SC nonstarting

I have a '54 SC,SN 8038729 that has always been a good starter until this year. I have had issues with damp and wet weather because of carbon core a wires on a magneto ingition.
I have good spark at the plugs,they are gas fouled due to flooding from the engine kicking back against compreesion when the starter is disengaged.Compression is good in all cylinders.
I have had the carb apart twice,it is clean,thought the issue might be the needle not seating, or a leaking float. The float checks out ok ,did the suck test and the needle is working properly.
I was taught that the initial mixture screw adjustments were to seat the screws and back off 1&1/2 to 2 turns to start. I have done this to no avail,played.A little both ways,checked the plug gap,and closed.them up a touch,now have the occasional fire ,but still have flooding when starter disengages, have tried with the fuel completely shut off.
Question. There is a raised boss on the bottom off the carb just behind the choke,looking at it from the bottom, it looks to have had a very small expansion plug(frost plug) staked in place , which is missing ,in this areas,there is a very small drilled hole into the throat behind the choke plate,should this be open to none filtered air?I
The firing order is 1 3 4 2, what be the normal position of number 1 plug on the mag cap? According to this unit it is at 4 o clock. The oil filter is in the way to pull the cap the check the rotor with number1 on top dead center.Mag has counter clock wise rotation .I thinking some one has change plug wire position on me.
What is the proper plug gap?
What is the proper mixture screw adjustments, and what sequences?
Point gap of mag if the cap has to come off to determine rotor position.
Thanks for you help, Dane.

If it's kicking back on startup you have a timing issue. I would look over the magnito for a problem

Mag cap may be moist or carbon-tracked causing "backfire".

Also, coil insulation "back in the day" (and, likely that of the cap and rotor) wasn't what it is now and it is NOT recommended to used anything but copper-core sparkplug wires with the old magnetos.

Jon,it's not kicking up trying to start and.it doesn't back fire,the kicking back occures when the starter disengaged and what cylinder is on compression at that time.
Also should add ,it is Zenith carb, also occurred to me, that if the wires were crossed it would either backfire or pop through the exhaust, it doesn't do either.

The missing plug in the bottom of the air in of the carb is supposed to be a sintered bronze plug. It's purpose is to let excess fuel to drain out, and to prevent dirt from entering. It is not essential to run, not sure where you would find one, maybe in a master rebuild kit or rob one from an old carb.

The dripping fuel following a failed start is completely normal for an updraft carb. When the engine fails to start, any fuel on it's way up the intake comes draining back down once the vacuum stops. Your mixture adjustments are close enough for an initial start.

I think you have an ignition problem, points out of adjustment, firing order wrong, carbon tracked/wet cap, weak coil. Do you hear the impulse mechanism clacking when cranking the engine?

Kicking back like that sure sounds like a timing issue to me.

The impulse works as is working as it should,healthy blue spark ,jumps about 1/8 to 1/4 inch

Impulse works,healthy blue spankings 1/8 to 1/4 inch

#1 is at the 1 oclock position someone has changed the gear that drives the rotor for yours to be where you say you need wires to have copper centers for best results. Does the rotor line up with the tower when it impulses. I would get #1 on compression then change the gears so the rotor will stop at #1 before it fires then get wires in correct order I have several mags and tractors start perfect

I'm a old guy novice so not very smart so my thoughts maybe way off base. Pull a plug wire, crank it over, as soon as you see the spark stop cranking. Trace the wire back to the cap and and now you will know the next wire to have spark. Check the cylinder that wire leads to, to see if it is coming up on compression, if it is not find the cylinder that is. That is where the next firing wire needs to go. Then you follow the firing order to check your wire. Ofcourse the normal way would be to take off the cap and and watch the rotor and send that wire to the cylinder coming up on compression. I believe you are not firing on the compression stroke. Wires all in wrong place. The rotor does not care which post is number one, its going to send spark to each wire in rotation, the spark must go to the cylinder on its compression stroke. But I am sure you know all this. Wishing you the best on your project. Remember anytime spent with your tractor is quality time. gobble

Possibly a cracked distributor cap, with all your damp weather it has accumulated moisture. Spray the inside with WD-40 and then after a few minutes wipe out the excess, worth a try.

Is it just rocking back from compression or firing back from being timed too early? Change the wires to copper core for a magneto.

Set the gear on the C mark & leave it.

There is room to get the cap off on every one I ever worked on.