here's my way of doing it, boring a throttlebody or carb is the easy bit, but if you have equipment making larger butterflies to fit isnt too difficult. might be of interest to someone.
1) first you need to work out the angle of the standard butterfly. this you can do by basic trigemometry. you need to know the diamter of the throttle bore (A), the distance from the base to the highest (B) and lowest (C) point of the butterfly when closed. you also need to thickness of the depth gauge on a vernier (D) which you can use to measure these. the tangent angle of the butterfly is (B-C)/(A-D).
when you know this angle, take a small wooden block and machine a wedge to this angle using a rotary table.
then screw the wooden wedge to a small faceplate (cant remember what I made it for originally)
then turn the wedge down to a slightly smaller diameter than the new butterfly size
then stick a butterfly blank onto the turned wedge with double sided sticky tape. I normally use 1.0mm thick stainless steel for the butterfly as its something I have kicking about.
then centre drill the disc and use a revolving centre and turn the disk to the required butterfly size. go gently or the heat will make the sticky tape loose grip.
you can use the throttlebody with an enlarged choke as a gauge to check for fit. you'll also need to make a note of its orientation so that you know how it will sit in the throttebody
when your happy, assemble the new butterfly in the throttlebody spindle (it may need filing longer so the new butterfly will fit). then scribe the holes that the screws need to go through.
the old and new butterfly next to each other (after drilling the screw holes)
ready for final adjustment. I normally make shims fill the gaps between the new butterfly and the slot in the spindle. in this case the slot is 2.2mm wide and the butterfly is 1.0mm thick, so I used two 0.6mm thick aluminium spacers, with one each side of the butterfly.
1) first you need to work out the angle of the standard butterfly. this you can do by basic trigemometry. you need to know the diamter of the throttle bore (A), the distance from the base to the highest (B) and lowest (C) point of the butterfly when closed. you also need to thickness of the depth gauge on a vernier (D) which you can use to measure these. the tangent angle of the butterfly is (B-C)/(A-D).
when you know this angle, take a small wooden block and machine a wedge to this angle using a rotary table.
then screw the wooden wedge to a small faceplate (cant remember what I made it for originally)
then turn the wedge down to a slightly smaller diameter than the new butterfly size
then stick a butterfly blank onto the turned wedge with double sided sticky tape. I normally use 1.0mm thick stainless steel for the butterfly as its something I have kicking about.
then centre drill the disc and use a revolving centre and turn the disk to the required butterfly size. go gently or the heat will make the sticky tape loose grip.
you can use the throttlebody with an enlarged choke as a gauge to check for fit. you'll also need to make a note of its orientation so that you know how it will sit in the throttebody
when your happy, assemble the new butterfly in the throttlebody spindle (it may need filing longer so the new butterfly will fit). then scribe the holes that the screws need to go through.
the old and new butterfly next to each other (after drilling the screw holes)
ready for final adjustment. I normally make shims fill the gaps between the new butterfly and the slot in the spindle. in this case the slot is 2.2mm wide and the butterfly is 1.0mm thick, so I used two 0.6mm thick aluminium spacers, with one each side of the butterfly.