With the flywheel, if the clearances are different side to side i do not see how you will be able to fix that. It is not adjustable. The only thing you can do is adjust the coil closer too or away from the flywheel.
I would suggest you leave it be.
To tell you the truth, i have never checked both clearances, use a business card on the square ones and im done. Once the clearance is set the only way it can alter is if it is hit or the bearing gets sloppy.
The fault is most likely to be carb or fuel related. I would suggest you put the mower back together so you can test this side of it.
As grumpy said, fuel flow.
It sounds as if it is running lean.
Another trick you can also try is pushing the primer button while it is running, see if that makes it pickup in revs.
Leave the mower sit with the fuel tap on. Start it and turn the fuel off. Time how long it takes for the motor to die out.
Then fuel on, leave it sit again, start the mower and leave the tap on then time how long before it plays up. If its a similar time frame you have a fuel flow issue.
If/when you do pull the carb apart, take great care not to loose the little needle that goes above the float on the back of the primer/carb face plate.
Stretch your new O-ring onto it, dont roll it on.
A good known spark plug. Keep in mind that it is not unheard of for a new spark plug to be faulty.
And regular checks for a consistant spark while it is giving issues will tell you if it is ignition related.
