What is your low speed thing? I can tell you without a doubt that the keyhole carbs do not need large jets, not on a stock engine. Even an 836 kit with a ported head does not need larger jets.
How many turns out are your idle mix screws? Have you pulled the idle jets out and cleaned them?
And if you have cleaned them, how did you do it? The idle mix screw stock setting are 1.5 turns out for the K and 1.75 for the F. Out makes the mix richer, in makes it leaner.
A 4-1 pipe barely requires any change in the mix. A lot of people don't like the keyholes, but once a person knows them, they are amazingly flexible and able to take a lot of changes with out a hiccup. Being clean though is super important.
And I also am of the school that you don't drill jets. Here's why, a jet has larger opening but a smaller inside. This is to create a venturi effect. When you drill it, it loses this and fuel doesn't vaporize as well.
If a larger jet were indeed needed, I would either find new press ins, or gently tap the jet body for screw in jets. I knew a person who did that and it seemed to work just fine.
My guess is that the accel pump is not working right and when you roll on, it starts lean initially because it is not getting the fuel from the pump.
I ran through a lot of this years back till I figured it out.
The ride IS the adventure. The destination is just to get gas!
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