What is your starting procedure?

Pump up fuel, throttle to first indent, turn key and choke ?

Yeah, that's pretty much it.... Pump the fuel primer bulb up until it's hard, advance the throttle slightly so that the throttle butterflies are slightly cracked open, turn the key to the START position and engage the choke at the same time..... hold that position until the engine fires/starts at which time the key is to be released to the ON position.

With that procedure, if all else is as it should be, the engine should start within six (6) revolutions.

Spark plugs should be Champion QL77JC4 plugs gaped at either .030 (long life) or ,040 (strong spark)..... Compression should be in the range of 100+ and fairly even on all cylinders..... The spark (with "all" s/plugs removed should jump a 7/16" air gap with a strong blue lightning like flame, a real SNAP! NOTE that the air gap of 7/16" is important!

Remove the carburetor face plate so as to make sure that the electric choke butterflies of all three (3) carburetors are actually closing when the choke is engaged..... Your explanation indicates that the engine could be initially attempted to start in a cold condition with no choke action being applied.

Also a posibility...... Keep in mind that a cold engine will crank slightly slower than when it's warm, and the engine must crank over at least 300 rpm in order for the stator to generate the proper AC voltage to the powerpack capacitor... a slow cranking engine will emit weak, erratic, and perhaps no ignition/spark.

A engine that sits for a long period of time always results in fouled carburetors, however, you say that the engine runs fine for the rest of the day once it starts That sort of eliminates the carburetors..... however, I'd suggest that you clean and rebuild the carburetors, and if nothing else... clean the six (6) high speed jets that are located horizontally in the bottom center portion of the carburetor float chambers way in back of the drain screws.

That "running fine" mention.... the engine has a smooth idle and no high speed hesitation?