Honda-SOHC

Obvious check, starter clutch sprag, it's engament and ability to turn the motor.

There was updated material specification for them  .... should be in there on later build apparently  ... no way to really check parts. It should though engage instantly, not hear it dragging without crank turning (slow moaning / screech) and fire it pretty promptly.  Not helped if battery is low or not up to scratch in giving the sprag a good "hit" to get bite and rotation.
If slow from battery, don't keep doing it as it won't help lifing.

Fork seals, check for weeping as they can get a bit gummy.

Rear brake caliper gers alot of water as underslung .... give pedal a good squeeze, then roll it back and forth to asses release.  Not hard to service, with two slide pins and lubrication of pistons etc. It will need reasonable and regular inspection if used year round, though not a show stopper.

Tires, check for alignment as wheel beeds are very well toleranced and resistant to popping the tire up onto correct place to run true. They're tubless too.

Must run antifreeze all the time, not water.

Generally tough and reliable overall.

Great torque from motor, like a diesel bus  don't need to rev much for pace. Feel very different to many fours in character, can feel a bit top heavy with full tank, though steer very neutral considering they're not race rep type bike. Easy going to ride, you've got to load the pegs and muscle the bars if you really want to get them moving fast.

Nice bike to own.

Not owning a 500, I've not that specific experience to offer.

A big BUT though, genuinely I've never had problems with plug "fouling" on four stroke engines, that's across all bikes and cars I've looked after since early 1970s.

Another observation, the Boyer ignition is very, very competent in it's delivery. I've no problem working with the std system, believing it to be mechanically almost peerless in it's design, application and component specification .... the Boyer i can see by comparison does offer tangible benefits if that's chosen by the owner though.
Among those benefit appears the ability to fire more adverse mixture range more of the time with exceptional repeatability.  This seems to support a more optimum idle mixture that may give benefit in any residual unburnt component/fuel that may be affecting the fouling determination this topic is about.

Very loosely, NGK plug heat range on these engines could be approximated to continuous load under wider throttle for extended periods of time.
 A 7 giving ROUGHLY decent coverage for 7000 rpm running.
Likewise, an 8 for predominately covering an 8000 rpm consistent load type riding. Etc, etc
Basically its a balance of number of actuation per minute against heat clearance from the engine, ambient air temp also playing a part in air cooled motors too.

From a wheel installation point of view ... usually I roll the wheel in, put chain on sprocket (not usually split) then roll wheel back and put spindle in that side, push through to brake plate, hold the spacer in there to fully locate wheel and then complete the whole thing by holding wheel rim at top as it's still located on chain side to facilitate alignment through right side of swingarm.

Mostly worked on 750 and always with them on right side of bike.

Amusing that in a original Honda 750 manual, a double page spread illustrated with exploded build diagram on RHS, picture of rear with it on RHS, also another picture installed opposite  LHS.

As Julie notes though, it doesn't make a difference from a clamping point of view, the spindle is "floating" and will effectively impart tensile load the same from whichever side you put it in.

You could file the flats to give a little better chance with a smaller sized socket firstly.

The WD doesn't usually do much as they aren't usually stuck in conventional "corrosion" terms, more that they've been tightened too much to distort the filter housing and just making the torque required too high to turn easily.

The housing will rock clock and anti clockwise within it's pegged location, just a little. Turn if fully clockwise first, then while putting pressure on the bolt to undo it, try and turn the housing WITH the bolt, until it reaches it's furthest anti clockwise position.
Then hold the bolt and turn the filter back fully clockwise again, then repeat this to get the bolt moved. It's just fidgeting it (doesn't move much) but gradually ratchets the bolt to help it loosen. Once the pressure comes off the housing the bolt will usually move easily.

Are the moving taper part of the valve brass or rubber tip Dave ?

If brass, you could spin them in lathe and use a emery stick fine finish to dress the taper again.

Adjusted floats all within 1mls now no leaks at carb rubber it was still popping check exhaust copper washer it was leaking now no popping and air screw set to 1 1/2 so I think it was exhaust washer

Good to get to the bottom of it, probably beneficial to have that odd one out float chamber now matching the others too.

The idle airscrew adjustment does allow for difference in individual flow within Honda set-up routine, being the real icing on the cake in regards to carb balance and smooth low speed running.

This is exactly what modern fuel injection systems do for smooth idle rpm, modulation of cylinder mixture by monitoring of the cylinder speed individually through ecu. Honda did this way before by observation of rpm and singular adjustment of idle circuit to get them smooth.