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Old 17th April 2007, 03:45   #1
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Default Setting the Air Fuel Ratio on a bike?

how does one adjust the air fuel ratio in bikes....that screw which increases engine rpm can that be used to make a bike run lean or rich.or is there some other adjustment
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Old 17th April 2007, 05:05   #2
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you can play around with different jets, needles, air screws etc.
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Old 17th April 2007, 09:45   #3
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Quote:
Originally Posted by lancer_mania View Post
how does one adjust the air fuel ratio in bikes....that screw which increases engine rpm can that be used to make a bike run lean or rich.or is there some other adjustment
Long time since I did this, but thought there were two different screws on old carburettors, the one which projects out (you can use your hand on this one) is the idle adjuster, and the one which is placed low (need screwdriver) was the air/fuel ratio adjustment. You also can change the needle position (move "E" clip to a different position), adjust jet size or use a different carburettor, change intake filter etc.
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Old 17th April 2007, 10:33   #4
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me and my friend were trying to do this on the unicorn but it ddint quite seem to work an one know ow it to be done on the unicorn.
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Old 17th April 2007, 10:45   #5
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Quote:
Originally Posted by lancer_mania View Post
how does one adjust the air fuel ratio in bikes....that screw which increases engine rpm can that be used to make a bike run lean or rich.or is there some other adjustment
Normally the air/fuel ratio is supossed to be 15:1,
Most carbs have a air screw and an idle adjusting screw!
The air screw is normally set " so many half turns out (approx 1-1/2)" from a lightly seated position. By adjusting this screw you make it run rich or lean!
The screw which increases eng rpm - is the idle adjust screw and that is adjusted so that the engine idles with the throttle fully closed.

Hope this helps!
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Old 17th April 2007, 11:23   #6
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if i want to increase the air fuel ratio for a particular ride and then change it bak to what it was how should i do tht
will adjusting the idle screw increase fuel flow ?
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Old 17th April 2007, 13:30   #7
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Air screw setting on the bikes:

Generally on all Mikuni carbs where the adjuster screw is near the rear of the carb it controls the air entering the circuit which leads to the emulsion tube. Tightening this screw makes it richer by blocking the air enetring the circuit and loosening this screw leans out the mixture.

On some carbs where the adjuster screw is towards the front of the carb it adjusts the fuel in the pilot (if i remember right) circuit. But these carbs are not quite common.

The most common position of the airscrew is 1&1/2 turns anticlockwise from the fully seated position.

Idle screw in all carbs acts on the slide in case of normal carbs and butterfly valve in case of CV carbs (and car carbs). It doesn't affect the afr. If you tighten the idle adjuster screw it just prevents the slide or the butterly from closing fully and it holds the throttle valve at that desired position.

Keep this in mind when you're trying to tune the bike:-

The pilot jet controls the idle to low end of the powerband.

The slide cutaway and needle controls the low end to mid range to a bit of top end of the band.

The main jet controls the midrange to wide open throttle of the band.
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Old 17th April 2007, 18:51   #8
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so where can i locate the screw to change air fuel ratio settings ?
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Old 17th April 2007, 21:23   #9
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there is no single screw that you can play with to make your engine run rich or lean. Tell us what you are trying to achieve and people will be able to help you better.
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Old 17th April 2007, 22:01   #10
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i want to make my bike run rich for a certain ride and then want to get back to what it was
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Old 17th April 2007, 22:19   #11
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why would you want to do that? And it would help if you told us what bike it is. All bikes dont come with the same carb.
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Old 17th April 2007, 23:57   #12
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all bikes are set ridiculously lean by manufacturers to increase FE.in the city its not that bad but on longer rides when i max out on the speed the engine gets very hot and at tht speed with the amount of air flowing in im sure that a little more fuel would be better.
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Old 18th April 2007, 01:12   #13
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they arent set "ridiculously" lean. maybe just a tad on the leaner side. you still arent saying what bike you want to do this on. Any mods it may have that changes things etc etc.....
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Old 18th April 2007, 01:57   #14
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ahh yes im sorry i have a ZMA but i want to do this on my cuzins bike which is a pulsar 180 cc dtsi
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Old 18th April 2007, 04:19   #15
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i think the 180 comes with a bs29 mikuni (correct me if i am wrong) and while i dont know the exact settings on it, maybe you should start with shimming the needle up a bit. The needle is found below the black plastic cover on top and you should put maybe a 0.1 mm shim below the needle retainer to start with. This will richen up the mid-range (the range you ride in the most) and you may have to play with the airscrew a bit to make it run right in the low end. if you cant get the right sized shim just raise the needle by one slot and see what happens.
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