Hey friend! I own a small engine shop but I wouldnt call myself a professional lol. Fuel solenoid usually either sticks all the way shut or all the way open. When in deep grass, stop and see if blades speed up. That would be a slipping deck belt. If it has loss in moving forward too, you could check both belts but I'd check cylinders. Compression test will tell you a little but a leakdown test will tell you exactly where compression loss is. Position piston on tdc (easier with valve covers off and spark plugs removed). Turn flywheel manually until intake valve closes then stick a clean screwdriver or rod into spark plug hole and slowly turn. When piston stops that's tdc. Rocker arms will have slight wiggle. While feeding air into spark plug hole, listen for hissing out of dipstick tube which would indicate worn or damaged rings. It's normal to have a little air but not too much. Also make sure engine is warmed up good as the metal will have expanded. Listen for hissing out of exhaust and carb which would be sticking, burnt, or damaged valves. Also listen near head gasket area. Hope this helps!
