I have a list of things the shop or rather clown with tools did. Wrong plugs, missing screws, broken choke, loose oil filter, didn't change fuel filter.
That or the "other shop" was a buddy who had a couple of plugs laying around and just put them in. After all, one plug is as good as another.... isn't it. LOL.
Heck, the parts store counter person could have likely just give them the wrong plugs just to make a sale.
I've seen that same thing and more stuff just as bad on a really nice onan engine. (as one example of several)
The wrong plugs had been run so long that I had to chase the threads for the longer reach plug threads due to carbon build-up.
Engine was on a really nice looking Case 448 that I bought non-running. Bubba had messed up several other things, carb, ignition due to wrong plugs being installed.
Gets really disgusting trying straighten out a Bubba mess.
Order a carb kit and accelerator pump kit for a Rehlko Command CV730 last week and just got around today to pulling the carb. and found that some yahoo had replaced the carb with one from a Courage twin. So different carb kit, which won't be here until middle of next week, and now I am not even sure how it will run after repairing the Courage carb.