Carburation
A friend races a 350 Manx I am familiar with. This year he couldn't get the carburation right, and kept leaning it out. At the last meeting of the year, with 30% smaller jets than standard, it seized. What the dyno had apparently not shown was that the problem was the 2MTT magneto, which was producing a big enough spark to start the bike, and run it, but not enough to burn the fuel completely, hence the sooty plug.
I had a similar problem with my own 500 at the same meeting, but I
knew that the timing was out, so left the jet (on a Gardner) where it was although the plug was dark brown - usually it is "dark white". My new PVL ignition system needs an approximate static setting to get the motor to run, but needs a dyno for a final setting, and i hadn't done that yet.
140 main jets in a Rapide is 5 sizes down on standard. That's a lot. 2 sizes is quite a lot. I suggest therefore that you make certain that carburation is the problem before you suffer ventilated piston crowns.
Every bike will be different, and it depends on so many factors.
I have bought a real time wide band fuel air meter to take the guess work out of it.
With the standard 276s on my Rap I am down to 140 main jets and the mixture is still rich.
I have checked several Vins and they are all way to rich.
And yes I do ride the bike hard and have done approx 1500 miles with those main jets in, that included thrashing around the IOM for a week.
The only way to do the job is either buy a meter or pay for dyno time.
Don't be tempted to buy other peoples set ups (I have heard of some people selling there results on to try an cover some of the cost) because as above every bike will be slightly different.
Remember most of the advice in the books is old now and the octane rating has changed, this will have an effect.