1956 640 starts hard question

Let's do some searching with some simple tests.

How are the plugs? are they gapped right and, are the wires new or old? Are they fouling or burning rich or lean?

How is the fuel tank and fuel valve? are they clean? and is the fuel line updated?

What is the wet and dry compression? Are you burning any oil? or are there any gasket and seal leaks on the head, covers, carburetor or manifold?

When was the last time the valves were adjusted and cleaned?

How old is the wiring? regulator and coil?

Is there any wobble to the rotor in the cap?

How is the battery? old? new?

6V or 12V system??

When was the last time you rebuilt the carburetor and checked the float adjustment and cleaned the needle valves or replaced the gasket, and looked at the choke plate or air intake plate?

Let's get some answers to these questions and let's problem solve this.

Replacing old wires with new wires is always a good thing and easy to do. One wire at a time. Make sure the battery cables are the heavy 4 gauge, and use heavier 12 and 10 gauge wires and connections. I do it just to be sure.

A carb rebuild is always good but good to THOROUGHLY clean it and soak it in lacquer thinner for a day, all apart in pieces, and if you have a calibrated wire set, GENTLY clean out the jets or replace them. Check float adjustemnt, and all moving parts. Blow it out with a can of compressed air or 120 psi compressor after you soak it in lacquer thinner, brass brush out the bowls and covers, make sure mating halves are mating flat with no debris. blow out the sediment bowl connection to the carb and ALL ports and jets. Resoak it in lacquer thinner until there is NO residue or coloration, or fine dust and debris that settles out. Make sure you soak it in lacquer thinnner bowl so that it is completely submerged.

Readjust needle valves. Turn in all the way, and back out 1.25 turns for starters.

Double check point gap and adjust timing and idle. Also make sure the lead wire into the cap from the coil is properly inslulated.

Use the NAPA distirbutor cap and rotor with brass contacts. Best one out there.

AND last but important clean out the air filter oil cup, and soak the brillo pad material in the air cleaner in gas and beat it against a rock and clean it and keep cleaning it until it is clean.

Clean air, clean carburetor, new wires and contacts, fully charging baterry, tight manifold seals and no pin holed manifold, and clean fuel lines help. Squeeky clean.

Summary:

Fuel delivery ok?
Air delivery ok?
Ignition up to snuff? clean bright, and tight?
Recharging system checking out ok? Battery holding a charge?
Clean points and plugs? gapped right?
Timing on the mark?
Valves adjusted?
Tight seals?
Fresh oil? and filter?
Colling system checked and functioning right?

Overkill I realize but good to be thorough.

Sometime 6V systems just start to wear down and 12 is acceptable as long as it is not masking hidden problems of poor engine maintenance or performance.

When were the valves last adusted?