This is John’s Bugeye, which came to us with an erratic misfire under high load. The symptoms were the same as we’ve seen when valve guides are too tight (we’ve written about this in the past). I thought you might like to read about our diagnostic process. We make mistakes too.

Under extreme heat and load the engine would break up and misfire. Once we pulled off the road and the temperature dropped just enough, then the valve guides would relax and the engine would run beautifully once again. You could drive the car around town without this issue surfacing but once you were under the highest loads (on the highway, downshifting to climb a hill, for example), only then would this problem resurface. So we did what any good shop would do… rip off the head and send it to the machine shop for a proper milling of the guides to ensure adequate clearance for the valve stems no matter how hot they get. The machine shop verified that the guides were indeed too tight, and they honed them out to make sure the problem wouldn’t recur.