Just a little thought! Have you had the compression checked? low compression on one or more cylinders would cause low rpm problems.
I think the carbs you have will have a “Starter Jet” which is not a choke as such, just a enrichment system which is much better than the old “Choke flap” It may or may not increase the RPM.
Many people used fully synthetic car oil in the early days (also branded as bike oil) which caused problems, this is because cars do not have wet clutches so manufactures add PTFE (Polytetrafluoroethylene) as a friction improver,(Google it) It is this which caused problems with bike clutches which if worn or only just up to the job in the first place causes slip.
Fully synthetic oil designed for motorcycles is far superior to that sludge that comes out of the ground, oil change intervals can be doubled and the biggest advantage to our big twins is when we get stuck in traffic and the temp rockets quickly synth oils do not burn and clog the rings so easily as it can take an extra 25oC or so before it brakes down.
Would you put crossply tyres on your bike if you could fit radials!!
However I just put the cheapest #OOPS# I can find in my bikes, My Moto Guzzi I have had for 35 years and over 125,000 miles with no problems on mineral oil 😉
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