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First time posting! I hope someone has a good suggestion for this problem. Other clues I noticed:
Black smoke while starting and running. But starts easily.
Engine slows even with fuel lever at max while mowing.
Engine power recovers when drive belt for deck is disengaged.
This has been a strong goat of a machine. This power problem just started today. Any ideas? Thanks for your time! Don in Washington State.

If the plug is black & wet looking then prime suspect is failed float valve
Remove carb & clean it
If the plug is black & fluffy looking then prime suspect would be a fractured timing key in the flywheel.
Prior to his happening did the engine stall ?

Donzmatic said:

Black smoke while starting and running. But starts easily.

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Choke stuck on or air filter plugged up. Could be both.

Donzmatic said:

But starts easily.

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Should with the choke on.

Donzmatic said:

Engine slows even with fuel lever at max while mowing.

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Either this is a gutless engine, needs valve adjustment, possible overheating or most likely low carb float bowl setting.

Wait you said max lever setting. Are you running it with the choke on?

Most throttle levers are normal running up to 3/4 throttle. 3/4 and higher gets you into choke mode.

Donzmatic said:

Engine power recovers when drive belt for deck is disengaged.

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Normal as you have no engine load then. Unless you are driving around the weeds.

See the setting on the top right there? Shows a choke action? That diagonal line representing a choke plate? Should have a detent before it goes into choke.

bertsmobile1 said:

If the plug is black & wet looking then prime suspect is failed float valve
Remove carb & clean it
If the plug is black & fluffy looking then prime suspect would be a fractured timing key in the flywheel.
Prior to his happening did the engine stall ?

Click to expand...

No it didn’t stall. The black smoke only happens when the engine is started or when it is reved when operating. I was going to change the gas filter. The loss of power is slow but the recovery (disengage the deck) is quick. Thanks for your reply.

On start up and when reved during operation, the throttle opens fully and sucks hard on the fuel ( we call this a signal to sound smart )
IF the float level is too high then the same amount of signal will draw more fuel thus the black smoke.
An ignition that is too far retarded will do the same .
Running slightly rich will cause a drop in power output

Pull the carb apart and check for a loose main jet ,