"Just my opinion" and I'm by far a super mechanic but try a batch of marvel mystery oil through the gas for a while, it may clear it up, and definitely won't hurt anything! I use it all the time and it really seems to help clean out the fuel system.
I've had some luck with sea foam. It always seems to run worse on the tank it's in, then better on the next couple. How much should I use per gallon for this application?
I don't recall the recommended dose of Seafoam/ gal but I'd just double it up for more than just a "maintenance" cycle. 2 oz/ gal sounds like plenty, or just use my measuring method of 2 glugs per gal.
I put about an ounce in the lawn mower tank plus I used the spray also on the Carburetor, usually it's one ounce per gallon, but you can use more. HOW 2 Instructions
It says one ounce per gallon but it won't hurt. You can run it on pure seafoam, even 2 stroke, I would dump at least a can in or two per tank if it were me.
For sure. I had to just go into the carbs on my johnson 150 GT (outboard ). Haven't taken it out in 3 years...since before son. I have started it that's it. Man I couldn't beleive the crud? Like black paste? It's gets E free gas since I have had it but also don't think my father in law before me did that. Had some soft fuel hoses too . It has 3 carbs each serves 2 cylinders. Some of the Jets had all kinds of crud in them. The worst 4 I pulled out and soaked in pure seafoam over night. All kinds of just came off the 12 high speed jets I pulled out of those. One was better. Must of been the worst one I did like 6 years ago. It had a new gasket set so I'm sure it was. After I soaked them I sprayed them with carb cleaner and compressed air. Once back togeather it ran better than it had in years it always had a stumble after idle from the dirty carbs. They just weren't so dirty that it wouldn't kick into gear off idle like when I tore it down. My point to all this is that it had seafoam in it and run through at a high does. Didn't help that issue.