[ssg]

Problem:
My bike is RE Standard 500 with Carburetor model.
It started almost a month ago. At around a speed of 23 km-30km the bike was experiencing a jerk as if it was not getting fuel properly.
After that during 1st start in the morning the idle RPM and noise remain stable but as I throttle the accelerator minimum it experiences as if a flat spot where it lacks the linear acceleration.
The bike starts without any hiccup in the 1st and 2nd gear but starts jerking  at 30 or 40km speed repeatedly. Once I cross 40 km speed there is no hiccup.
After running 3/4 km the bike has no problem and runs like a real bull. I thought at warm stage it behaves normally. But soon I found that at a warm stage if I keep the bike for a few minutes and have a cup of coffee it again starts jerking. So warm stage theory is not fully justified.

What troubleshoot we did so far:
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*Checked the fuel ....no presence of water droplet.
*Changed stock standard BOSCH spark plugs with the same brand.
*Checked the Ignition coil
*Changed Spark plug cap.
*Checked the ignition and the electrical wiring.
*Battery is fine as it has no issue with Self Start or any other electricals.
*Cleaned carburetor and found there was slight worn out spots on piston valve slider.
https://ibb.co/cJf3Y3r
https://ibb.co/h8L41pt
According to RE Mechanics this was common and not a factor here. But to confirm the same I installed a brand new UCAL UCD33 carb.(same as the stock). And found the jerk gone, I was delighted.
After 2 hrs. of rest when I started my bike I again felt the jerk. My goodness! and I soon realized it was not a carb problem and returned that and again installed the my old carb.
* Checked CDI
*Intake Manifold is new.
*Air-filter changed.
*The fuel tank is clean and no rust.
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RE Mechanic's findings:
They found nothing wrong. They changed settings in Carb. and adjusted to a richer mixture. Again no issue in running the bike initially but the issue returned again like a nightmare and they returned me the bike.
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What next?

What shall I do now please suggest. I am totally confused with this bike.