Carb Fact Sheet   (by Larry aka Half_Crazy)

The straight skinny on carbs.

Let's start at the beginning of the air path.
The Airbox: The airbox on the LC/C-90 is one of the absolute best airbox designs I have seen to date. It draws air from as far away fron the engine's heat as it can. It has 6 liters of plenum area and a huge almost 70-square-inch filter. The engineers designed this airbox to resonate at certain frequencies to help with cylinder filling. The rubber boots that attach the airbox to the carbutetor throats have two nice velocity stacks on the opposite end of them. 
The only issue with the airbox is that the actual air inlet is tiny. This is done for sound deadening and to help the motor make as much low-end torque as possible. It works. A stock LC makes strong low end... the problem is the torque peaks early and falls off. By opening the air inlet (like with an open top K&N air filter) There is more air available than the motor can draw. At 6000 RPM the air this motor can flow is less
than 170 CFM and the airbox will flow well over 250 CFM.
It has been said that the skins of the bike (fake tank, neck covers) restrict air to the air inlet. While it may appear that this is true, there are plenty of openings under there and I have never seen a bike run any different with the skins on or off. Yes, it sounds different, but that's it.
As you may know, I ran my LC with just the velocity stacks on top of the carbs for a while (No air filter or airbox). Yes, this allowed hot air off the motor directly in the carbs, but the entire area under the fake tanks was open, so there was nothing to trap the air. Anyway, the point is that the jetting did not change at all. #167.5 main jets. However, the mixture did change at part throttle/bottom end because the airbox is a whole lot more efficient at keeping the air velocity high with its large plenum and resonating frequencies. The bike LOST torque without the airbox, like 6 ft/lbs.

After much experimentation, you can't beat the stock airbox and open top K&N air filter. It's as good as it gets on one of these bikes. Most owners of other motorcycle brands WISH they had an airbox this well engineered. That's a fact.
How are you gonna hang 6 liters of plenum space and velocity stacks out the side of a Harley? Where will you put your right leg?

Moving on to the carbs: These things are so absolutely simple. It's an air passage and has 3 fuel passages. There are jets that meter how much fuel is flowing into the air stream as air passes through.
The problem: Each of the 3 fuel circuits bleed across each other. They all flow fuel all the time. This makes someone tuning the motor want to comit suicide. You have to anticipate what will happen when you make an adjustment to just one fuel circuit.

NEEDLEJETS: Just a funny shaped block of metal with a hole through the center. That's it. No one needlejet is better than another. Factory Pro says their needlejets are patented "high dispersion"... Funny, I have replaced like 50 of those things and they look just like the OEM or the replacements I made. Just a tube.
The difference is only in what they are made of... or... will they last?

 If you are fortunate enough to have the stock, OEM, Factory Suzuki/Mikuni needlejets... KEEP THEM! NO replacement is any better. They will last FOREVER.
The object of making replacements is to replace those that are worn out by aftermarket needles that come in jet kits from Dynojet (they make Cobra, V&H, Thunder Mfg., K&N, and a couple of others). If your needles have never been replaced with aftermarket, you will never need new needlejets.

No one's replacement needlejets work any better than anyone else's in metering fuel. To say that this one or that one delivers better mileage or performance is total bullshit. The question then becomes how long will they last? The BEST needlejets are the ones that came in the bike. That's a fact.

Jetting/Tuning: There are many ways to skin a buck. What you are looking for is the best compromise of all three fuel circuits to get the most seamless and strong delivery of power to the ground. You see people say to use:
#32.5 pilot jets, Mix screws ar 3-3/4 turns out
#35 pilot jets, Mix screws at 3-1/8 turns out
#37.5 pilot jets, Mix Screws at 2-1/2 turns out
IT'S ALL THE SAME!!! You put in a bigger jet, you close the screws some... The amount of fuel flow is the same. Now... why use different sizes? To keep the screws close to factory spec where they have good spring tension.
When you go past 4000' elevation I like to use the #35's because the screws are less sensitive and its easier to keep a handle of the fuel mixture. Pretty much personal preference only.
The larger your pilot jet the more sensitive the screws become. With 37.5s when you turn the screw 1/8 turn it matters, with 32.5s 1/4 turn doesn't do much.

Now for the kicker: Changing your mix screws by 1/4 turn (37.5's) will make your main jets off by a size. That's right, the pilot circuit feeds fuel at full throttle too.
The point being that tuning these things is not so simple as it may seem.

Many times people will write to me and say, "My stage 3 runs great. It pulls like crazy, is smooth, never a hiccup, and gets 40 MPG consistantly... but when idling I get a little 'poof' out the pipes every 6 to 8 seconds. How can I get rid of this?".

My answer would be not to compromise the way it runs over an occasional 'poof'. Call it character. How much time do you spend idling?

I designed my jetting packages (G-Man sells them now) to be the best compromise for all riders, work with many exhaust systems, be easy to tune, easy to install, and return decent fuel mileage. I designed my replacement needlejets to be as close as possible to an exact diplicate of what came in the bike when new. Much much research and R&D went into these. They were tested many many miles on several volunteer's bikes and re-done through several generations using different materials, different platings, different bore sizes, etc. before being sold to anyone. Once for sale, the buyer could rest easy that if there was ever an issue I would make it right at no cost to them. I eventually replaced most of the early generation parts with later designs (for free) which I didn't mind at all. I had ONE customer that was unhappy with what I did. I gave him a FULL refund, inculding shipping, even though I knew he was unhappy because his bike had other issues. I was the one who had to sleep at night. My guarantee let me sleep well, knowing I had taken the high road.