Flat spot 1982 GS550EX CV rubber diaphragm carbs

[Isetta]

Hi. Bike is 1982 GS550EX with the CV rubber diaphragm carbs. I am in the United Kingdom. It is original UK supplied bike with standard jets and airbox/filter. Motad 4 into 1 exhaust which was on it when I got the bike. About 40k miles.
Bought the bike in 1987. Used it for 10 years during which time it always ran fine with no problems. then I put it away. Got in going again in 2016. Carbs fully cleaned in 2016 but it didn't run all that well. Did about 50 miles and then put it away again as I got a bit fed up with trying to get it to run well.
Got it going again recently, needed another carb clean. I also replaced the pilot jets with same size jets (they got a bit butchered whilst removing them).
Bike pulls away fine but has a flat spot around 3-3,500rpm, noticeable on flat and hills but more so on hills.
I have the mixture screws (which are top of carbs towards the cylinder head) 3.5 turns out, which i read somewhere as being recommended. I have tried different settings of these but it does not seem to help the flat spot. Can someone clarify - is more turns out making it richer or weaker? (UK bikes do not have caps over these screws).
The needles in the slides have 5 slots in them. They were set at the middle point. I have moved the circlips down one notch, making the needle one notch higher. So it is richer.
I think it feels better but it's still not right. It might not be better, I might be imagining it.
I am tempted to move the needles another notch, but it's not a 5 minute job to do that.

So I am asking here if any one thinks moving the clips on the needles sounds like I am doing the right thing to cure this flat spot?

I know different parts of the carbs have more or less effect at different rpm and throttle positions.

Does it sound like the area I have the flat spot in is most likely needle position as opposed to pilot jet or main jet?

Any ideas on any other points, for example do the slide springs ever lose their strength from age?

I would just like to get this going as well as it did when i put it away in 1997.

Please let me know your thoughts. thanks

[Isetta]

To add some more info -
I have read this http://www.factorypro.com/tech_tunin...m_engines.html from the sticky post.
I have recently put in new rubber plugs where they go over the pilot jet holes.
I have checked float heights which were fine.
The rubber diaphragms on the slides look good.
In 2016 I installed new genuine suzuki rubber carb mounts (the bit between cylinder head and carb) with new O rings.

[tom203]

3.5 turns out on mixture screws seems excessive….but fiddling with this does nothing?
how are the o-rings on these mixture screws?….they need to seal

[Isetta]

Hi, 3.5 turns out does sound a lot compared to other bikes I have had but the haynes manual says 3.5 turns out. I read 3.5 turns out somewhere else also, but of course that could have originated back to the haynes manual. I checked the mixture screw o rings and they seem fine. Screwing them in (so less than 3.5 turns) seems to make the bike worse. More than 3.5 turns out does not seem to make it any better than 3.5 turns(about the same).
So today I thought I may as well move the needles another notch. So I have now done that. The more you do something the quicker you get at doing it.
So it now has the tiny cirlcips on the lowest notch, so the needles are now at their highest settings (with the plastic spacers on top still).
It seems to ride fine now. before today, the last time it rode fine was in 1997 when I put the bike away. In those days in the UK we were using leaded petrol, probably with higher octane rating. I guess the problem may have been down to the different petrol we are now using in the United Kingdom.
I'll have another ride on it tomorrow to check that I still think it seems OK.
Tried mixture screw adjustment again on various settings. 3.5 turns out does seem about right (but I don't have a colourtune plug ).

The big circlip inside the slide that needs releasing to get the needle out was a bit tricky. I had some circlip pliers but the tips were too big, so I had to file them down to get in the holes of the circlip, hoping that I did not go so far that the tips broke off. I bought those circlip pliers in 2016 for that job and then found the tips were too big, took me 6 years until yesterday to get round to filing them them down. Not really sure why I gave up so easily in 2016.

[Grimly]

Yeah, the change in fuel formulation over the years has caused some headaches. You can't be sure what is in modern fuel and how it's going to affect carburettion or fuel system components.
Still, it mostly has the desired effect once it's sorted out and the differences accounted for - twist throttle and go, happy grin.