Oil ratio?
First, drain the carb of gas if you can, and let it refill with fresh gas. I found in my experience that when I left a bike sit and it was just warm enough the bowl will fill with gas, and evaporate the gas leaving oil behind, and then more gas in, more oil left. It would feel like it was impossible to start if the fuel was left on in certain weather.
Also examine your tank a bit? Caster oil falls out of gasoline at low Temps. Depending on your oil type and weather your oil could be settling out in the carb and tank, giving you just too oily gas. Shake the bike back and forth a bit to mix up the gas before you attempt to start if you think that your oil is settling, run some fresh(er) fuel to the carb and give it a go.
If it's not your fuel then carb is going to need help. If it suddenly got warmer were you live and ride, the carb being tuned in for the cold will run kinda blah... You mentioned not touching it after getting the idle right. Could be clogged? Check for metal bits in the carb bowl if needed.
I had a problem like yours, really did the run down for a leak. Turns out I had small metal bits and dirt like stuff in my carb bowl. It made the bike run very bad.
Filter? How's the air filter look? Sometimes if your too rich or too much oil (black spark plug) the engine spits back and coats the filter in oil. This is especially bad with the Dellorto clones with the cloth/paper/screen filter it usually comes with. It litteraly soaks up oil and chokes the bike out on wide open throttle.
Maybe take the filter off for a moment and try to start, if it looks like it could be oily on the inside of the filter then it might be just that. Even my lawnmower has this problem when the dust caught up in the filter. Same with atvs and many other vehicles.
I do recommend getting a new filter if yours is dirty. Or get carb clean and clean it out. Wait a few minutes to let the filter dry out nicely before you try starting.
Sometimes it's just not where you think it is. Spark plug connected right? Gotta love forgetting to hook that up and try starting... Pass the inhaler please.
Check how tight it is on the plug. You certainly pulled it off to take the engine apart right? Double check that just in case, maybe the spring clip inside turned sideways