My motor dies without so much as a miss at full throttle but runs smoothly if wound on smoothly to two thirds throttle. Could this be worn main jets? It has electronic ignition.

• If unit will not run without choke , the main jet in carb is clogged and the carb needs to be cleaned out.

If you are lucky, you might just need new fuel filter and the fuel bowl cleaned out, and maybe new air filter. If not so lucky you need carburetor rebuilt. Backfire is usaully caused by engine running too lean ( too much air for the amount of fuel present). Fuel bowls get gunk in them and the fuel needles and fuel jets get varnished up. The accelerator pump gets worn out gaskets and/or bellows and doesn't give the right amount of fuel mist when you accelerate.

Full Choke is indicative of a motor starved for fuel

This sounds like a clogged main jet. or else if it is an older mower, the jet is not sized big enough for the ethonal gas. Try thoroughly cleaning the carb and jets, or drill out the jet to the next larger size.

Running only with full choke is a fuel problem. Unhook fuel line from carb. it should run good. If not go back the line. Check the fuel cap vent.

Does it idle correctly ??? You have an idle mixture screw that might need to be tweaked a little or you have an idle jet that is dirty. When you turn the throttle past that point you are pulling fuel through the main jet and bypassing the issue. Try to partially apply the choke and see if it runs better at your problem spot, this will help diagnose the issue...

I would say you have a blocked or damaged jet. I would strip down the carburettor and replace any badly worn jets and clean thoroughly. When refitting the tick over mixture screw , screw it home and back out about 2 1/2 turns. That will be somewhere near it.
Hope that helps.

It sounds like the fuel mixture is running very Lean..There are usually two adjusting screw screws on the carburettor that control the mixture. A Larger one is for the Main Jet and a smaller one for the Idle Jet. There is also an adjusting screw on the throttle butterfly to control the idle speed.
To adjust setup, start with at least a half tank of fuel, and have engine at operating tempurature (That is warm but not overly Hot). As a starting point, loosen (anticlockwise) Main Jet screw about half a turn, to richen mixture,so that engine will run without choke. Run engine at full speed and adjust main jet for highest smooth speed without engine missing due to mixture being too lean or rich.
Allow engine to idle and the adjust Idle Jet Screw for smoothest idle. Idle Speed screw may have to be adjusted if idle speed is too high/low.
Sequence above may have to be repeated to get best running. If engine hesitates sometime when sudden load is applied but then picks up, try adjusting Main Jet by 1/8 to 1/4 turn richer until it handles the load without faltering.

Sounds more like a vacuum leak, check any vacuum hoses for dry rot. Next, I would start by removing the carbs and pulling the fuel bowl checking for some rust or crud on the jets, ck for any small ports on the jets. A worn jet would let in more fuel and should not cause a shut down at wide open throttle.

PLOGGED PILOT JETS AND FUEL SCREW PASAGEWAYS