I thought this was helpful information that Matt at Tyga provided me in a recent email exchange.

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You willneed to increase the main jet if you blank the power jets.

Work out the area of the main jet. Keihin jets are marked in metric sizes. A #150 main jet for example is 1.500mm. A #152 is 1.525mm. A #158 is 1.575mm. etc. etc.

The power jets work the same way. A #75 power jet is 0.750mm

So add the two together and you get total flow area.

Say #150 main and #75 power jet.

Main Jet Area = 1.767mm^2

Pwr Jet Area = 0.442mm^2

Total Area = 2.209mm^2

Now work backwards:

Sqrt(2.209 / Pi) x 2

Required main jet = 1.677mm

This corresponds to a #168 main jet, but always err on the safe side and go a size or two bigger than this.