Good news! In my attempt to make my string weed-trimmer run, I'd cleaned the wrong parts.
The shop mechanic told me the exhaust system (muffler) was plugged up, not the air cleaner or spark plug as I had suspected.
So no parts were bought, and only a half-hour of labor was charged for the cleaning. It runs good as new now!
(And Karyn had the nerve to accuse me of keeping items far past their normal useful life!)
Of course, next week the trimmer head will probably fall off when the rusted-through shaft breaks, but I'll get another week out of it!