Runs but back fires under load

there is detail missing here that is essential for a solution
what engine is it , v or straight, no of cylinders
what work has been done prior to the problem
what ,make and model, year of vehicle
you have changed spark plugs so assuming it is a v8 I would be rechecking the firing order of the HT leads ( 2 cylinders crossed )
They will start and idle ok but backfire under load

Injectors may be clogged causing lean fuel misfire under load.

you mean not firing no spark on 1,2 cly.or no change on rpm when you pull 1,2 if you have spark on it ,means no gas or your plug totally worn out

Could be fuel or ignition.
If the coil or ignition is weak, you may not have enough power to fire the plugs under load.
And if the fuel pressure is low, you may not have enough fuel under load.

several things to check for this one....since its happening under a load.

- timing
- ignition (spark plugs, distributor, cap/rotor)
- Idle controller
- exhaust system

back firing???? out the exhaust or the carb.
my first guess is the spark plug wires;(under load). any thing in the ignition system; plug too.
back fires out the exhaust under load sounds like low fuel... rebuild carb or star by cleaning the fuel filters.

have your fuel injectors checked especially if you have ever ran it out of gas...you may have a sticking injection needle

Sometimes when the ignition leads get older, they can misfire under a load.
If they haven't been changed in a while, I'd recommend this first.

you must have the throttle at full at all times when under load driving and mowing, back fire is do to much fuel. if your running throttle at full than look at the carburetor and then the wiring (short)

If this just started, maybe after a hard run, you may want to check your sparkplugs. I've melted the insulator on one of my plugs after a prolonged, spirited run causing same symptoms.