Truck won't start doesn't have 12V

[pound41]

Hello I have a 1971 chevy blazer. I had to redo the fuel system then mess with carb and a fuel pump putting out too many psi. So I have been cranking and starting it a lot lately and it has been fine.
The truck has a Mallory Unilite distributor that I didn't install, I have the same unit in 3 other trucks so I am familiar with the unit. I was looking at how to test the module According to Mallory with the key on I should have 12 volts on the negative side coil. The coil is brand new. I have 6 volts on the negative post.
Any ideas on where to start. Thanks

[Rust_never_sleeps]

Bump the motor and test again?
Could be you caught it right at the point where it triggers a spark to the coil. Not sure how that type of optical trigger works, but with points, if it's sitting on the lobe, it'll show voltage like you describe.

[pound41]

What they say is it should be 12 volts on the neg of the coil. Once you confirm that. Place a Credit card between the photo optics and the reader to essentially block it. When you do that the Volts should fall to 2 or less. If not the module is open and must be replaced. I didn't try that since I didn't get 12 at the coil.
Thanks

[dmjlambert]

Show a picture of your wiring to the coil. Perhaps you have the yellow wire that goes to the starter and through the white cloth-covered resistor wire to the firewall connector, as if it's wired for a points distributor.

[HO455]

First off was the truck running before?
Does it spark when you crank it over?
Is it a nice hot blue spark?
Since you've been cranking it over a lot I would check the grounds first. From the battery to the block and to the unit. Something may have gotten disturbed while you were working on the carburetor. I learned long ago to always go back and look at the last thing I messed with when these kind of failures occur.

Good luck and keep us informed as your progress.