only seven cylinders
 

I have no spark on cylinder number four in my 72 mod. 350 slc.
Have new plug wires, new points, new rotor, new rotorcap. Starts easy warm and cold but use a lot of gas. Tried with another plug wire but still no spark. Any ideas.....???

I suspect the distributor cap terminal for No. 4 cylinder is defective, consequently no voltage is being distributed to that spark plug.

How do you know you're getting no spark on that cylinder? It has to be either the cap, the wire or the plug.

I would have to agree with the above statement. I have seen caps that had cracks that were not evident to the naked eye, but would prohibit a particular cylinder from firing. Good luck

Thanks for your advice. I will change the cap but it's only when the engine idles. When I rev the engine I got spark on all cylinders.

Soooo, the pertinent question, already asked is, how do you know you don't have any spark at idle?

I connected a timing gun to the wire. No light when the engine idles. Only when I rev it. Light in the gun on all the other cylinders.

I'll wager you are fouling the plug and the firing voltage is too low to trigger the gun. Pull the wire take an old plug and open the gap some. Put it in the wire connector and set it on the valve cover and run the car.

Yes!! There is spark. It's a yellow spark. Looks weak. The car only have 90000 miles in 36 years. I think it still have the original ignition coil (blue Bosch) and points. Should I uppgrade some of the ignition parts? New coil? Pertronix ignition?

Do the same test to a different cylinder and compare. The same ignition system is creating all the sparks to the activation level required. The voltage to fire a plug out of a cylinder could be as low as 1kv. A plug in a shorted/fouled cylinder could be 3-5kv and the normal idle kv would be around 10kv.

Without a scope to verify, I would do the comparison and then look at your plug. Switch it with a different cylinder after comparing them.