A couple of things..............

I see that you live in the cooler part of the country...falling leaves. The fact that your choke is not hooked up will greatly affect the way it runs. That carb requires the choke spring type of choke. That mainfold has the heat riser system. I would put a manual choke on it.

You said that the carb is leaking...It may be from the throttle plate rod shafts. Quads are known for this. Have a good look for this.

If going with a new carb...600 cfm should be about the max, as it appears to be a stock engine other than the intake.