The first thing to do, just to say you did, install new plugs.
I have chased problems needlessly just like this and the problem was solved by replacing perfectly good looking plugs!
Choke assist can be deceiving, it can remedy both lean condition and weak spark.
Wouldn't hurt to take the carb off, ease the bowl off, take a look, clean it out, clean the main jet with a stiff wire. Just spraying and blowing will not remove dried ethanol residue, it has to be scraped out of the jet, down to bare metal without actually enlarging the hole.
But, usually a clogged carb will not be intermittent, it will run or not run. Fuel delivery can be intermittent. Water in the fuel can be intermittent. Look for rotten fuel lines, pieces of rubber can flake off the inside and clog the fuel pump, cracked lines can suck air. If there are 2 tanks, the change over valve can suck air.
The spark... Removing either wire should make a drastic difference. At idle removing one wire may kill the engine, but up to speed it would make a noticeable difference. If removing one wire does not make a difference, check that wire for spark delivery. I like to use a test light, it won't make the light light, but it makes a safe insulated handle (with the other end grounded) to stick inside the boot and observe the spark. Running up to speed it should easily give a 1/2 inch or more spark. Remember, you have to catch it in the act to prove anything.
If you do find weak or no spark, open the top cowling and unplug the kill wires from the coils, replace the cover, take it for a trial run. Just be aware once started the ignition nor safety switches will not stop the engine! If that solves the spark problem, look for a shorted wire to ground, or some have a diode in the wire between the coils. A shorted diode will kill one or both coils.
While the cowling is off, be a good time to clean out the cooling fins, and check the intake bolts, be sure they are tight an no sign of vacuum leaks.
Another possibility could be valve problems. A loose valve seat can cause intermittent compression problems. But save that for last! Not sure if that has solid or hydraulic lifters, need the model and serial numbers to research that. If solid be a good time to set them by the book procedure, don't guess!
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Today's Featured Article - Listening to Your Tractor - by Curtis Von Fange. Years ago there was a TV show about a talking car. Unless you are from another planet, physically or otherwise, I don’t think our internal combustion buddies will talk and tell us their problems. But, on the other hand, there is a secret language that our mechanical companions readily do speak. It is an interesting form of communication that involves all the senses of the listener. In this series we are going to investigate and learn the basic rudimentary skills of understanding this lingo.
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