Jet kits VS shims.

[Red01]

The 2" mod applies to the 1G B12 only.
From Fast Larry's site:

Intake Modifications
The stock opening in the airbox is very restrictive (only 1-1/2 the area of one carb throat -- I've seen bigger on some very small engines).  Adding another 2" hole in the airbox cover may allow for as much as 10-15 hp gain.  Substantial main jet changes are necessary though.  K&N airfilters (#SU7593) are a matter of choice.  The fact that they don't require rejetting proves that they don't increase airflow that much.  They also let larger particles through than the stock paper elements (something they don't advertise); this is probably the worst possible thing for engine wear, but may be worth it for racing situations where you do frequent engine rebuilds.

Adding 2" hole:

Remove seat and tank.
Remove the airbox cover.
Using a good perimeter type hole saw, drill a 2" hole on the side of the cover opposite the existing hole (You'll see a flat spot on the cover).
You (I think) can buy an velocity stack for the new hole (if you want one) from your dealer.
You'll need to rejet the mains:

You can remove the float bowls from the carbs while in place, but it's much easier with the carb bank removed.  See pilot screw adjustment section for details on carb removal.  Remove the screws holding the float bowls in place and pull off the bowls without fouling the floats inside.  This is very hard on #3 carb with the carbs on the bike, but can be done.  Again, altogether easier if you replace the screws with allen headed jobs.
The mains are the jets lowest in the float bowls and in the middle of the bowl too.  They will also be stamped with a 102.5 number.  Remove these with a Mikuni jet tool or flat blade screwdriver.  Install new jets and torque to 0.8 lb-ft.
Depending on altitude where you live, you'll need a main jet anywhere between 127.5 and 132.5.  If you use dynojet jets, use a #126 (I think); they're different size/shape of orifice.  Remember it's best to jet rich if you can't find out the optimum one on the dyno; your engine will thank you if you don't run lean at full throttle and high rpm.
You'll definitely need to readjust pilot screw setting and may need to shim the needles for best performance.

[CWO4GUNNER]

I need your opinion on intake and exhaust theory because I am going to attempt a stage 2 modification for a 4-stroke single cylinder bike where no after marked kits are available except exhaust. This bike engine stock burns the correct light brown spark plug color in pretty much all ranges of the throttle circuit under load with an after market exhaust already installed. When the stock air filter is removed or even the air box exposed during operation, the engine burns hot with the spark plug burn very white during no-load testing only. My basic question is isn't the fact that this engine's lean burning response to de-restricting the air box a clear indicator that there is still HP that can be unlocked by enriching the carburetor circuits (pilot, needle, and main jets) free lance? And given the low cost in doing so wouldn't it be well worth it since the engine is a bit of a dog.

[Red01]

Yes & yes.

[CWO4GUNNER]

The bike make is a 06 125 Yamaha Vino scooter which does 45 MPH wide open on level ground and 30 up the slightest grade with RPM dropping too fast. The community experts tell me no-way as they are predominantly into making all modification to the CVT transmission, changing rollers and tensioner springs and reduction gears to make things easier on the engine. A few have holed or melted their pistons after air box mods attempts (K&N), without follow-up mods to the carb. I am not looking for more speed, just better sustained speed and power. I am almost out of warranty so I think its worth a try if I am careful. I plan to use the Dynojet kit changes I did to my Suzuki Ozark 250 single as a model to figure the ratio difference I will need for the 125 jet sizes and needle shim hight as both are Mikuni. I would appreciate and thoughts or recommendations you may have, please.

[pmackie]

Your on the right track, but remember, for these smaller bikes, you are better to start off a little rich. If holed pistons are a potential problem, you want to start with higher octane fuel as well. Because this is a scooter, the choice of main jet is likely the most important, as I assume it is usually WFO most of the time.

If cost of jets is going to be a concern, you may be able to get someone to drill out the stock main jets with a numbered drill set. Once you determine what percentage increase you need, increase the "area" (dia x 3.14) of main jet by the same percentage. You can also solder them up and redrill.

Can you increase airflow to the cylinder to help keep it cool? More horsepower always means more heat, and you need to keep the little bugger cool.

Just some thoughts...

[CWO4GUNNER]

The carburetor is a Mikuni BS26, main jet #97.5, pilot jet #22.5. My hope is that jet and pilot jet sizes are standard and interchangeable between Mikuni carburetors and that they are not custom for a particular model carburetor so I can just order the jets. The engine has a full time mechanical fan that blows pretty hard. I will also order an air box I can sacrifice as I will be drilling holes while increasing main jet size and adjusting the pilot jet size and adding shims to needle jet until I reach optimum burn in all 3 carburetor circuits with an open air box and K&N filter. Thanks, I will keep an eye on engine head which I hope the spark plug will be a good indicator maybe having to drop plug heat ranges also to control it.
Thanks

[Ranger]

[CWO4GUNNER]

Actually no. Although I know that either Ivan or Dale would be willing to help me on this as I have always been able to hit it off with them on the phone intelligibly, I'm not going to take up there time over basic information I can get on this site. Especially if I have no intention of making a purchase.
    Look Ranger, quit baiting me with these subliminal innuendos or Ill have to hire a long robe bearded wizard with big feet to follow you around between eternities!
   

The carburetor is a Mikuni BS26, main jet #97.5, pilot jet #22.5.

Have you called Ivan about something like this?

I understand he's big on giving out free advice to non paying customers  

[Ranger]

[CWO4GUNNER]

Facilitating the original topic. I would like to say that for me although I realize shimming is an alternative to a tested carburetor kit for the Bandit 1200, I cant see why anyone would want to go though the agony of pulling and re-synchronizing the rack of carburetors more then once to test out a given shim application or jets for that matter. The cost effective work has already been done (only $130) and to the degree that the slide needles are custom made, something you most cannot duplicate. If there were no kits available it would be different, but in this case the work involved with re-pulling to get the shims and jets correct is more expensive in time and frustration against the cost of a kit to get it right the 1st time, especially when these same folks spend the most money on the can which although necessary provides the smallest part of the benefit for the cost.

[Rocketjock]

If you're just shimming, you don't have to pull the carbs. Just pop the tops.

[CWO4GUNNER]

I have to admit I have pulled the tops off of many a single cylinder dirt bike to raise or lower the needle clip and change the jets by just loosen the boots and turning the single carb 90 degrees to access both the slide and jets from either side. But I have never tried to access anything on any 4 cyl rack with my big hands without pulling them out. I even had to install permanent plugged vacuum lines to occasionally syc because I cant fumble the vacuum caps and test hoses on and off to get it done.
   OK so help me understand, do most of you find it easy to change needle positions, change jets, plug and unplug sync hoses without benching the entire carburetor rack? If so, how so?

[Ranger]

[CWO4GUNNER]

But the main point here is you had pulled and bench the rack (normal) to do the work. You diden't somehow fast finesse a short screw driver above and below the mounted carbs with one hand while slipping in shims and jets with the other while looking away at the TV to watch Monday night football. I'm pretty dam good with tools, I just wanted  a reality check on this, thanks.

OK so help me understand, do most of you find it easy to change needle positions, change jets, plug and unplug sync hoses without benching the entire carburetor rack? If so, how so?

Honestly, I find it easier to just set it and forget it  

Seriously, I pulled the rack, drilled the plugs out and set the screws, popped the floatbowels to set the float height and swap jets, flipped them over and did the needles.  One thing I did do was put my "Made by Blade" vacuum block on there so the only thing I need to do is connect lines to the block, out in the open.
I'm still running the stock airbox w/o any holes drilled
I come from the old school of thought when it comes to engines and carbs:  Set it up right the first time and don't tinker.  If it's running right, it's time to ride.

[Red01]

If you need to get to the bottom of the carbs for float bowl removal and/or drilling the EPA plugs out, the carb rack will need to come out.

If you're just shimming the needles, the rack can stay in. The carb tops can be taken off with them in place.