...From My Experience, also check:
1. Choke! Check closing and opening. When COLD, the Choke should Never be COMPLETLY Closed, it should be Open ONLY a 1/8" Minimum.
2. Lean, open Idle MIXTURE screw(s) 1/4 turn. BEFORE attempting this, do a Test by Pumping the Accelerator 2-3 Times, then Start the Engine. Fuel Pressure?
3. Weak Spark!...Inspect, Cap and Rotor. Check and re-gap Spark Plugs. AND, 'If' just one Spark Plug WIRE is NOT making a good connection (as in NOT Snapped onto it's Plug, completely) This will cause Hard Starting, and possibly, a back-fire 'Pop'. Take the Time to Inspect ALL 8 Wire Plugs, at the Spark Plugs and at the Dist. Cap!
4. Test for weak Coil.
...since the Engine ran well until recently, indicates a problem increasing over time. I go with Spark Plugs!! I run Autolites #25 gapped a .045".
As Larry advised, a Bad Ground WILL cause a 'Weak Spark'. Last, it's been a known fact that a Starter Motor that Draws 'Excessive Current', WILL ROB Voltage From the Coil...again, Weak Spark, but Only upon Starting! Check this with a Volt Meter AT THE COIL, as You run the Starter!! There should be a 12v 'Bypass' Wire from the Solenoid to the + side of the Coil. This bypasses the Ballast Resister, Only when Starting. Check for it...there should be a Minimum of 10.0 Volts on that Coil.
"...How can You have a Good 'Positive', when You do NOT have a Good 'Negative'!?"
Good Luck with it!
MJ