Mishimoto Electric Fans in Your 67-72 C10?

[Mike_The_Grad]

Your idle mixture screw adjustments depend on what carburetor your running. I run my original 1972 quadrajet 4mv on my 350. I had my screws at about 3/4 turn out from seated. This was way off. I chased every possibility of why I was having hotter coolant and engine temps. Like I previously posted it was due to a few things happening at once.

First off I had a big vacuum leak at the rear of my factory cast iron intake manifold. I didn't put enough silicone down to seal the intake along the rear of the engine block. That was my fault for not taking into consideration of having my engine block decked by the machine shop.
I also had a leak at my carburetors base gasket. Relatively easy fixes.

Also like I mentioned previously, burping my coolant system properly made an obvious improvement.

I learned that most quadrajet carbs respond well to around 4-5 turns out. This may seem like a lot, but it doesn't hurt to try, you can always turn them back in. Mine are set now at about 4 3/4 turns out.

Once I addressed my vacuum leaks and tuned the carb through trial and error, it straightened my temperature fluctuations right out. The engine idled smoother, responded better, and I won't say it "had more power" but it was utilizing the power it already had more efficiently. It wasn't easy, especially without a shop or a mentor, but I figured it out. That's how I'm able to hopefully provide some insight into your issue.
I hope yours is a simpler and quicker issue to fix because summers just getting started, and Vegas isn't somewhere you want to have overheating issues anytime of year.