Odd 350 problem.

The 56 Chevy grain truck has an odd issue. Years
ago I had a 350 out of a suburban put in it. It has
run like clockwork since.

About a week ago it would not start on an
intermittent basis. I replaced everything in the
distributor when I saw it had an anemic orange
spark. It runs like a scalded dog now, but still has
the problem.

If you start it and keep it running for the first 15
seconds you are set. If it dies in that time it won't
restart. It has an automatic choke that isn't set just
right so it's pretty normal for it to be fussy those first
couple of seconds. It used to always fire back up if
needed.

When it won't start I can fix it if I crack the line
coming out of the manual fuel pump. I let maybe
three tablespoons of fuel run out and then tighten it
back up it will fire right up. There's an inline fuel
pump right before the pump. If it were a fuel issue
there wouldn't be fuel there. I took the cleaner off it
and found the accelerator pump wasn't spitting out
fuel, so somehow it isn't getting fuel but it has it at
the outlet of the pump. No blockages in the line. I've
never seen a situation like this you could fix with a
"blood letting".

Which needs replaced - the fuel pump (kit)or the
fuel filter?

I would think maybe an outlet check valve in the fuel pump not working correctly. When you release the pressure it can open and work right after that.

I agree, JD, and I would "T" into the fuel line and connect a low pressure fuel gauge, and verify no pressure, then replace the fuel pump if that confirmed it.

Electric pumps sure do work nice on those older trucks that sit between uses, hit the "prime" button and fill the carb before cranking.

MUCH less wear and tear on the starter and battery.

A couple things to check:

Fuel pressure and volume. With the electric pump, you should get a steady stream of fuel, no air, with the pump running. When cranking the mechanical pump should give a strong shot each time the pump cycles, about 3 to 5 PSI depending on the pump.

If there are air bubbles coming through, check for rotten hoses between the pumps and the tank, or possibly a defective pump. Watch for leaks at the vent hole on the mechanical pump.

And take a look at the filter in the carb fitting. For whatever reason the engineers decided the filter needed a spring loaded check valve in it. Sometime, especially when they sit, the valve will stick. You can either take the valve out, or get a filter without the valve.

I also wonder where the fuel is going. Even if it sat for a month or more, there should be enough fuel in the bowl to get it started. Could be in internal fuel leak, especially on a Q Jet carb. They had plugs that leaked, especially if it has a history of backfiring through the intake.

Which carb?
If q-jet does it still have the small filter that goes in the fuel inlet?

Float could be getting stuck due to corrosion on the arms and/or pivot. The "blood letting" drops the line pressure and the next pump pulse bumps the float loose.

There's also a check valve in the accelerator pump circuit that can stick.

Me, I would crawl up in there and at least pull the top off the car and see if I had trash or corrosion in there.

Two questions. Why is there an additional inline fuel pump? And have you checked the filter on the carb itself where the fuel line connects? It's a sneaky little sucker.

It's only got the mechanical pump. 1950's didn't have anything other than mechanical.