Nice looking jeep! I have driven M38A1s with the stock Carter carburetor and with a Solex. In my experience, there is not much difference in performance. Since it lacks a conventional choke, the Solex equipped jeep is much harder to get started if the vehicle has been sitting more than a few days, or if it is cold outside. Personally, I stick with the factory carbs.
If you want to make sure you're getting the most out of your jeep in it's original configuration, be sure that the valves are set right, that the points and plugs are clean and gaped correctly, and that the air cleaner is clean.
One thing I've found that makes a noticeable difference in performance is timing. The factory spec for timing is usually fairly late for most military vehicles so that they can run on low octane gas (68 is the minimum spec listed on the M38A1 data plate, if I remember correctly). Since the lowest available these days from the pump is 87, the timing can be advanced from the factory setting. I usually do it by feel, advancing it in small increments until I get a little spark knock when under load at a low RPM, say, going up a hill. Then back off on the timing a little until you don't have any spark knock. This is your optimum timing setting.
After you have made sure all of the above listed items are correctly set, there are a few more things that you can look into. As Zout pointed out, friction from engine and drive train oil creates significant drag, especially when cold. The previous owner of my M35A2 (IornHorseTheGeneral) put all synthetic oils in the engine and drive train. I acquired the truck in the summer and didn't think that much about it to start with, but when winter came around, there was a big difference! The engine cranks over quicker and starts easier due to the viscosity of the oil. The transmission shifts easier, and there is much less rolling resistance when cold. If I were using one of my old jeeps or trucks as a daily driver, it would get synthetic oil in the engine and drive train.
Locking hubs would be on my list too. That is something that can easily be returned to original.
I'd do the above listed things and drive it a little while. If you still need more power, you can look at the intake and exhaust systems. The factory oil bath air cleaner works great, but is somewhat restrictive in its design. Peter Debella offers a kit that retrofits a dry paper cartridge into the stock air cleaner housing for WW2 jeeps. I don't know if there is one available for M38A1s or not, but it would be worth asking. Also, a little larger diameter exhaust could help some, but to me, a jeep should sound like a jeep when it is running. Different exhaust might make it sound different.
One thing that is often overlooked, be sure to keep your tires properly inflated... One or more low tires make a difference.
Good luck!
Joe
