I just completed installing a Weber 38 DGES on my 4.2L YJ. Since there is a lot of discussion on Webers, I thought I would summarize some lessons learned here to assist others who might be considering the Weber upgrade.
1. The Weber 38 added substantial amounts of power to the engine, which was otherwise in good shape even though it had 115K miles. I mean a lot of power.
2. It is not necessary to upgrade the manifold and header to get more power with a Weber, although they will add to the benefit. It gives large horsepower gains on the stock manifld and header.
3. The 38 is not too big for the 258. Working out rpm calculations based on 5,000 rpm, your get a 412 cfm demand from the engine. Both the 390 cfm holley and the 390 cfm Weber 38 fit this requirement - almost as if they were designed for 6 cyls of this size.
4. Jetting is critical in the Weber. Get the jet kit with your carb.
5. Setup is critical with the Weber - follow the instructions exactly. Any little assumption made or step skipped will change your results.
6. As the engine warms up and is operated, your idle speed goes up. When they say to make adjustments on operating temp motors - they mean it. If you adjust on a cool engine, it will be off.
7. Webers tend to run lean after setup - what you think is a rich misfire is probably a lean misfire. Experiment with your jets to confirm and remedy.
8. The primary circuit in a Weber operates up to 2200-2500 rpm. At this rpm, the main circuit begine to come in. There is often a "gap" between the circuits that feels like a rich stumble when driving. You crack the throttle all the way open to get over it. This injects fuel from the accelarator pump to cover the gap where the primary leans out before the main is fully operating. The jet kit comes with instruction on how to remedy this - get the jet kit. (Did I already say that?)
9. You may never get the transition from primary to main to be totally smooth. It is much like when the secondaries open on a 4-barrel. The Weber is a racing carb and is designed to be operated and make its power at high rpm. If you need to get to 3000 rpm smoothly, get the throttle open.
10. Call the Redline tech support guys for questions - they are very good. You knowledge of other carbs does not necessarily apply to Webers - they are different.
11. Learn your vacuum/emissions system - you can retain everything except the warm air intake with the Weber. They sell an adapter to keep the stock air cleaner and therefore the warm air doors if you want...
12. Get the Haynes Guide to Weber carbs and the Weber tuning manual. Read them before beginning. Know how the Weber works. It will save you trouble.