OZ ATV :: The Australian ATV Forum

[Banshee250R]

hi guys
im thinking of putting a new set of carbs on my shee in about 2-3 months when i completely pull it down,powdercoat it and rebuild it

it will have fmf fatty chambers and powercore2 silencers,boyesen rad valves,light porting,k&n filter and no airbox lid

i was thinking of either a 2 into 1 setup or twin 28mm carbs

can anyone steer me in the right direction with opinions of the 2 into1 setups vs twin 28's??



cheers
luke
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Doin skids for the kids and doughnuts for the grown-ups
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[4stroker]

i too would like to know this, surely there are some banshee "gurus" on here??
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2002 SE Banshee

[rapmat]

heres a run down on carbies to the best of my knowledge


Motorcycle carburetors look very complex, but with a little theory, you can tune your bike for maximum performance. All carburetors work under the basic principle of atmospheric pressure. Atmospheric pressure is a powerful force which exerts pressure on everything. It varies slightly but is generally considered to be 15 pounds per square inch (PSI). This means that atmospheric pressure is pressing on everything at 15 PSI. By varying the atmospheric pressure inside the engine and carburetor, we can change the pressure and make fuel and air flow.

Atmospheric pressure will force high pressure to low pressure. As the piston on a two stroke engine goes up (or goes down on a four stroke engine), a low pressure is formed inside the crankcase (above the piston on a four stroke). This low pressure also causes a low pressure inside the carburetor. Since the pressure is higher outside the engine and carburetor, air will rush inside the carburetor and engine creating a vacuum effect until the pressure is equalized. The vacuum going through the carburetor will pick up fuel and mix with the air.

Inside a carburetor is a venturi, fig 1. The venturi is a restriction inside the carburetor that forces air to speed up to get through. A river that suddenly narrows can be used to illustrate what happens inside a carb. The water in the river speeds up as it gets near the narrowed shores and will get faster if the river narrows even more. The same thing happens inside the carburetor. The air that is speeding up will cause atmospheric pressure to drop inside the carburetor. The faster the air moves, the lower the pressure inside the carburetor.



Most carburetor circuits are governed by throttle position and not by engine speed.There are five main metering systems inside most motorcycle carburetors. These metering circuits overlap each other and they are:
* pilot circuit
* throttle valve
* needle
* main jet
* choke circuit

The pilot circuit has two adjustable parts, fig 2. The air screw and pilot jet. The air screw can be located either near the back side of the carburetor or near the front of the carburetor. If the screw is turned in, it reduces the amount of air and richens the mixture. If it is turned out, it opens the passage more and allows more air into the circuit which results in a lean mixture.



The pilot jet is the part which supplies most of the fuel at low throttle openings. It has a small hole in it which restricts fuel flow through it. Both the pilot air screw and pilot jet affects carburetion from idle to around 1/4 throttle.

The slide valve affects carburetion between 1/8 thru 1/2 throttle. It especially affects it between 1/8 and 1/4 and has a lesser affect up to 1/2. The slides come in various sizes and the size is determined by how much is cutaway from the backside of it, fig 3. The larger the cutaway, the leaner the mixture (since more air is allowed through it) and the smaller the cutaway, the richer the mixture will be. Throttle valves have numbers on them that explains how much the cutaway is. If there is a 3 stamped into the slide, it has a 3.0mm cutaway, while a 1 will have a 1.0mm cutaway (which will be richer than a 3).



The needle affects carburetion from 1/4 thru 3/4 throttle. The needle is a long tapered rod that controls how much fuel can be metered into the carburetor venturi. The thinner the taper, the richer the mixture. The thicker the taper, the leaner the mixture since the thicker taper will not allow as much fuel into the venturi as a leaner one. The tapers are designed very precisely to give different mixtures at different throttle openings. Needles have grooves cut into the top. A clip goes into one of these grooves and holds it from falling or moving from the slide. The clip position can be changed to make an engine run richer or leaner, fig 4. If the engine needs to run leaner, the clip would be moved higher. This will drop the needle farther down into the needle jet and cause less fuel to flow past it. If the clip is lowered, the jet needle is raised and the mixture will be richer.

The main jet is where the jet needle slides into. Depending on the inside diameter of the needle jet, it will affect the jet needle. The main jet and jet needle work together to control the fuel flow between the 1/8 thru 3/4 range. 99.9 % of the tuning for this range is done to the jet needle, and not the needle jet.



The main jet controls fuel flow from 3/4 thru full throttle, fig 5. Once the throttle is opened far enough, the jet needle is pulled high enough out of the needle jet and the size of the hole in the main jet begins to regulate fuel flow. Main jets have different size holes in them and the bigger the hole, the more fuel that will flow (and the richer the mixture). The higher the number on the mainjet, the more fuel that can flow through it and the richer the mixture.



The choke system is used to start cold engines. Since the fuel in a cold engine is sticking to the cylinder walls due to condensation, the mixture is too lean for the engine to start. The choke system simply adds fuel to the engine to compensate for the fuel that is stuck to the cylinder walls. Once the engine is warmed up, condensation is not a problem, and the choke is not needed.
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getting your quad around just got a whole lot easier www.ozcustomquadcarriers.com.au

[rapmat]

ps... i didnt author the previous post it was plagerised from bansheehq forum

http://www.bansheehq.com/forums
a lot of realy good stuff in there ... i could copy and paste more and you guys would think i knew heaps but i couldnt be bothered

Matt
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getting your quad around just got a whole lot easier www.ozcustomquadcarriers.com.au

[Banshee250R]

my eyes hurt

halfway through reading it i thought hang on i was reading this bansheeHQ lol

but good reading

thanks mate!



cheers
luke
_________________
Doin skids for the kids and doughnuts for the grown-ups
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[4stroker]

yeah some good basic info there but still doesnt really give us anymore insight into the best actual set up to use on our banshees.

i know there is heaps of info available on banshee hq but i would really like to hear from someone a little closer to home.

thanks, mark.
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2002 SE Banshee

[Davo]

If you just have bolt ons i wouldn't worry about going bigger, unless going to a single carb for a smoother powerband.
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banshee

[4stroker]

Davo wrote:

If you just have bolt ons i wouldn't worry about going bigger, unless going to a single carb for a smoother powerband.

this is the sort of info i was chasing...

however my engine will be ported if/when i get carbs, and surely the newer 28mm pwks would have to be "profanity removed" loads better than the 20 year old mikuni 26s??
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2002 SE Banshee

[Banshee250R]

thanks for the info guys i will stick with the twin 28's



cheers
luke
_________________
Doin skids for the kids and doughnuts for the grown-ups
------------------------------------------------

[Davo]

4stroker wrote:

Davo wrote: If you just have bolt ons i wouldn't worry about going bigger, unless going to a single carb for a smoother powerband. this is the sort of info i was chasing... however my engine will be ported if/when i get carbs, and surely the newer 28mm pwks would have to be "profanity removed" loads better than the 20 year old mikuni 26s??

yeah pwk's are a better carb, easier to work on etc, with porting 30mm. The stock carbs can be used on modded motors you will just loose some top-end.
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banshee

[bansheebuzz]

single is the only way to go in my opinion, will produce alot more torque of the bottom and make the bike alot more ridable.

I tested the banshee i had down the quarter with twin 34mm flat slides and also a single 34mm flat slide setup

both carb setups produced same top end of 90mph, both doing high 13.7 passes.

[4stroker]

i have gone with 33mm pwks and v force 3 reeds for my heavily ported RZ motor...

they should arrive in the mail in a week or 2, let you all know how it goes! hopefully it doesnt go bang!

thanks, mark.
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2002 SE Banshee