Would expect the vacuum balance to be consistent across 4 carbies for all rpm. idle and up to 1/8 throttle opening, balance dependent on mixture screw and throttle valve balance settings, and for larger throttle openings, only throttle valve setting(mixture is set by jet sizes). Unbalance could be air leak, possibly carby boots, or fuel flow restriction.

Cylinders 3 (right rear) and 4 (right front) do not share an ignition coil, so an ignition fault unlikely. A cylinder 1 & 3 or cylinder 2 & 4 problem would be a possible ignition fault. Do the four plugs fire when resting on the rocker covers. NCs can be a little temperamental to start after standing for a few weeks, their chocke ( fuel enrichment) system is a crude. Was it hard to start after standing between race meetings or only at the last meeting.

It's always been quite easy to start, never had much issue except at the last round. I've just changed all 4 carbie rubber boots, so that shouldn't be the source of an air leak.

All 4 spark plugs fire when laying on the rocker covers, I've swapped spark plugs multiple times without any change.

I've just realised that I haven't cleaned the mixture screw orifices, I only made sure they were all equal. I'll give that a try, there may be something stuck, but it doesn't explain why number 2 was playing up, and now cylinders 3 and 4 don't want to play.

You had the carbies professionally cleaned but continue to suspect blockages? If fuel bowls are full and jets clear then fuel should flow. Fuel tap is vacuum operated or HRC modded? If fuel or easy-start is directly sprayed into each carby throat will it start on 4 cylinders?

Because electrics aren't really my strong suit, I've been concentrating on doing/checking what I can which is basically the mechanical things such as carbies, valve clearances (as well as swapping plugs, leads, coils, etc), but I'm fairly confident the carbies are nice and clean. The fuel tap has been modified so is no longer vacuum operated and I've confirmed that fuel is flowing through.

When the bike was running on 3 cylinder I was able to get the fourth running by spraying some carbie cleaner down the throat. Now that it's only starting on 2 cylinders I'm struggling to get it to rev high enough that it doesn't stall within seconds. I'll have another look at it in the next couple of days and trying swapping things around.

Cranks well, what is battery voltage, have you tried another battery?

Yep, it cranks well and shows 13.59v. I haven't tried a different battery yet but might pull one out of my roadbike to check.

Do you get full battery voltage at the ignition unit and ignition coils when cranking.

Unfortunately I had to admit defeat and take my bike to a mechanic who is very familiar with VFR's and RVF's. I've already missed round 1 of the 2019 race season and I wasn't getting anywhere fast with solving the issues I've been having. Round 2 is only 3 weeks away so I'm hoping they'll be able to sort it out quickly. It'll cost me more than I'm happy with, but not having it running was costing me the fun it gives me when I can go racing.

Once I have the bike back I'll update this thread detailing what was wrong so it'll hopefully help others in the future...most likely to warn others not to do whatever I did wrong :-)

After all the stripping and cleaning of the carbies, swapping coils, leads and plugs, the mechanic ended up finding the cause...one of the pilot jets was blocked! Honestly I couldn't believe it after the carbies had been ultrasonically cleaned, had heaps of cleaner sprayed through all the jets and were even soaked in acetone for 5min, there was still a blockage.

They replaced the #38 pilot jets with smaller #35's as they were fouling the plugs easily. It's now starting, idling and revving so much cleaner than it ever has.

Thanks for all the help, especially from Magg, it's been much appreciated. Now it's time to see how she performs on track. I missed round 1, round 2 is in 2.5wks and I can't wait :-)