I've just started the bike following a period of storage and found the bike was only running on three cyclinders, No.3 cylinder wasn't firing. Cold exhaust. So checked the plug and found it to be OK and sparking when earthed against the block. However, the spark plug was dry, so I guessed that the fuel wasn't getting in the cylinder. Oh well time for a carb overhaul as they may well have got varnish or gel build up following the storage!
Having stripped down the carbs everything appeared to be normal. There was no obvious blockages in any of the jets etc. All the needles, valve seats, gauze filters, diaphramps , tubes etc are good. But something must be stopping fuel getting into the cylinder from this particular carb?
It's a standard set of Carbs VDE3D B, with the standard 118r and 115f main jets and the carbs are without any modifications. (A quick question while I'm on the subject of carbs, what is the difference between the front needles, J51B, and the rear needles, J51A, as they look virtually identical?)

Before I put them all back together after cleaning them, has anyone had a similar problem or any other thoughts on what I may have missed or forgotten?

Thanks.

If it's occurring at idle it's usually due to a blocked pilot circuit. The usual (and probably only!) way to clear this is to strip the carbs and clean in an ultrasonic bath.

These narrow passages block very easily if the bike is left to sit around for longer periods with fuel in the carbs.

The front set of needles is slightly longer (leaner).

Thanks Cammo and yes it is on idle as well as throttle blip, so from your suggestion, I'm guessing it's this pilot circuit. Therefore as the Forums Carb expert can I pick your brains further.

The carbs are stripped at the moment and I'm going through everything, checking and cleaning, but there doesn't appear to be any varnish and all of the obvious holes are clear.I've removed the Pilot jets from the carb bodies, (all individual parts kept seperate for each of the four bodies), do you know where the pilot circuit runs? I planned on using some Carb cleaning fluid and putting it into the hole where the pilot jet was removed from. Leaving it for a while and then blowing it through with compressed air. However, as I'm not sure where this port goes or exits, I wasn't sure where to look to see if anything did come out, dirt, varnish etc? More importantly I don't want to blast air or put fluid into a hole and cause possible damage elsewhere!?!

So if you are able to explain where this pilot circuit runs and where to look I'd be grateful, thanks.