New to me '69 Silverline with Johnson 85hp outboard. Has an Airguide Sea Speed tach installed. At idle, it will often read ~3000rpm, and anything above idle it will jump to the high peg and stay there. I can confidently say that there's no way this motor is spinning 3K at idle, and 7K+ RPM just off idle, lol. Are there any common failure points to look at with these things?