Evening all, long time lurker first time poster!

Anyhoo, down to business...and before I start I'm not expecting any good news to come from this thread. I expect the inevitable bad news so this is more a blind hope post than a bike newbie fumbling in the dark.

NC29-L of a 91 vintage. Picked up for a song on fleabay as a "spares or repairs" job. Generally tidy - smokes badly when warm was the description. That didn't phase me (too much to go into now but I'm sure I'll explain why later).

Obvious problem when I went to see it, cylinder number 1 (nearest the front brake lever for clarification) wasn't firing at all. Cold downpipe etc. rough as a badgers bum when revved.

Scenario A - quick fix, plug/coil/HT lead knackered.

Scenario B - not so bad, carb/fuel related problem.

Scenario C - uh-oh, no compression.

Two guesses which one it is... I was first alerted that this wasn't looking like it was going to be easy when I pulled the plug out and found a brand new one in there perfectly clean...

"Alarm bells are ringing Willy..."

Plugged in the HT lead and checked spark - yeap all present and correct. Looked a little weak though... "Just so happened" to have a spare set of coils in the lock-up so for sh!ts and giggles (oh ok, in desperation) swapped them over. No improvement.

Tried to do a compression test but the thread on my tester to large for the diddly little plugs on these bikes. Ho-hum...

Off with the carbs then. Wasn't too hopeful about this, all the butterflies were opening evenly and a smell of petrol in the corresponding inlet made me think it was probably ok. On inspection the float bowls were all full and no evidence of any problems.

Application of the palm of the hand on the inlets and turning the engine over gives suction on cylinders 2 through 4 but ziltch on 1.

Satan himself has vomited in my kettle... we have no compression by the looks of things...

Looking down the inlets into the cylinder head and turning the engine over you can see the inlet valves opening and closing, so guessing they're doing their job properly.

So, the nub of the post... what's it most likely to be before I start taking the head off or dropping the block out.

Exhaust valves?
Flipping great hole in the piston?
Rings?
Or something my inexperience of these particular engines doesn't know about...you know... "a really really really easy and cheap as chips fix..." (he prays with a tear in his eye).

As I said, I'm expecting the worse so you won't be the barer of sad tidings if it is one of the inevitable problems rearing it's ugly head.

Any advice gratefully received. Muchos gracias!

Do a wet compression test
If you google wet compression test it will give you a guide on what the problem will be from the results therefore telling you wether it's piston rings or valves

Do a dry test to make sure first so get your hands on the correct adapter

Drop the sump off, pretty sure you can peak up the cylinders from there with decent torch. You'll also notice any metal in the bottom...

Also pop the cam cover off, and turn the engine manually, see if any of the valves aren't moving freely on that cylinder.

There was fuel in the carb, but did you check that the pilot jet and main were clear and not blocked?

You're firing on the other cylinder off the coil so you can eliminate dodgy coils or a blown CDI. That's a good start...

Regarding the cylinder numbering.. (you state No.1 is rhs.)

Cylinder numbering is always left to right from the riders/drivers position on an ILE......thus No.4.

It bears no relation in most cases (an exception may be on vacuum port location)....just informative.

4 stroke engines rarely hole a piston.....that's 2T territory.

Before you pull the motor completely......you are going to have to remove the carbs and the exhaust anyhow......
see if you can rig up a means to fill the inlet / exhaust inlet channel with paraffin/kerosene...thinners is good, its does not evaporate like petrol, up to a known level with the corresponding valve fully closed.....see if the liquid is leaking past the valves.
This is better with the head removed and inverted but I have done it using nylon/plastic plates and sealant with motors in place as a quick means to verify if the motor was worth further time/money.

Yours is more likely stuck rings/corroded cylinder bores if it has stood in a damp location
An ILF engine when stopped nearly always rests with 2 pistons up protecting the bores and 2 at the bottom of the stroke allowing damp and hence corrosion into the 2 other cylinders. Depending on valve position ...corrosion may be much more prevalent on one of these two.

Even with no compression tester....a few squirts of oil down the plug hole should raise compression enough to test
the cylinder by feel.....don't get an eyeful of oil !

No compression whatsoever when dry indicates a valve issue....even the most worn cylinder will produce some compression even without rings fitted....the oil splash/supply forms some sealing.

I would check the valve clearances before taking it all apart.
You might just have a tight valve.....