JW51, you are 100% correct, the manifold design is terrible for even distribution on an inline six. V8s are much better, but the intake type will determine just how even the cylinders are. I know race engine builders alter the jetting in the four corners of the carburetors to help even them out.
My manifold on the 250 inline six has dual carburetors spread apart to feed three cylinders each. I can see the difference in fuel ratio's with a very slight turn of the mixture screw of one or both carbs, so the gauge is very responsive to changes. I recently installed the stock intake and single Monojet carb just to see how it compared to what I had been running. The Monojet was a stock rebuild by me that came on the engine, the fuel ratio was off the gauge lean. It ran OK but not great. I raised the metering rod till the fuel ratio came down below 17:1. It ran pretty good, but I could smell the rich exhaust and fuel mileage was way down. I have three of the same carburetors, so swapped them around and all three ran about the same, jetting was very close to each other. The manifold design was creating the lean conditions, the outer cylinders run much leaner then the inner ones. I didn't need to check plug color to know what was happening, didn't run it long enough to color them anyway. Last weekend I went back to my dual setup, The truck is much happier now! An infrared heat gun reading exhaust manifold temps will show you the same thing, the outer cylinders are always hotter.
