If black smoke is present during the bog, look closely at the air filter/air intake system.
If not black smoke, look at your fuel system, specifically the supply side (tank to lift pump). I like to do a flow test. Easy to do. Rig up a nurse tank and connect it to the injection pump's inlet. You'll have to bleed it, but it takes 2 seconds (knob). Nurse tank can be an old gear oil bottle flushed out really good, the one with a nipple on it. Put a piece of fuel line on the nipple and connect to the injection pump. The loose end of the fuel line that you took off of the injection pump, run that into a jar and have someone start the engine. Watch your flow closely. Should see it come out solid fuel, and the stream should be the same size as the ID of the fuel line. If you're seeing bubbles in the stream, there's an air leak somewhere. No bubbles and low flow (it's usually obvious) there's a restriction. I see a lot of leaves get into the tank, then down onto the tank's outlet, restricts it, and the lift pump will starve, then of course engine bogs under load. You can't really get down into the tank on a lot of them to remove said debris, so the only option is to remove the tank and clean it. Fun.