I recently acquired an 83 F150, 302 with manual transmission. The guy I got it from didn't have it long or know much about it. He claimed the prior owner had just put $5k into having the motor rebuilt but the truck would not start because "the timing was off"..... The motor has what appears to be a relatively new set of Edelbrock e street heads, performer rpm intake and a holley 4160. Its also running an hei distributor.

​​​​​After digging into the motor I found that the cam bolt had loosened up and the teeth on the cam gear were all chewed up. The timing chain obviously jumped teeth which bent every one of the exhaust valves. I tore the heads down, replaced the valves, put everything back together and got it running again.

I now have 150psi compression on all 8 cylinders. The truck fires right up and idles just fine but has very low but steady vacuum. The truck will idle all they way down to 500ish rpm but sounds lopey and only pulls about 5" vacuum. Setting the idle around 750rpm I get around 7-8" of vacuum. I've checked for vacuum leaks with soapy water and propane and found no leaks. I thought I might have adjusted the valves too tight so I pulled the valve covers and readjusted them with the truck running and up to temp. Backed them off until they knocked then tightened them up until they stopped and gave them another half turn. Vacuum stayed the same after all that.

I have no idea what cam was installed but I imagine it would be pretty aggressive to cause that low of vacuum. But I don't think that aggressive of a cam would idle that low. The timing set I put on has notches to advance the cam +/- 4 degrees but I had it set to zero since the timing set I took off was just a stock set. Could the cam need to be advanced? Or do I have something else going on?

 

A cam with a lot of overlap can have low vacuum. And can idle that low. If it runs good, leave it alone.

 

This. I can venture a guess because of cranking compression, but without knowing mechancial compression, its only a guess. If it will idle stable below 800 its not "that big", and big cams can still pull decent vacuum. I assume you have ignition timing set? What is it tolerating for base timing and how much does vacuum change with it?