Tuning Q after a tear-down ordeal...

[colo]

Hi guys, new member here.

I've had my current '72 C20 for about a year now. The engine appears to be out of a 1975 vehicle, likely a Nova.

The truck had a few issues under the hood I wanted to take care of:
-Pinhole radiator leak
-Rochester 2GC carb that was not in sound condition, even after rebuild and electric choke upgrade
-The 2bbl intake had EGR and other unused emissions equipment on it (my truck was non-emissions from the factory it appears)
-Water pump bearings were failing
-Clutch on mechanical fan was failing

Since I was pulling out the 2GC, I figured why not replace the intake and go to a 4bbl carb. So I took it upon myself to replace everything in one swoop, since it all involved draining and flushing the cooling system. Carb, intake, radiator, electric fan, starter (had distributor off by 180 and the engine kicked back against it and broke the nose cone off), fuel pump, fuel line, vacuum lines, thermostat, water pump, and a cheap $25 under-hood tach to assist with tuning.

Now it's two weeks later. I've replaced all the items and have the distributor back in, everything buttoned up. With the help of these forums I also took care of the resistor wire ignition issue that was left over from when the PO switched to an HEI distributor. I ran a new 12v unfused ignition wire to the cap.

Now I am down to tuning the carb and timing. I had never altered both of these components at the same time before. Also, I'm not certain if the heads on the engine are stock or otherwise. I've never had it out of the engine bay to pull the numbers.

I tried timing the engine with the vacuum advance off to roughly 4 degrees advanced as a starting point. The timing mark is jumping around a bit, about a 6-8 degree variation. So I set it as best I could, approximating the middle of the movement with the timing marks.

I took some time to try and reign in the idle mixture and idle speed on the carb but I'm having a lot of difficulty getting the engine to run well enough to get meaningful vacuum readings. Unfortunately the carb is second hand and I haven't identified the rods and springs installed yet. This is on the list for discovery tonight. The carb is from a local carb builder with a good reputation, so I don't have any reason to suspect the carb is damaged or improperly setup yet.


Should I spend more time investigating the timing issue, or hook up the vacuum advance since engine speed is unstable right now? Should I spend some more time trying to get a good idle mix and speed (and if so, any tips on getting it to be stable enough for a good vacuum reading)?

I suppose it is possible I have a vacuum leak, having replaced the intake. How can I determine if that's the issue?

This is ridiculously long... sorry. If anyone can help out I would be very grateful. Some equipment details are in my sig.