Had a 3T starting engine that didn't want to start.Turned out the valves were hanging up.Easy to check by taking valve cover off.While in there make sure they have proper clearence.
Had a blown head gasket once too.
Just my two cents

lots of great info here...along with Fuel, Air, Spark...
-i would add timing of Spark, as well as Compression. the Spark has to arrive on time, within a certain window...as well as having a fuel mixture that is compressed and ready for the spark to ignite..
-once these basic requirements are met....something will happen....

Since Bvaleri has had the engine running previously, lets not mess around with the magneto or timing.
I am afraid that will just compound the issues and make this a much more difficult process.

Pulling the valve cover is a great idea, at least you will know they are opening and closing.
A compression test would be in order too.

I still suspect there is an issue with vacaum leak somewhere or the float level is too low. It sounds like FUEL issue and he has said previously, he had good spark.

Long range diagnosis is like shooting skeet in the dark, near impossible to connect.

CTS

I'd agree, almost certainly fuel/air issue. I'll keep messing with it pretty positive I measured float height correctly and it moves freely on the pin. Hopefully some needle adjustments get it working but I'm thinking air has to be getting in somewhere just need to find it.

Just did some tinkering and fuel was the problem. Pulled the top and the bowl was dry filled it a little got it to fire it died right away but could then here the gas filling the bowl from the tank. Stopped messing with it for now because my choke plate won't stay sealed now so I can't get it to start again. Rebuild kit didn't come with a choke plate or rod and I'm not having any luck finding replacements.

Float level is critical.
Bowl should be FULL without running over the top of the bowl and flooding the system.
Again, when needle is closed (do not overtighten, you will mess it up - gentle to stop), then open 1 1/2 turns. Adjust from there. At least it should fire, if running rich, tighten slightly, lean -open slightly.

Ensure your gasket between the Carbuetor and intake manifold is sealed, no leaks! Each time you remove the carb, you may be damaging the gasket, you can coat with Anti-sieze compound, grease will also work. Don't overdue the coating materials!

Any leak between the Cabuertor and Intake Valves including the valves not seating correctly can and will cause the air fuel mixture to be too lean, not support combustion.

CTS

Bvaleri -
Sent you private message, check your mail box on this site.
CTS

Gasket looks good still and should be sealing hard to tell without it running so I can check properly. It may be good now if I could get the choke to seal but now that it isn't too much air for it to fire up. Trying to find a new choke rod and plate, pretty sure the interior component in the rod is broken. I pull it and it only turns to half closed and then I can push it shut but it opens back up a bit as soon as cranked. Afraid I'm going to need a new carb if I can't find those parts.

Will do

Check the fuel tank cap if the vent hole is open.