Un anglais à fait ça:

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I opened up the air box by making a 28mm hole in the other side, the original hole was kept the same size but tube & outer snorkel totally removed & bell mouths (old cut down air horns) fitted to each side. This combination works well, as I found by only enlarging the original hole it created turbulence in the air box and the bike ran like crap (wouldn't rev over 3500 rpm) & the same if you make the new hole too big.

To make a hole in the other side of the air box I had to move the CDI box to up under the seat, the wires gave enough slack so it just straps in where the toolkit usualy go's

Fitted #115 main jets, tried #112.5 but they were a little lean at full revs, drilled hole in throttle slides out to 3.5mm & set idle screws 1.5 turns out.

The bike now performs really well from idle to redline with no obvious flat spots. Before I modded the airbox I couldn't get it to rev over 8000 rpm, I put this down to not enough air flow to meet the demands of the engine with bigger jets it was just over fueling itself at full throttle, but I feel it important to add that you can go too big with holes to the air box, this creates a problem at low rpm.