Sounds like it is starving for fuel under load. What is the make and model of the engine?

If your carb has an adjustable main jet screw(on the bottom of float bowl) turn it CCW 1/2 a turn,then test it in the dirt ....If you do not have an adjustment screw .....you either have a float out of adjutment or a blocked idle passage ....in other words,possibly new carb time....Sorry ,Jim

In the many years that I have done all the small engine work for a store, I've never had to replace a carb yet.

NAPA makes a carb cleaner that you soak the carb in for 5 minutes and wash it with water. It has worked 100% of the time for all varnished or blocked carbs.

I agree that it's a blocked jet or passageway or a screw that's a little too lean. I would adjust 1/8th turn at a time.

i have a tiller thats been past on to me from my grandfather all i can find on it is garden way and serial #471161 i would like to find an owners manual

Try the MTD site. Can you engage and disengage the rear tine assembly? The lever is on the left near the tine's drive shaft.

I still have the manual in pdf format if it will help. Just email me and I will reply later today or tomorrow morning