Brad, the tach in my Laser(502Mag/mpi) got erratic as you have described, but it did not affect in performance in any way. I sent the tach in toFaria for a rebuild so I could keep original gauge with Powerquest logo. The idle RPM always shows high I think, but this year it started skipping around in the 3000-3500 range while cruising. I pulled the dash and rotated the cylinder select switch back and forth through the range and it seemed to fix that problem. (must wipe/clean contacts in there)
The switch is on back of tach, just make sure to get back to original position. If I have more problems I’ll replace the tach. Not real happy with the rebuild.

Guys,

I validated my engine's RPM with the TechMate over the weekend. It seems to be reading about 200RPM low at about 3500RPM, and maybe 300RPM at 4500. Is there any calibration that can be done? Maybe a gain metering for needle position or whatever?

I have not done the selector switch "wiping" yet. I plan to, just haven't taken the time to remove the gage panel to get to the selector switch yet. I did, however, do the old "gas gage knuckle tap" test and, wadayano... It worked! Over the weekend, we were out cruising, and it was reading significantly low, so I tapped on the gage face with my knuckle, and BANG, the needle jumped up about 7-800RPM, with no change in physical throttle position or audible engine speed change. What do we read into the results of this very scientific experiment?

Thanks. Brad.

It could be as simple as a loose wire connection on the back of the tach.

Griff,

I'll definitely be inspecting connections while I'm behind the gage panel.

Thanks. Brad.

What tachs do you have?

My Livorsi tach's read about 200-300rpm low on my 496HO's.

Emailed Livorsi and they said they can rebuild/recalibrate them for $88/Ea.

Concept,

Gaffrig.

Thanks. Brad.