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Have a look at any modern mx bike and you see the answer. As big a box as possible for the frame and with be biggest filter and cage you can fit inside. The old twin-shock frames are much easier than any modern trials bike in this respect, so it's just a couple of hours of relaxing thin sheet metal work, for example 0.75 mm aluminium (or as thin as your welding skills allow). Try finding a cage and filter from any mx dealer and you are almost there. The cost for all this is......very relevant for what you get, in terms of looks and (often) performance. Perhaps the difference to your wallet can be justified, after all...?

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As we are talking Fantic here, not disc-valve engines, the bigger-is-better rule applies. Regarding Aprilia, SWM and others with Rotax disc-vale engines, you are right, they are less sensitive to airbox volume because of the dampening effect of that long inlet tract and the precise timing of the disc.

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I had a 303 fantic years ago with the air intake on the side, riding upstream it was very easy to get water in the box. So I blanked the holes off and drilled two holes the same size in the top of the box.

It was terrible,  big flatspot and a lot less torque. I then cut the tubes or trumpets off the original holes on the inside of the box and glued them onto the new holes. This made a huge difference.  So those tubes act as a baffle or restrictor and taking them off made the bike worse not better. On the original design when the side cover was fitted the other end of the tube was very close to the other side of the filter box. I got the same effect by glueing the tubes at an angle so they were close to the back panel of the box.

On this basis I don't agree that more air is always better on a trials bike, it certainly wasn't in this case.

 People were making larger air filter boxes for air cooled Yams but they needed re jetting to run properly. So it may be more complicated than just making the box bigger.   

Finally Nigel Birkett used to reduce the air Intake on the Tyz with duct tape which made them easier to ride. So I think you need to try one first, it's by no means guaranteed to be better because it's bigger.

Edited by baldilocks
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