If you want to mod the motor and think you can get it inspected, by far the biggest bang for the buck is putting a 4.0 head on your 4.2 block. Even staying with the weber, you will have a huge increase in power, far above anything you can do with the 4.2 alone.
418 miles on 19 gallons of gas. Can't beat the BBD !
Before you get too deep into building on the Weber, make sure you know what smog check shop to take it to when the next smog check rolls around. Technically the Weber is illegal on the 1990 4.2L Wrangler, even if it did pass smog. All it takes is a savvy smog tech to notice that the carb isn't stock, and you'll fail the visual inspection for smog. You have to be able to pass visual and emissions to get the smog certificate.
Just to reiterate:
Here in CA, our street legal options for the 1990 4.2L YJ are:
Sticking with the Carter BBD. Its a decent carb, but it takes alot of work and polishing to keep it running well. You cannot [legally] swap in any other carb.
Upgrading to the California Howell TBI kit. Pricey, but from my experience with it so far, its worth it.
Upgrading to Mopar MPFI kit. Expensive. Really expensive.
Engine swap to something fuel injected like the Jeep 4.0L
Smog laws are really strict in California. As new new Jeep owner wanting to mod your engine, you might want to get familiar with the smog laws. Any aftermarket modifications technically have to be California Air Resource Board (CARB) certified. This gives certain aftermarket parts exemptions for use on specific vehicles. The Executive Order # (EO#) applies to select vehicles for each aftermarket part that is approved.
Headers, catalytic converters, ignition modules, ignition coils, fuel delivery system - everything fuel or emissions related must either be stock or CARB approved in order to comply with the law for street legal vehicles.