14A Starting Engine Question

The starting engine on my 14A will only run just over an idle with a lot of choke and it pops, sputters and will make a loud backfire often. I had trouble with the original gas valve on the dash leaking so I bypassed it with a gas hose and a new brass shutoff valve prior to the issue with the starting engine. Thinking that it wasn't getting enough gas I changed it back to original with another gas valve I had on the shelf. Now its back to the steel factory fuel line with good flow and it the engine still won't run correctly. So I pulled a new rebuilt carburetor off the shelf and installed it. No change and the engine still won't turn the diesel over fast enough.

Having several tractors with starting engines over the years, I have always known with the temperamental ones it's either fuel or spark. Is it possibly the magneto? A few times it has straightened out after 5 minutes of jacking round with it, and it will take off and run properly.

Any ideas?

What's the read on the spark plugs? fouled with excess fuel? or dry and light brown? and whose plugs are they?
Could very well be the mag, do you have another to change out?

I will pull the plugs out and check them. I may have another mag and will look for it.