Hesitates off idle

Hi all you clever people, got a little issue on the bike doing my head in.

2001 CB500S Bog standard 35k miles.  Generally good fettle, loads of power, recently serviced.

Great fun to ride but...

It does this weird thing, whenever it's been idling for a few seconds and you go to move off or blip the throttle, it hesitates a bit like it misses a beat and stutters before revving.  It gets me at the lights, I'm always thinking is it going to stall, (it never does).  Also when I'm trickling through the villages using engine braking, then I go to give it a wiff of throttle it does it then, a little stutter for a second, giving rolling gently on the throttle a choppy feel.

Two other possible clues. 

-If I hold the throttle open at about 5-6k revs a standstill in neutral, I can hear a light misfire not detectable while riding.

-It starts from cold without choke and HATES choke, even after only running a few seconds, giving it choke kills it, so i have to start it with a little throttle and hold some revs until it'll idle or it dies.

Carbs have been off and jets cleaned out a couple of months ago by my local bike shop while it was in getting tyres as I had a big misfire that was traced to a leaking needle valve flooding one cylinder, (bike was laid up a while before I bought it).  I chatted to them recently about the hesitation off idle and I don't think they're interested in sorting it, putting it down to "a quirk of an older machine don't turn it into a moneypit"  Yeah I like FI bikes but I like my CB it fits me well and the power is just right.  Plus I've put new tyres on it...

I've put a few tanks of fuel through it with some long rides thinking that might sort it but no still doing it.

So I think my next step is to have the tank off and see what the plugs look like.

But any pointers before I get the spanners out??

Many thanks in advance, I'll report back on any outcomes.

Forgot to mention, it's had new spark plugs as well.

Well I had the spark plugs out,  The right looks fine a light tan but the left is quite sooty.  I don't think the bike burns any oil, so maybe a rich mixture or a poor spark?

The mixture screw on the sooty cylinder is currently 3/4 turns out, am I right in thinking turn it in to lean it out?  3/4 turns out already seems on the lean side to me but I'm happy to be corrected.

Update - Well I checked the compression, 190psi so that's fine, checked the diaphragm of the carb, no splits there, I'm at a loss, there must be something causing the black plug/misfire/hesitation off idle.

I think there is intermittent misfire in one cylinder. Try exchanging the spark plugs and if it doesn't solve it, exchange the spark plug wires and if it continues the same, you can already suspect a coil, exchange the coils and observe.

Thanks Burudi, I just went and took the plug cap off the problem cylinder, unscrewed it and inside the HT wire looked a bit corroded so I cut 10mm off and screwed the cap back on.  It does seem a bit better, it actually accepted some choke which made the revs rise where before it just bogged and died.  Not much choke mind, too much and it still dies stone cold.

I think I'll replace the ht leads as you can buy it by the metre, plus I've seen instructions on how to strip/clean the caps as well so I'll do that while I have the tank off.

I have tried swapping the brand new plugs and that made no difference.

Also Local bike shop set the mix screws to 3/4 turns out but the manual and the internet reckon start at 2 turns, can it hurt leaving them at 3/4 turns out or should I change to 2 turns out?

OK, so I had the coils off, stripped and cleaned the spark plug caps, replaced the leads while I was in there, cleaned all the coil connections, swapped the coils and spark plugs round round and guess what?  Still doing it.  

I'm starting to suspect something funky going on in the carb.  It's all cleaned out in there and the idle jet is the stock size, but I guess there's nothing to say that jet hasn't been abused with a drill or something in the past.

Out of interest, while the engine was hot and running after the ride, I screwed in the idle mixture screw on the problem cylinder to just seated and the idle speed rose slightly which I guess means that the engine liked that.  I might buy a new stock idle jet and replace the one that's in there to see if that does anything.

Edit - I was out for a ride today and while on a dual carrigeway, I tried a plug chop after 5 mins or so at 70 mph, the plug looked fine, so to me that narrows the rich running problem down further to the idle circuit.

Is it possible to suspect the state of the carburetor membranes?
Back in the day I bought some spare ones on AliExpress just in case. I don't know its quality

Hi I inspected the membrane on the problem carb, no splits there and it seems normal, though I don't have anything to compare it to.

OK I solved this in the end.

I wound in the mixture screw on the rich cylinder until it ran right, it only ran right when wound all the way in, (lightly seated).

I had the carbs off and inspected the idle jet on the rich side and it looked rough so I replaced it.

Then it ran rough so I wound the mixture screw out again to 2 turns out and it runs great now.

So the idle jet had maybe been roughly cleaned in it's past which oversized it and caused that cylinder to run righ.

I then balanced the carbs and it's so nice at low revs now.

Hopefully it helps someone out in the future.

Thanks for the feedback and glad you sorted it.