My guess is that 391 is a stratified charge design two cycle engine. I find the Stihl ms251 and Dolmar 6100 I have which are strato design tend to kind of jump up to high rpm and I wonder if they really are suitable for help or guests to use. You might have done high rpm with little load which I have over the years tried to avoid. I have never touched the adjustment of the carb on the Stihl one. The Dolmar one is likely hard to kill as I have had the high speed setting essentially all over the place.
For the sake of discussion I will type some reply. For the sake of discussion I will not discuss plug in or battery.
The 4300 model, which was Dolmar 421 and smaller bore 351, before that 420 and 350. These are an inboard clutch with lateral chain adjuster well closed off from crevices for debris to accumulate. They use the Poulan Mount Oregon code 041 but the small husky mount and even the small 3005 mount can be made to work. There are even universal mount bars to cover the three of these. These are an open port design that seem to run more to my liking that the strato designs I have. These can have the 50cc rim drive put on them with a real thin washer like .2mm to span a void.
The 50cc ones like Mowers at Jacks throws ads at me for low 500's if I recall. like this one
https://www.mowersatjacks.com/product-details/makita/ea5000prgg
What is wrong here? A closed port non strato design, ok only one ring, lateral adjuster not in the clutch cover. I don't have one. I can see comparing it to the Stihl ms261 but that is a whole different bar mount and drive link count loops from where I am at. I can get parts on line maybe not instantly, most on here likely have spare saws anyway.
The Makita 3601 dual reed strato 35cc 31mm stroke 38mm bore model. Have you tried one of these and formed an opinion? It is outboard clutch with the adjuster in the clutch cover and some cost cutting measures like no oil input on a line in the tank but if one has a collection, they should think of this one. Not sure what they were thinking with the ridges on the starter handle, I filled the area other than that no mods yet anyway.