If it has engine I had a similar problem a while back. Pulled okay on level but no power up hill and smoked under power. One of the plugs was badly fouled. Replaced plugs and it ran better but still ‘stumbled’ a lot. Found the plugs fouling again from too rich a mixture. I removed the air cleaner cover and found a mouse nest around the air filter. Cleaned it out, replaced the air filter and it’s running fine again. My suggestion, try new plugs and air filter.