I hooked a Sun tach to my 65 when I was tuning the carb. The Sun tach indicated 850 rpm while the tach in the gauge cluster read 1000 rpm. Assuming the Sun tach is correct (i have to check that) is it possible to recalibrate the tach in the car?
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Re: Accuracy of Tach
The Corvette's mechanical tach works like a speedometer, so you can have the gain altered by changing the magnetic flux of the head magnet. This will alter the reading across the range by approximately the same percentage, but I don't know of any way to change it with the instrument installed. You'd have to remove it and take it to a qualified AC speedometer service station.
I had an electric tach calibrated once, but the way they had to do it was remove the needle from the pin a rotate it a bit. This changes the "offset" and alters the reading by the same number of revs at every indicated speed. Prior to the work the tach read about 200 revs high at idle and 400 revs high at 7000. I had it calibrated right on in the upper range, so now it reads 200 low at idle.
You should also cross check the reading with another test tach. The error you report is 18 percent at idle, which is quite a lot. My experience is that mechanical drive Corvette tachs are quite accurate with no where near the percentage error you are reporting. Also take readings at 2000, 3000... as high as you want to get an idea if the tach is off by the same percentage at higher revs.
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Re: Accuracy of Tach
When I use my dwell/tach/volt/etc tester, it must be calibrated each time. This is a simple twist of the calibration knob. Every time I use it, it requires a minor tweaking. Did you adjust yours before using it? GaryComment
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Re: Accuracy of Tach
Yes it was calibrated but is a very old unit. I'll check the idle with another unit after I get back from Bloomington. I am sure that the factory tach is more accurate than the Sun unit indicates. In any case the car is running very well since the carb adjustment.Comment
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