I did change my petcock to the Yamaha one, still the fuel delivery is very unstable more so lately (1997 model in Europe):

Engine starts OK cold, runs even without choke soon,
in the beginning accelerates OK sometimes for many minutes,
sometimes only problem at full throttle high RPM,
then can (nearly) stall, but when this problem it runs better when warm using choke, but worse and worse.

I use fuel without ethanol, previous owner used E10 for 1 year/last year.
I checked using start spray: no false air leak.

I suppose there is a need for carburettor cleaning, maybe a dirty float bowl problem? Not too high or low like described here: https://www.vintagebikebuilder.com/setting-mikuni-float-levels.html

Can the bottom of the carburettor be removed while in MC?
Maybe first only remove the float bowl drain plug to get out some dirt?
Not spraying in carburettor cleaner through drain plug hole, because it destroys rubber gasket.

I did buy these rebuild kit and jets but wonder why my Mikuni carburettor underside looks different than most pictures?

Maybe my carburettor is not standard and parts will not fit? (see below)

Has anybody tried low cost alternatives like these?
"unbranded alternative type" Mikuni VM38mm VM38 VM 38mm Round Slide Motorcycle Carb Carburetor VM38-9
https://www.aliexpress.com/item/1005003070271940.html
https://www.aliexpress.com/item/1005007039437665.html  40 mm?
https://www.ebay.com/itm/184829373498
https://www.mapcycle.com/vm38-9-mikuni-vm38-round-slide-38mm-carburetor.html
https://mikunioz.com/product-category/jets/main-jets/?v=13b249c5dfa9
https://www.vintagebikebuilder.com/mikuni-tuning-and-jetting-guide.html

left side my carburettor

my carburettor rebuild kit (not tested yet) and jets
https://www.aliexpress.com/item/1005004516162406.html
For Suzuki 1986~2019 version LS650 Savage (Boulevard S40) motorcycle carburetor repair kit with vacuum diaphragm

https://www.aliexpress.com/item/33018387858.html
Replacement Large Round Carb Main Jets For Mikuni N100/604 Carburetor #155 160 170 175 180

https://www.aliexpress.com/item/1005006288806849.html
Pilot Jet Idle Slow Jet N151.067 M5x0.8mm Thread for Mikuni CV Carb

What Verslagen said!

Most people would caution against using cheap knockoff jets like in those Ali express kits.  Buy genuine Mikuni is the main suggestion.

If you decide to go the cheap knockoff route, those main jets are the right style Mikuni jets.  BUT they are all too big.  The smallest one in the kit is a #155.  It might work (but on the rich side).  A #150 is about all you need even if you have a better flowing muffler.  If you live in higher elevation, a #145 might be just right.

The pilot jets (slow jets) are the right style, but you’ll only need a #47.5 or #50.  All those other ones are just a waste.

For quality genuine Mikuni jets, try JetsRUs.com.

If you want to try a cheap knockoff Keihin slide carb, I’m running a PWK38.  Link to my trials and tribulations here.  —> http://suzukisavage.com/cgi-bin/YaBB.pl?num=1714330191

This style slide carb requires a different throttle cable (or modified stock), and you’ll have to use a different air filter or intake tube.  Same as the Mikuni VM36 slide carb needed changes.