A drill is not the best choice for opening up a hole that large, grabs and is not very accurate. The best thing is an actual boring head and a mill. Assuming that you don't have that, I would use a reamer for small incriment sizing. You can buy an adjustable reamer that will do several sizes. I bought one for a job specific purpose from someone in Europe I found online for about $10.
Carb boring help please?
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