Small Mikuni VM Carb Owners Tuning Tip
If you need a richer jetting and the steps of main jet sizes are too big then you can reduce the shroud height little by little till the jetting is just right. The "steps" % change between a #60 and #65 (next biggest) for a small VM carb is 8.3% which is too much. For a larger carb the % change between a #120 and #125 is 4.15%. I think the ideal step for all sizes is 2.5% which only exists for the larger VM carbs. So that step up from a #60 should be #61.5 which doesn't exist. So being able to increase air flow over the needle jet by lowering the shroud is one trick a tuner has available. It has to do with the Bernoulli Effect which states that the higher the air flow velocity, the more vacuum. Lowering the shroud allows more air flow over the needle jet which creates more vacuum and brings up more gas as if your jetting was richer.
Shroud portion to cut away is marked black. Grinding with rotary tool/cutting disc. Half height shroud.
By lowering the shroud from 5mm to 2.5mm I increased the flow area by 6%. So the obvious question is "why do these shrouds exist if they obstruct air flow?". I think they exist for 2 reasons; 1) they up the range of needed main jets on small carbs so the manufacturer doesn't have to make very small main jets, 2) For undersized carbs the air velocity over the needle jet would exceed the sweet range for gas/air mixing if the shroud wasn't used. For example, a 34mm used for a 250cc racer that should have a 38mm carb. How to know if yours has too much velocity that the shroud is correcting? Just download my free Excel spreadsheet for figuring carb air velocity.
The incremental main jet sizes for the Keihin carbs are half that of Mikuni VM and since the Keihin's smallest carb is a 20mm they have enough selectability in jets to not have to resort to this trick. And with Dellorto carbs the main jets from #50 to #60 are in 6 different sizes which is perfect. So it looks like this tuners tool is mostly for small Mikuni VM owners but any one can use it if they don't want to wait for a larger main jet to arrive in the mail or the non-Mikuni VM carb has a shroud when it shouldn't for the size and type engine it is mounted to.
Don't take the above graph as gospel. It is just an approximation based on my own limited experience. It's best to lower the shroud a little at a time, maybe 1.5mm each time and then see how the jetting is. Look at both the plug and the piston top. The plug should have a medium dark color and the piston top should be blackened except for areas close to the transfers.
One last thing - this affects the mid range jetting by the same percentage so don't worry about it only affecting the 3/4 to wide open throttle settings. It actually affects the needle/main jetting from closed to WOT by the same percentage.