Help me understand this 21mm Mikuni

Hoses are vent tubes, second cable is choke.

http://www.mikuni.com/fs-carburetor.html
hopefully the info you need is on that page. I wish I could be more help than that.

the vent hoses are there so excess gas can flow out all over your bike. you don't need them, but they help to route the gas away from your leg. this usually only happens if your needle and seat gets clogged or your floats stick.

the choke you can do without. just make sure it's not stuck on all the time or your bike won't run. i think you can remove all the parts for it.

On your carb, the choke is actually an enrichment valve. There is no plate blocking airflow like a normal choke. Yours works by allowing more fuel when the valve at the carb end of the second cable is raised. There is a brass barrel that acts as the fuel valve and on some carbs you need the barrel in the hole to avoid a huge air leak. Not ALL need the valve installed but MOST DO.
You can easily use a thumb shifter for a choke lever and mount it off the grip area to save space and allow a shorter cable.

You can also just use the valve with a finger loop of cable sticking out for starting so don't let the second cable worry you too much.

as far as I understand to idea of those vents you want to ends of the tubes open and held in place above the carb.

but I am not entirely sure about that, some googleing also turned up instructions to have them plugged into brass nipples attached to the air box.

hopefully you can find the right carb on that page, I am not sure exactly what one you have so I couldn't.

Just don't forget you're getting a LOT more air going into.your engine so be sure to jet accordingly... what works for a 14mm nt carb wont work for a 21mm carb...
You should have 2 jets in the carb at the bottom and possibly a 3rd jet... the big one is the main jet, this is the one that restricts the fuel going in above idle... you already know that... there will be a smaller one that's the idle jet and it restricts the fuel mix at idle and low speed... also known as the slow jet. This one shouldn't need to be changed if you can dial in your idle mix somewhere between one and 2 truns out from a soft bottom out. You turn the idle mix screw in until it just bottoms out then adjust it out a turn and a half, this should let the engine start and idle. Then adjust it out until you hear the engine stumble counting how many turns it takes before it stumbles. Next you adjust it in until it stumbles and count the turns. After you know how far to go each way you set the idle right between the 2 stumble points. If the engine wont stumble by adjusting the idle too rich then the slow jet is too small, same the other way, if it wont stumble going lean the jet is too big... jet this accordingly. For the main jet, this is still done best by doing plug chops until the plug is the right shade of tan.... you're donenand ready to ride.
now if it stumbles or hesitates when you crack the throttle or if it surges at cruise speed this is where you adjust the needle clip position.
The third jet( if your carb has one) is the fuel enrichment jet and only feeds fuel when the choke cable is pulled, if you're not going to run a choke cable this jet will be inactive. You may need to put a small spring above the choke (fuel enrichment) plunger to keep extra fuel from going into the mix when not needed.
There may be a few small brass jet looking things at the carb inlet and these are the high and low speed air bleeds, don't mess with these or block them off, these may not even be removable, but these send in a precise amount of air to control the fuel mixture at idle, cruise, and full throttle. A smaller air bleed will allow for anskightly richer mixture and a larger bleed will slightly lean the mix, these shouldn't be messed with but if your carb does not respond to jet changes or if you tried everything else and it just wont tune, this can be the problem.

Now to get that carb to fit on your engine, the easiest way to do so is to get one of those " billet" intakes mzmiami sells on ebay and use a short section of 3/4" or 1" hose and 2 clamps to connect the carb to the inlet... the inlet I mentioned has an 18mm inner diameter and a 20mm outer, its meant tot the 18mm mikuni carbs but ive tested it with a 21mm dellorto and it worked great. My only problem was frame clearance so I'm swapping out the 21mm carb for a 18mm mikuni so everything will still fit in my frame without having to make a custom intake.

Looking at your lice again... your main and idle jets are in the middle and the small tube is where it gets the fuel enrichment from... if you choose not to use this (the choke for all who don't know) you'll need to make sure that plunger cant come up either by adding a small spring above it to hold it down or by installing a short cable andnlocking it out somehow. Most the time there will already be a small spring in there to keep it shut.
The vent tubes just route these so any fuel that might come out the vents wont get on you or your back tire...

Jake's also has the big intakes designed for the 18mm Mikuni carbs that should work good for the 21mm carb linked with fuel hose as Dave mentioned.

Yes... just lock it in the closed position... you will need to use the 2 jets in the middle as one is the main jet you already know about and the idle jet which is the one reign next to the main. The idle jet supplys fuel at idle and also thru out the rpm range and the main jet comes into play shortly above idle and contributes more as the throttle opens and rpm increases, the needle regulates all part throttle mixture in addition to what the idle jet is already feeding up to nearly full throttle. At full throttle the main and idle jet are both feeding unrestricted.
So you tune by getting the smoothest idle possible, then once this is done you can do your plug chops to dial in the main. After both these are dialed in then if there's any issues at cruise, then adjust the needle hight or even try a different needle since most carbs have different needles to make them compatible with several different engines.
it is tedious work to dial one in all the way but possible to make it run very well at all throttle positions.

the idle jet is just that. it's to tune the bike at idle. use the idle screw to set the idle as smooth as you can get it, then tune in the idle jet by "blipping" the throttle, meaning crank the throttle with the clutch in til there's no hesitation or stumble.

you may have to do this while riding, too. if there's hesitation or stumble when you accelerate, turn the screw in or out to find the sweet spot. mostly this happens at full throttle starts instead of just cruising up to speed.

it's a three step process between the 2 jet screws and the idle screw to keep your bike from stalling when you let off the throttle, and to take off and run smoooth throughout.

bairdco said:

the idle jet is just that. it's to tune the bike at idle. use the idle screw to set the idle as smooth as you can get it, then tune in the idle jet by "blipping" the throttle, meaning crank the throttle with the clutch in til there's no hesitation or stumble.

you may have to do this while riding, too. if there's hesitation or stumble when you accelerate, turn the screw in or out to find the sweet spot. mostly this happens at full throttle starts instead of just cruising up to speed.

it's a three step process between the 2 jet screws and the idle screw to keep your bike from stalling when you let off the throttle, and to take off and run smoooth throughout.

Click to expand...

^^EXCELLENT guide. All I can possibly add is that most of these screw adjustments take VERY SMALL changes to fine tune.
If you need more than 1/2 turn on anything but the idle speed, your jets may still be off a bit, depending on the particular carb.

Fully agreed as well... there is a difference how it behaves just idling and when trying to accelerate under load. The engine can idle really nice but hesitate or stumble when blipping the throttle or taking off from a stop if the idle mix is off either way.
The same can be said about part throttle performance and the needle setting, if it cruises nice at one speed but stumbles or hesitates when opening up the throttle from part throttle cruise. Raising or lowering the needle can fix this area. Sometimes a small washer can be installed under the clip if its too rich or lean after raising or lowering the clip setting, the washer will offer adjustment in between clip positions for finer tuning... this may or may not be necessary. Needles can also be sanded to clean up a part throttle issue but I don't recommend this unless you got a spare or 3...
First thing to do is get it installed and tuned the best you can then ask if you have any issues after you get it running. It may work just fine but there are a lot of little tricks to fine tune if there's any bad spots in the rpm range...
Definitely good advise on blipping the throttle to fine tune the idle mix tho.

i have a 22mm mikuni on my race bike and just overhauled everything for the upcoming race this weekend, so i just went through this annoying tuning process.

luckily i have cool neighbors, since i spent an hour ripping up and down the street on an irritatingly loud bike with a small screwdriver in my pocket, repeatedly stopping to fine tune it.

and i'll be doing it again at the track.

i have the manual choke lever attached to the carb, and it sticks out almost 2". at the last race i had to ziptie the lever down to keep the choke from opening when my knee bumped it.

this time, i fixed that by cutting a 1/4" piece of fuel line with a slit in it and inserting it under the lever on the plunger so it can't be bumped open.

another stupid but common problem is with the needle and seat that controls fuel to the float. i somehow put it in upside down (the pointy part goes up into the carb and the flat part pushes against the float.)

putting it in backwards will make your float stick and you'll see the full effect of what those vent tubes are for, as gas pours out all over your bike, leg, driveway, etc...