Do you feel the compression is low when you pull the starter rope? It useually have quite a resistance.

You use compressed air to blow dry when you clean the inside of the carb? Your trimmer is well used, but I do expect it to last longer though. You try to adjust the carb ( not the rpm). You sure you are not running too lean?

If you are not complaining about the power, why not get the SRM225 again and keep the old one for parts.

How many hours are on that trimmer? It may just be worn out. I agree with buying one similar then you have spare parts. Those are good trimmers.

Take the muffler out, check to make sure exhaust holes/arrestor screen is not plugged up (I am assuming it has two stroke engine)

Just a comment: You should NOT get a shock from touching the plug wire. That indicates that the insulation is cracked or worn, and you're leaking electrical impulses. Try wrapping the plug wire with electrical tape, and see if it runs better.

You might also try running it in a dark area to see if it's "leaking" electricity - you should NOT see any sparking.

Do what esemilio suggests for the original problem.
I have a 210 that had the same problem.

Remove the spark plug and plug it with your thumb and pull the cord. if the air pushes your thumb off, then you should have enough compression.
If you changed the carb and still have the same prob, then exhaust is the last stage to get it to work.