Motorcycle Repair: Ignition troubles 1980 CB750F, air fuel mixture, few more seasons

Question
Hi Rich, Hope you can help. The bike takes forever to turn over (over 5
minutes of cranking). Once it does start it runs and idles fine with decent
enough power for a 30 plus bike. Trying to troubleshoot the starting problem
I have replaced the ignition leads and plugs, no improvement. The interesting
thing is if I take the ignition lead off of cylinders 1,2 or 4 the bike will stall
(normal). But if I take the ignition lead off of the 3rd cylinder the idle hardly
changes. If I take the plug out of the cylinder, ground it and turn it over it
generates a spark so it's not the coil. Something that might be related is if I
engage the choke when starting it does nothing to the RPMs but if I engage it
when it is warm it increases the RPMs. Any suggestions would help since it's
my first bike and would love to get a few more seasons out of it.

Answer
Drew, your problem is in the choke system.  For some reason, it's not enriching the air fuel mixture when the engine is cold.  The engine needs a richer mixture when cold to start and once it's starts to warm up it needs less fuel. It's possible the choke plates aren't closing all the way, or the choke passages are plugged.  You can confirm my guess by pouring a very small amount of fuel onto the air filter and if the bike fires right up, I'm right.

Regards
Rich