You take a $80-$100 (initially) older trimmer, blower, etc.and it needs a tune up and carburetor clean and new fuel lines. By the time you charge for labor you are around the initial cost, $80-$100. How is that cost effective when you are polishing a turd? Then you do all that and it sort of runs, but just isn’t right. Hard to start, etc. The customer is throwing good money after bad.
It's probably a good practice to take in at least $30 for diagnostic fee for everything. Even 2cycle stuff. That pays for the tech to do initial checks, like compression, spark and fuel flow.
Obviously, it doesn't take more than about 10 minutes to check those things. If the equipment has more than 100lbs of compression and the cylinder isn't scarred, then it's usually worth fixing.
I found that a lot of times, the customer simply didn't realize that you have to prime the air out of the carb, or it's just something else pretty simple. Like a bad on/off switch or fuel lines.
The Amazon carbs (which aren't any worse than what they're putting on these cheap models) are usually about $20.
A cheap weed eater at Walmart is now $100 to $150. So if the customer can get theirs running for $50, they'll be satisfied. And you'll pocket $30 for 20 minutes labor.
But, If it's low on compression (less than 100lbs), or or some time consuming issue, most customers understand it's not going to be worth fixing.