Jim Sohl
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I'm going to make a guess here, but I've seen similar things in the past in engines with mechanical oil pressure gauges, or some other gauge that responds quickly to slight changes in pressure. The culprit as Judson suggests, is simply wear. Possibly, end-play in the crankshaft. Try to sight from above the engine while it is idling, across the crankshaft pulley to a mark or reference point on the floor. Without moving your head – line of sight – run the throttle up and down. It won't take much change in RPM. At the point where the RPM changes from up to down or vice versa, in a worn engine, the crankshaft will move in or out of the block by roughly the end-play clearance. In a new or fresh engine, end-play will be quite low, 5 thousandths or less, and that much movement is difficult to see by just sighting across the crankshaft pulley. However, worn engines will show more end-play, sometimes 15 – 20 thousandths or even more. This much movement is relatively easy to see, at least with some practice. This much movement, if indeed you have excessive end-play, can effect oil pressure, especially at the time of change to or from run-up to over-run. There is no cure other than a general rebuild that includes use of appropriate thickness thrust bearings on the crankshaft to bring end-play back into spec. If the pressure change is slight, something like 5 - 10 percent of the highest reading (hot) or less, don't worry, just be informed and warned that track-day racing is not advised on an engine showing end-play that can actually be seen. Good Luck!
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