Heres the deal.
My tacho and my shiftlight don't operate in unison.
When My shift light lights up, (set at 7500) the tacho is barely reading 6500. This may have had something to do with why I killed my old motor. used to try and screw it out to 8000. (I had cam and valvesprings to those revs) But to my frustration it went flat at 7500. I wonder how many revs it was pulling?

Anyhow, I tried another tacho today. But it was sticky, and erratic.
Then I noticed on the back of the one I pulled out, that theres and adjustment screw in the back. Has anyone had any experience with adjusting one of these tachs?
Even with the shiftlight, I dont like the possibility that I'm going to rev it past my selfimposed ceiling of 8000. I spent 2K building this one, and if I break it I'm giving up on it.

I guess the hardest part is getting a reliable signal. With a 4 cylinder the signal frequency will be 4 signals / rpm.

I'd use a CRO (scope)
Using the following information:
400Hz = 100revs/sec

Multiply both side by 60 for rpm
24kHz = 100rpm
240kHz = 1000rpm
2.4MHz = 10,000rpm

First benchmark the current settings,then start fiddling

Interesting to see how you go.

I have a 1200 tacho and a CRO. I'll try it out some tuning next weekend, and write up an article.

I had thought of knocking the shiftlight back to 5000, rev it up till it lights. and then monkey with the adjuster screw, and see if i can make it read 5000. Then go back to 7500, and see if it's reading 7500 when the light lights. I need to know which way to turn the screw.