When I purchased my first Healey, I was able to start it and let it idle but couldn't drive it at that time for I had no working brakes. As I was going about tuning it up and becoming familiar with it, I noticed how there was a compression tester extension attached between one spark plug and a ignition wire. Obviously, someone was in the process of testing the compression but never removed the extender. Amazing how revving and idling never gave a misfire or clue of something ever running amiss. And because of that, I was worried there was some legitimate purpose for that being there. Gladly, there wasn't but noticed no difference in idling before or after. Guess that fooled the previous owner, too, obviously.