Ryobi S430 4 cycle trimmer

I wonder if vapor lock is an issue. Have you tried idling it for a few minutes before shutting it off?

This just got stranger. I have two of this model, one at each of my properties in different cities. So the idling before shut down did not help. Brought one from the other place this weekend so I could swap carbs before jumping to conclusions and buying a carb or kit. Had the same problem so I started using the other one and found that it does the same thing. Then realized that I had only recently changed the order of my work at this property and that’s when I noticed the trouble. Previously I had always done my trimming and blowing straight through at both places. Since I started alternating between mowing and trimming at this place I’ve had the problem. Wondering if it’s an inherent defect or if there might be something that fails after a couple years. These were purchased about 8 months apart.
One more thing, when they are running poorly, I can rotate it to the tank down position and it will run better for a while. But after a few minutes this fails as well. Doesn’t matter how full the tank is either. I’ve tried topping it off and checking the filter position to be sure it’s in the fuel. Also noticed that when this is happening the primer bulbs don’t work well at all.

The one that I just learned has the same operating trouble after sitting a while had never been operated in the wrong orientation so I’m thinking it’s unrelated to a sticking ring. Plus I’d think a sticking ring would affect performance start to finish. Guess I’ll just chalk it up to characteristics and do all my trimmer work straight through from here forwards until I wear them out. Thanks for the input!