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I was thinking TB's from a 600
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Then you were thinking
wrong. Most "Super Sport" 600s use a 38mm diameter throttle body. I know you might be doing the basic math and you're thinking that 600cc divided by 4 equals 150cc per cylinder which sounds close enough to 125cc. Makes you think you could get away with swapping two of those 38mm Super Sport 600 throttle bodies for your two EX-250 carbs, but you can't. It would be an air-flow disaster.
Every one of those Super Sport 600s have an ECU-controlled secondary butterfly system. This is the reason the design engineers can put 38mm throttle bodies on 150cc cylinders. The ECU-controlled secondary throttle bodies guarantee good low engine speed (off-idle) performance so the engineers can use a relatively gigantic (38mm on a 150cc cylinder) throttle body that makes really big horsepower at high RPM.
There is no secondary butterfly control available in MegaSquirt I, MegaSquirt II, Microsquirt, or MegaSquirt III.
Even non-Super Sport bikes are being equipped with ECU-controlled secondary butterflies. Two examples of "Sport" level bikes with secondary butterfly systems are the European EX-250 fuel injected version and the Ninja 650R. These bikes don't really need them but I think that the motorcycle engine design engineers are "spoiled" by having ECU-controlled secondary butterflies as a tool to fine-tune the bike's performance (especially idle and off-idle).
The European EX-250 fuel injected model uses 28mm throttle bodies. I used one of these Euro EX-250 throttle bodies in my 2005 EX-250 FI conversion. I removed the secondary butterfly system and they work great, but that's because they're sized correctly (28mm for a 125cc cylinder).