I haven't any idea of the 6E compression ratio. Villiers just don't mention it in any of their handbooks and manuals
. As the cylinder head on the 6E was ground in to seal it with no head gasket fitted mine tend to have a slightly higher compression ratio from standard. This arises purely from me lightly skimming the head in my lathe to clean up all the years of previous owners grinding the head in to fit but never effectively cleaning up after themselves. I like to start with a nice new flat surface so I skim it just enough to clean up the bearing surface. I believe head gaskets are now available for those who can't get an effective seal. To be honest I've never found it to be that much an issue.
I find the 4/5 carb works well. However it gets very grumpy if you try to run it with the twist grip set to automatically shut off when you take your hand away. If you look at the spare parts book you will see the 4/5 was designed for a lever throttle which is not self closing but stays where it is set. It doesn't shut itself off. The original AMAL
twistgrip fitted by James to replace the outdated throttle lever had a friction adjuster underneath which you tightened up so neither did it shut down of it's own accord. Set like this it replicated the Villiers factory lever action .You have to close it yourself. Back in it's day there were good reasons for this style of throttle set up. The bike has neither turn signals nor an ignition key. Also it was built for a country where they drive on the left hand side of the road and most hand signals are given with the right hand, the throttle hand. A bike that lost all power immediately you tried to give a hand signal obviously has a bit of a problem. You stood a good chance of being hit hit up the proverbial every time you signalled..
It was also expected that the engine would stop when you completely closed the throttle. This got around the lack of an ignition key. End of journey? You want to stop the engine? Simple, just close the throttle. Some two strokes of the day were also subject to spluttering, banging and four-stroking on the over run or down long hills. Some did it, some didn't. Some did it sometimes. A throttle set up to completely shut the engine down at those times cured all that.
So, the 4/5 carb works well but needs to be set up 1951 style not 2019. That will nicely solve your idling issue.
It's all great fun isn't it. Who knew that riding a 1951 bike would be so different from riding a current one.