[TR] Tachometer calibration

[Dave Connitt]

I had a problem a week ago or so where the tachometer in my TR4A was making a lot of screaching noises and figured it was a dry bushing from 50+ years of life. I figured I would try fixing it myself. What could happen??
As I was taking the tach apart, I noticed there was a black paint mark on the drag cup that was connected to the needle so I just noted where the mark was with the needle "at rest" against a point on the frame and continued taking it apart. From reading information on the topic, I knew that that little spring attached to the drag cup was very delicate and was very careful to hold the tach upside down and lift the "drive" end off the drag cup, not the other way around. Once I had separated the frame, I just set the drag cup half on the bench in that same orientation. 
The other half of the "frame" contains the magnet, shaft and bushing which connects to the tach cable. Sure enough the little shaft that the magnet was attached to was dry as a bone.  The bushing and the shaft looked OK so I figured it just needed cleaning and lubrication. After careful cleaning of the shaft and it's bushing with some carburetor cleaner, I lubricated it with a few drops of 3-1 oil and put the magnet and shaft back into the bushing. MUCH BETTER.. The shaft would spin very smoothly by hand. 
The assembly went well. Keeping the frame half with the drag cup upside down, I dropped the "drive" half back on the drag cup half and reinstalled the 5 little screws. It took a little trial and error to get the two pieces together where the drag cup would move smoothly around the magnet. The two pieces can only go together one way so all the screw holes line up and there are two keying pins to help hold the thing in alignment but there still is a slight amount of movement so it's not a quick assemlby.  Next, I aligned the mark on the drag wheel to the spot on the frame I found before taking it apart at pushed the needle back on the shaft. By the way, I didn't put any lubrication on the drag cup bearings.
The tach is now running smoothly and the best of all, no noise.
The only problem is that somehow the tach is running about 500 RPM below what I think it should be. At idle, the needle just floats off the pin from "0". I think I can get a pretty accurate reading of the actual engine RPM using a Harbor Freight digital tachometer ($39.00). My problem is that I am wondering which way I should turn the needle on the drag cup? I have to take the tach out of it's case to move the needle as I have to hold the drag cup while I twist the needle on it's shaft and don't want to end up taking it apart 10 times..
One way I thought about doing the "calibration" was to pick a RPM, say 2,500. With the tach out of it's case and the cable out of the firewall, have someone hold the throttle at 2,500 RPM and paint a new mark on the drag cup. Stop the engine and move the drag cup to the new mark and install the needle pointing at 2,500 RPM. At least it would be accurate at mid-range. 
What do you guys think??
Dave Connitt
Thanks,
Dave Connitt
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