I will add this about your tach since it sounds like it may be bad.

My experiences repairing these tach circuit boards pretty much requires replacing every component on the board to make them accurate again.

The last one I fixed had a shorted transistor making it completely inoperative. I bought transistors via ebay from a German seller because they are nearly obsolete. Then it worked, but barely. Found a bad zener diode that had drifted in its turn on voltage, replaced that. Then it worked better but was way inaccurate. I started checking resistors and almost every one was way out of tolerance so I replaced ALL resistors with modern metal film resistors which are very accurate to rated values. NOW the tach worked and was within 100 RPM throughout its range (I only checked it to 5K).

So unless you have someone who can do this type of repair, I suggest you buy a replacement board from RT Engineering and install that. They sell 2 different priced boards for your tach, the more expensive has an onboard jumper you relocate to a test position and then tweak an adjustment for 4K RPM on your tach meter. I tested this board using a pulse generator and O-scope and it is very accurate.

BUT first you may want to check your tach meter movement to ensure it is not open. This can be done with a 1.5v battery applied to the meter movement leads internally (got this info from RT-Eng web site).

I can give more info if anyone desires.