Good possibility but you probably will need more than the 1 oz per gallon of gas that the can recommends to do what you want to do, maybe a full can (16 oz. to 6 gallons....and it will take time as the SF has to soak in for the Naptha in it to loosen varnish. Like you take it out and run it, or run it on muffs for 15 minutes or so and shut it down for a day or so.....for a number. Then do it again, possibly again.
I am thinking since reading the post and here is a brainstorming answer. Your carb works on basically 2 systems: A low speed, controlled by the low speed jet, and a high speed controlled by the high speed jet. The jet is nothing more than a small orifice (hole) that allows the air moving through the carbs. venturi to suck fuel up out of the bowl. If things are working as designed, a certain flow rate of air will suck out a certain amount of fuel.
At mid throttle you are at the highest volume of fuel required from the low speed jet before you get up fast enough to activate the high speed jet which you use for your high speed operation which you said is ok.
So, I assume the low speed jet on your carb., one of your carbs....don't know if that 40 is a 2 cyl and if so if it has one carb or two, is partially clogged. If the SF won't fix it you will need to dismantle the carb and clean it up. Berryman's Chem Tool is made for just such an occasion, directions on the can at your auto parts store, WM has it. You will need a carb kit which you can purchase on this site as part of the process.
HTH,
Mark
