Can someone please enlighten me

[Duane.Hoffeldt]

Hey there guys, this is my first post and I figured that I would start off with more of a theory type question rather than a question about my specific FJ.
There is some theory around carburation that I still don't quite understand and I was hoping that an expert or two could enlighten me.   
This may sound like a completely silly question but why do so many members advocate increasing the size of their main jet?
I mean I completely understand that the jet meters the flow of fuel, however upstream from the jet is a tapered needle that fits into the discharge tube, and the position of this needle has the "ultimate say" over how much fuel flows through the main jet. If you put in a larger main jet but don't change the needle surely that larger main jet will still only flow as much fuel as the position of the needle in the discharge tube allows. And unless the size of the OEM main jet is so small that it begins to restrict the flow of fuel before the needle reaches its highest lifted position, fitting a larger main jet without changing the needle seems superfluous unless you ride your bike on the pin all the time.
Also, I was reading somewhere that it is incredibly important to be 100% accurate when setting the float height as the fuel level in the float bowl has a major impact on the air/fuel ratio - why is this?
I thought that air fuel ratio was controlled by the slide and needle position and the size of the main jet at full throttle.
And I am not referring to the extreme scenarios where the fuel level in the bowl is way off so that it can either "run dry" at maximum revs or where the level is so high that it simply decants from the discharge tube - the article that I read made it seem that even if you are off by a millimetre or so either side of the specified float height then your fuel air ratio WILL be affected.
Thanx guys
And PS - fantastic site

[Arnie]

Duane,

Welcome.  I am not the "Carb Guru" of the site, but the first question you pose is easily answered.  The reason so many people say to increase the size of the mainjet is because they believe that bigger is better in all things.  As you rightly understand, this is not necessarily so.

Your bike (any IC engine) will run best when all is in balance and the fuel-air ratio is correct.

Float bowl fuel height is critical since that will determine how much vacuum is required to lift the fuel up into the airstream.  It, along with jet sizes, needle position, needle taper, and "idle mix bleed adjuster" and carb sync will all affect the fuel air mix at various rpms and throttle positions.

If your bike is stock re airfilter, airbox, and exhaust, and capacity the stock sizes are usually very close to right within their adjustment range.

Cheers,
Arnie

[Flying Scotsman]

Arnie nailed it on the head.

[bcguide]

The main jet controls fuel metering when the throttle is wide open when you are wide open the needle has a small part in the fuel metering
Go the factory pro I think they have a great bit on how to jet your carbs.

[SlowOldGuy]

As Arnie said, the fuel height in the float bowl determines the amount of vacuum required to flow fuel through the metering circuits.  The carb float/fuel level, jet sizes and needle height are all adjusted from the factory to flow the correct amount of fuel and air for a given throttle setting/demand.

If you jack with ANY of the components, you "may" upset this balance (and probably will).  The main jet is only critical at WOT when the needle has reached its max height.  For a majority of everyday riding, the needle position is the most critical setting.

The Factory Pro site has very good information as well as the Carb Files right here on this site.  Do a little reading and continue to ask questions.

DavidR.