When I had my shop back in the late '70's, I had a Jeep 6 cyl. come in for a tune up. He had bought the Jeep brand new and it had never had any parts replaced in it's 40k miles, except for the exhaust manifold. It had already been replace 4 times due to cracks! He'd had tune-ups naturally, and oil changes, but no major parts. He had had the dealer do all the work in the past, but, was never happy with the way it ran (gutless) and was willing to give me a try. I started the tune up by pulling the plugs and doing a compression test. The plugs were wiped! Just ice white and melted. No wonder he was going through manifolds. This thing must be running too lean or something to make it run that hot in the combustion chamber. The coolant temp was still in the middle range on the gauge, so it wasn't as though that was the cause. I double checked everything else and completed the tune up thinking maybe they had done something wrong on the previous tune-ups at the dealer. I finished up, took it for a test drive, and sure enough, this thing was a dog. No power even at full rev. I'm scratching my head trying to figure out what was the story here. Compression was excellent at 150psi across the board. I decided to recheck the timing and it was spot on. I revved the engine and the timing advanced like it should........whoa, wait a minute here. The timing was going the other way! Dang, I missed that the first time. Instead of it having spark advance, it was having spark retard. What the heck? Well, I figured it out. Seems Jeep used Delco distributors that year and used the same one as some GM 6 cylinder, EXCEPT the GM version turned the other way! Well, the Jeep factory either ordered the wrong distributor to install in this Jeep, or, whoever built that particular distributor installed the advance parts backwards (my guess). I flipped everything around, reinstalled the distributor, fired it up, set the timing, saw it now actually had vacuum advance, and took it for a drive. Wow, what a difference! This thing now ran GOOD! Customer was thrilled, I had a customer for life, and he never had to replace any more exhaust manifolds.