This is one of the drawbacks from eliminating the factory air cleaner and riser from the exhaust manifold. Although cold air is ideal for an engine intake, the moisture can suppress combustion. With the factory set up pulling a partially heated air charge from the riser it can counteract it a little. Try using premium fuel and re-tune the ignition to be a bit more aggressive (another thing to take note of is the increased vacuum the Weber creates - it causes the stock distributor to over-advance timing so low down power is always a bit off. You either compromise the timing to get it running decent where you can, or get the distributor modded to match the overall timing curve for the Weber)