I think you are mixing up drill sizes with pilot jet diameters. a 19 pilot jet is 0.019" and not 0.166"
Don't know about starting but .002 would certainly make a difference to the low speed running
Believe it or not the area difference is around 25% larger.
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My understanding is that they are the same size (as you suggest) but in situ they do not flow the same amount due to the design differences. Hence the need for a bigger jet being required to get the same effect.
You did, Amal changed the route for the fuel to take when they added the accessable pilot jet but seem to have neglected to test what effect it would have on the flow rate in an installed jet as opposed to the laboratory flow in fixed repeatable conditions. So in the lab test both the 17 premier and the 25 pilot bush flow the same but installed in the carburettor the real flow rates diverge with the extra turns the fuel has to take to get to the premier reducing the flow, hence the need to fit the 19.
No, IMHO when you put the #19 in it will still be hard to start because that's not why it's hard to start. With either a #17 or #19 you can get the correct air/fuel ratio, but you get more of it with the #19 and most bikes like that especially when idling and when quickly opening the throttle from idle. It's more likely that you have the air screw out too far, are opening the throttle while kicking, or that you have an air leak.
They don't need to be identical, unlike main jets you can adjust the air using the pilot adjustment needle. As long as you get a good idle between 1.25 and 1.75 turns out your jet is sized correctly, the 17 takes you outside that range below 1.25 turns.
I had #17 and fitted #19 on my Mk3, excellent starting, no banging on deceleration, much smoother bike down low.
So true...
