Someone from Torrensen reported that the Tachometer for the 2GM20F was the same as the one used for the 3GM30F. A sailor in another forum reported that the number of teeth on the ring gear of the 2GM20F is 97. Now if both statements are correct the number you are looking for is 97. There is a way to find out the number of teeth and that is by removing the starter as through the opening you can observe the teeth. Paint a tooth with a dash of red paint so you may identify your starting and ending point. Activate the decompression valve and turn the engine slowly by hand counting the teeth until you have gone one complete revolution. Your Tach documentation should specify the range in number of teeth that can be adjustable. If 97 is within that range I would give it a try and see what kind of readings do you get.
The shop manual deliniates a test for the sender;
1) Disconnect the leads and with the engine stopped check the resistance between the terminals . Resistance should be 1500-1700 ohms.
2) Run the engine and measure the alternating current voltage between the terminals. The voltage reading should be at least 1 volt.
3) If sender fails either test, replace the sender.
Good luck.
