I dont have much money so i am looking for cheap/DIY/free mods on my MC21. I have mfactorywest cans and mmax derestrictor, but still havent put them on.

I want to try the airbox mod, but dont know where to place the holes or how many.

With the cans, the derestrictor, and the airbox mod, how much gain will i get? reliability? and will i need to rejet?

Thanks
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93' MC21

The cans will do little but sound a bit better.

The MMax box is rubbish, it's just something else to go wrong if you ask me, put it in the bin and do the wiresplices instead, detailed here.

A couple of 1 1/2" holes in the top of the lid (one either side) works as well as anything else.

60+bhp is available on a '21 with no other mods, so 30% over std.

Yes, you will need to rejet.
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Please do not PM me technical questions, if you can't find it on the Forum start a thread

Wire splice and holes are free, and buying a few jets to suit your setup won't cost much either. Best advice though is to spend some money at a dyno and have the bike setup by a pro. It will save you lots of money in the long run.

Changing the cans will have some small effect, but the MC21 pipes flow pretty well already.

With all three of those mods, and with conservative road jetting you should see upwards of 58hp. As you tune the bike higher and start making low to mid sixties you'll find the jetting becomes more critical, and the chances of siezure increase massively. And a sieze will cost you roughly £160 depending how bad it is.
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MC21SP Plaything
BMW F800GS Bumblebee
Triumph 9551 Daytona Big boys toy
FJ1100 Sporting relic
GTS1000 oddball

Would drilling holes in my '16 airbox make any difference? I have been told that these have smaller carb inlets and exhaust ports and this is why they don't make much power.
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it's a name, not a condition

No, since the 16 wasn't restricted. It only makes a difference on the post 89 bikes as the law in Japan restricted them, so there's power available but you need to modify the bike to get it.
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MC21SP Plaything
BMW F800GS Bumblebee
Triumph 9551 Daytona Big boys toy
FJ1100 Sporting relic
GTS1000 oddball

Opening up the air box will give a little more power. you will need to re jet through.
I cut holes in the lid of my old track 16 at llandow last year. It made the mixture weak so more air must have been entering the engine.
It will not give a lot more power, small carbs and ports as you say

Is it possible to attach some K&N cone filters to my carbs and remove the air box instead of using a normal filter? Power gains? Problems?
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93' MC21

As Jeff has said, not much better. The carbs and inlet ports will not flow a lot more air I'm afraid.

You could always try a supercharger...
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MC21SP Plaything
BMW F800GS Bumblebee
Triumph 9551 Daytona Big boys toy
FJ1100 Sporting relic
GTS1000 oddball