I used to have a '43 GPW so am familiar with the L-head but what you say about the YJ not going lower in RPMs is interesting. I have a '95 with MPI (I think your '90 has TBI) and am pretty amazed by how the computer automatically adjusts the idle speed to keep the engine from stalling. If I let off the gas slowly and put both feet flat on the floor the computer takes over and it will crawl up pretty steep, rocky slopes at idle. It seems to be able to vary the idle between 500 and about 1200 from what I have seen. If I let off the gas too quickly the computer does not take over fast enough and it stalls. Also, the ´95 can crawl in 2wd at idle on the street without stalling, just letting the clutch engage smoothly. BTW, the L-head is great, I think the low horsepower keeps the wheels from spinning, allowing it to maintain traction where other jeeps just spin the tires if the driver is not really good at being easy on the gas.