This other suggestion is so different from what I wrote above, that I decided to put it in a separate post.
Something I had on my old bracket dragster engine years ago, which really helped with tuning my large alcohol carb on a small displacement motor, was the anti-reversion function I built into my headers.
This neat mod tended to kill the reverse pulse coming back up the pipe and into the combustion chamber during overlap of the valve phase. This nasty pluse slowed down the intake charge velocity, especially at lower rpms, which made the signal (usually called "vacuum") lower on the fuel circuit. This causes a lean condition.
The way to build an anti-reversion device into your exhaust is to make a short (approx. 1 to 1 1/2 in. long) stub pipe which fits inside your main exhaust pipe, right up at the head.
You could do this by welding or even brazing the tubing onto a thin flange which would be sandwiched between the exhaust port head flange area and the flange of the stock or custom exhaust header pipe.
The added-on stub tubing should be the same size as the port (on the inside, of course), but smaller than the pipe it slips into. If the header pipe happens to be the same size as the exhaust port, then a larger end needs to be fitted onto it to allow the anti-reversion piece to fit down inside the main pipe.
Here's how it works: that stub pipe will catch the reverse pulse coming back up the pipe, in the area between it and the outer wall of the main pipe, thus killing it. It's like a oneway valve, in other words.
Yup, it's a fair amount of effort, but it works.
The other solution was mentioned above: get a carb with an accerator pump circuit. I've heard tell the Harley guys remove such a carb when going to a bigger one, and sell them cheap on e-Bay.
