[elhigh]

Too much carb is pretty bad for mileage, yeah.

If you're running wide open all the time for racing it's less of an issue as the air flow through the carb gets up to useful velocities. But when you're just motoring you need a more modest setup.

With too much carb, you're wasting some of the engine's energy pulling a vacuum against the inside of the throttle butterflies when you're tooling around at lower speeds. The low velocity gives poor fuel atomization (already mentioned) so you have to run rich just to get enough fuel to burn in the cylinder to keep you going down the road. It's a cascade of problems that all stem from too much carb and result in bad fuel economy. The rich condition might not be as bad as all that if the carb is set up carefully, but I bet it's a little smoggy when it's running.

You watch. You put the SU in and your torque peak should move pretty far down the rev band where you can use it more conveniently in daily driving. If the Weber was set up correctly you're probably going to feel the loss of some top end power, but the engine should be a lot more drivable. If you've been running smoky, switching in the right carb should reduce that.

For what it's worth, the Weber 32/36 is a recommended aftermarket replacement for my truck, which has almost literally twice the displacement your engine has. If it's a good carb for an engine that size, then you have to be either revving the nuts off a smaller one all the time to make use of the Weber's capacity, or else it's just more than a daily driver can use effectively.