 Carb Size and the Needle Shroud Two things cause the fuel to "come up" - suction from the engine and the sonic vacuum wave from the engine. Engine Suction: The bigger the carb the less suction is at the needle jet and so the higher the RPM needs to be to achieve the same suction (and same atomization) at the needle jet. Vacuum Wave: the needle shroud catches more of the vacuum wave and so the higher it is the faster it wants to pull up gas and so the smaller the main jet needs to be to keep the fuel volume per second the same. Small Mikuni VM Carb Owners Tuning Tip If you need leaner WOT 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 change suction at the needle jet by lowering the shroud is one trick a tuner has available. It has to do with the shroud catching less of the sonic vacuum wave from the engine. Lowering the shroud brings up less gas as if your jetting was leaner. Below you can see that lowering the shroud .5mm leaned out the WOT jetting 4% and only leaned out the mid throttle jetting 2%.  The shroud portion you want to cut away can be marked black. I made a big height change on my VM18 just for testing purposes in my "2 stroke laboratory". Grinding it down was done with a rotary tool w/cutting disc.    By lowering the shroud from 5mm to 2.5mm I increased the venturi flow area by 6%. So the obvious question is "why do these shrouds exist if they obstruct air flow?". They exist to increase air/fuel mixing by catching more of the vacuum wave for a stronger suction on the gas which results in more gas entry speed which benefits atomization. This is important for street and trail bikes that depend a lot on mid range power. Other Carbs 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. Shroud Height Changing It's best to lower the shroud a little at a time, maybe .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. Mid Throttle Jetting Change Via Shroud Height Change This affects the mid range jetting in a significant way. If the main jet is correct then lowering the shroud and enlarging the main jet lessens mid throttle jetting more due to your richening the main jet to bring it back up to the WOT richness it previously had. So this means you can use this trick to lessen mid throttle jetting if you can't find (or don't want to buy) a leaner needle. And the reverse is true that you can lengthen the shroud to richen the mid range jetting.  This shows the changes in jet sizes to maintain the same jetting richness at idle and WOT.  You can raise the shroud by soldering on a copper extension or just JBWelding it in place (not recommended for piston port intake engines since the suction wave is much stronger and will have more of a vibratory effect on the shroud). You can lower the shroud with a rotary tool and cutting discs. Precautions Don't lower the shroud if mid range jetting is already lean, and don't raise the shroud if the mid range jetting is already rich. But keep in mind that you can buy different needles to change the mid throttle jetting. A leaner needle is one with less taper angle, and a richer needle is one with more taper angle. Click here to see available needles for carbs. |