to confusion.
regarding point no1: It is better to tune when The engine is slightly hotter and not just little bit worm hence raise the throttle to cross 60km/hr speed during warming up ride.
First try to find the screw position where you get maximum RPM...once you find this then the stable RPM position will be normally close to highest RPM point.
regarding point no 3:The idle RPM setting while tuning has nothing to do with vibrations.it should be set at such an RPM so that you can easily recognize the change in RPM when you turn the fuel screw.so if idle RPM is set it too high then you will not notice the RPM change...same if its too low.
regarding point no 6: its true that after tuning,keeping Idle RPM low will give better mileage...but see that engine does not stall while at traffic stops or when headlight is turned ON.also it should start easily without choke under normal temperature.
do not tune the bike when fuel level falls in reserve neither it should be when tank full...i.e fuel level should be somewhere in between.about 1 lit fuel above reserve mark is good.
one important thing if you have noticed carefully around fuel screw...there is a small marking done with green/black color ink around the fuel screw.that marking covers half of screw screw circle....indicating the zone of Rich and lean mixture area with respect to screw hole line where you place screw driver tip for turning.the factory setting is exactly in between that marked zone.so this is a good indication to find if you are turning the screw beyond the maximum rich or lean mixture areas. In my bike this marking has now gradually vanished after repeated high pressure water washing. but it can be seen in all new bikes.

For CARBURETOR Tuning

First the important thing you must understand is the direction of rotation of the screws. The air-fuel screw turned clockwise gives a lean mixture and when turned counter clockwise gives a rich mixture. The idling screw when turned in (clockwise) will give a rich mixture and when turned out will give a lean mixture. Before you start tuning you MUST make sure the engine is warmed up. Take a quick ride which should have at least half of the top speed,before you start tuning the carburettor which should sufficiently warm the engine. Turn the idle screw till the engine rpm reaches about 2.5k. Now our aim is to make the mixture as lean as possible at the same time not stall the engine. We do this by rotating the air-fuel screw clockwise to the point where engine receives its leanest mixture. The engine might stall during this process but do not mind, with practice you will get it . Now the mixture is set to the leanest condition possible which means the engine is receiving a mixture with more air and less fuel. Next step is to set the perfect air-fuel ratio, we do this by slowly I mean very slowly turning the air-fuel screw counter clockwise. The rpm should increase gradually and at a particular rpm, the rpm needle will show become constant. Now that’s the point where we have our perfect mixture. Now try to start the engine and try riding it, the throttle response should be a little crisper and accurate. The method is all about trial and error, with due time you will be able to get the right feel from riding and from the engine sound.

Tuned

Sorry guys i was to update you all yesterday itself.. as i posted i had started at 9am.. thing is i ended up making a trip to vellore.. but that didnt stop me from tuning :P

I just made a minor change to the fuel line before i started .. i added a short pipe that would typically hold 200ml of fuel.. this was actually for ease in mileage calc :P.. so whn ever i needed to calc .. turn off the fuel know and dive till the engine gasps.. then odo reading from test start *5 give kmpl.. (may not be too accurate but is sufficient ).. also my avg test speed was 50-55 kmph , with frequent shifting and braking and also on rugged terrain to simulate 40%hwy60%city riding ..

OK now as you probable know my second twister (white DISC) was giving me 65-73 kmpl and this was the bike i wanted to tune.. and hurrah! mission accomplished after dozens of trial error runs :P

What i did ..
1*Hooked the tach ,by winding its pickup wire around the sparkplug..
2*drove about 3kms to warm up the engine
3*4*5 actually all pretty much as i had posted befor :P.

took me about 13 tries to get the right combination (at least i feel it right )
started by turning the fuel screw completely inward ..at that point the bike even refused to start.. thn slowly i unscrewed it till i could get the bike start .. now that the bike started i unscrewed the idle screw till the engine would stall , so at that point such that engine would be alive let the scre be.. thn test ride with fuel know off so just under 200ml of fuel in the line...
thn stop fuel opn engine off.. again the same process over and over , but with minor variation...
i kept note on what kind of every test result i got. (note am using approx rotation outward in degrees for fuel screw from the point its completely in )

Test No. FuelScrew rotation Mileage(200ml) Mileage(1ltr)

1- 615 9.4 47
2- 635 9.6 48
3- 650 9.7 48.5
4- 700 9.9 49.5
5- 725 10.2 51
6- 750 11.3 56.5
7- 765 12.4 62
8- 770 14.1 70.5
9- 790 15.7 78.5
10- 805 16.5 82.5
11- 815 17.3 86.5
12- 825 15.4 77
13- 835 12.6 63

so at the 11th test was where i got the highest mileage. after that economy stated dropping so i figured that was where the ideal point was so i decided to stick to that ..

By the time i had reached my destination in gudiyatham (near vellore) i had completed my tuning.. just to be sure i noted the mileage on the way back . but this time with exactly 1ltr fuel in the tank (had drained the tank and also had a 2ltr bottle with fuel in my bag for whn the test would be over so as to get me to a fuel depot)..

Bike indeed did return 87.6kmpl .. that is 15kmpl more than what it was giving me before ... So i can declare my TEST a SUCCESS

yeah ! now one difference i feel is that the throttle is not as responsive as it was but i can live with that :P, am getting the mileage i was expecting :P..

Now my friends who i went to meet after knowing this want me to tune their bike too .

I got back around 6pm last evening from gudiyatham to mylapore , driving in reserve :P and still there is fuel :P and whn told to my friend here they all are also shocked .. a couple want me to help them tune today .. two twisters three pulsars , dazzlers and a couple more.. lets see if can get the feat i achieved here too :P

mileage has improved by almost 15kmpl. ... and yes it does pull a bit at third gear but is really nothing that cant be handled. a little more throttle and its set :P

before tuning the bike, i was getting maximum 84 kmpl which was very good, now after tuning it has improved further by 3-4kmpl. which means its same as yours at about 87-88 kmpl.this the maximum i can extract on highway at constant economy speeds without pillion.although throttle response feels a bit low....but i can overcome it by giving little more throttle.

As you all knew my bike gave me a 93kmpl on highway at a economy speed. But in side city it returns me only 58kmpl. My office is just 3.5kms from my home so daily i come for lunch to my home so per day only 14kms+ 4kms for my friends home visit so totally 18kms per day. It took three days to complete 58kms. so is it good mileage for three day driving. But if i drive continuously 70-80kms in single day it gives me 70-75kmpl.

@2wtourer Please note, the highest mileage is also the range where there is more air intake and less fuel, this will damage the engine, its like feeding your horse less food and making it run, it will be good intially until you get the piston rings and cam heads damaged due to too high temperature. The ideal setting you must keep is around the 9 or 10 of your recorded test setting, this will also give your bikes normal performance back!

Yes its absolutely normal, i guess you keep less fuel in tank, so if you have 1 lt fuel, then naturally there is 7lt+ of air in tank, and over 3 days the petrol gets evaporated gradually. But when you put 1lt petrol and ride, the chances of fuel evaporation in an hour is very little. So try this by keeping your bike always above 50-70% or better still always go for tank full, this will return better mileage and save your money! And don't forget to do a full tank, run the bike normally for 3-4 days, again full tank and check mileage. This time you will surely get 60-65kmpl!

Tip: Its always better to do a full tank once in 10days rather than putting 50rs everyday going to the office!

Thanks prajnyan will keep that in mind.. and i did reduce the tune a bit after that.. i was aware of the stress it may create so instead ok having screw at 815 degrees i hav it at 800 .. and did a test once again today .. in city traffice.. Nugambakkan and T.Nagar area.. headin out till Teynambet.. 17times start stop .. in rush traffic.. got 78.4Kmpl.. :P ..

the traffic cop at t.ngr (near pothy's) came while i was filling 1ltr into my tnk and asked me what i was tryin.. it seems he had seen me do the same at VenkatNarayan road :P :P .. anyway a new cop acquaintance made.. a

Ya you are correct i always keep bike in reserve condition. I'll check it with full tank fuel.