Question..Does this truck have the seat fuel tank or saddle tanks.
Check the fuel lines for cracks..especially if it has the seat tank, check the fuel line under the truck where fuel shut off is. Also, check fuel lines to fuel pump for cracks. It may be sucking air in a bad fuel line.
Mine had a bad fuel line on the fuel shut-off valve under the seat tank, but did not leak fuel when not running, but sucked air when trying to start.
If it was running before, it should not be a timing issue. These things will start when out of time, just no power or fires back through the carb.
My practice when approaching one of these old GMC is to first, check fuel line and tank for bad fuel and leak in fuel lines. Disconnect fuel line at carb add a short piece of hose to fuel line put other end of hose into a coke can or plastic bottle. Then crank engine over a few times to see what come out. Do what is necessary to correct any problems replace fuel line into carb.
I automatically replace the battery to 12volts negative ground regardless of what is in there. Buy a new resistor, coil, rotor & distributor cap, points and condenser. I even carry a spare distributor that is rebuilt just in case.
I carry roles of automotive Red 10 & 12 gauge wire, clean all battery cables inspect them and replace what is necessary. Run new Red 10 gauge wire from starter to amp meter then Red 10 gauge from amp meter to switch, then new 12 gauge wire from switch to resistor, then 12 or 14 gauge wire from resistor to "+" terminal on coil, 14 gauge from "-" terminal on coil to distributor. check that insulator in distributor is okay and replace point and condenser, replace rotor with new and cap as new, make sure plug wire are copper core and no cracks or bad plug wires.
I have several GMC L6 engines to deal with, (2) 228s, (2) 248s, (1) 270 and (1) 302 the latest. I've had Chevy 216s and 235s.
The procedure is time consuming, but it is easier than spending a lot of time trouble shooting problems. The cost is less then $100.00 at my local NAPA store.
I always put a see-through fuel filter between the fuel pump and carb to catch any water, crud or rust flakes and I can see what is happening and fuel color.
It is about 2 to 3 hours work, but well spent time in the long run.
The only exception is if the person I am buying from gets into the vehicle and starts it for me. ...
I have also learned to replace the 2 rubber flex fuel lines with new ones. They are cheap, a pain to replace but worth it.
The valve from valve cover to intake manifold is an early type of PVC, but not for smog control.
It is part of the vacuum system for windshield wipers and in some cases Hydro-vac Brakes and Vacuum shift rear axle. It does help the engine to suck up crankcase fumes/vapors.
I hope some of the information helps.
Good luck again and happy motoring.
GMCPic
