My Homelite UT-20002A trimmer have a hard starting problem. I remove the muffler and cleaned the carbon in the exhaust port. I also replaced the spark plug, but still the trimmer is dificult to start.

Check for fuel to carb. Check for ignition by removing plug and laying metal case against cylinder head, pull starter and watch for spark. Check carb. Air filter and check exhaust port on muffler for dirt dobbers and carbon buildup. While plug is removed hold finger over spark plug port and see that you have a good bit of compression. If this does not help remove and replace or rebuild carb.

If the spark arrestor has been cleaned then it could well be the muffler itself, or the exhaust port blocked, remove the muffler and check the port for carbon build up.

This sounds very much like a blocked spark arrestor in the muffler outlet, or the exhaust port is blocked with carbon, remove the muffler and look up the open exhaust port for signs of carbon build up, somewhere in the muffler there will be a mesh gauze ( spark arrestor ) either clean this or remove it completley.

Remove the Torx screws holding the rear cover on. Then remove the larger Torx screws holding the muffler on to the exhaust port of the cylinder. There will be a gasket there. The spark arrestor will be mounted in the opening to the muffler. You can scrape or burn the carbon off of it. If the spark arrestor is real clogged up, then the muffler probably is too. It should also be cleaned. The inside of the engine probably is carboned up too( if arrestor and muffler are). They all should be cleaned of carbon. It helps the engine perform at its' best and breathe like it needs to.

check air filter if clean check spark arresterin muffler usually a screen under plate where exhaust comes out also tube shaped can be pulled out with pliers,if clogged canclean with wire wheel also can remove muffler tosee if exhaust port ic clogged with carbon if all is ok problem maybe with carb hope this helps

your muffler has gotten full of carbon from the oil/gas mix you use and now theres too much back pressure and needs to be removed and cleaned or replaced,also while its off clean out the carbon deposits on the reed valves at the exhaust port

Check your muffler to be sure doesn't have a build up of carbon from the oil/gas fuel. This is the problem with 99% of equipment that uses mixed gas. Take the muffler off and see if it runs better without it. If so there is carbon build up in the muffler and muffler port on the engine. Burn the muffler in an open fire for a while to remove the carbon, or burn it with a torch, toss out the screen that's is in most mufflers, (unless of course you live in California where they are required), Scrape out the port with a knife and after the muffler cools on it's own (don't cool with water as you will warp it and not get a good seal) reinstall and see if this solves the problem.

Hello:

Have you Removed the Muffler and Checked the Exhaust Port and Muffler for Carbin Restriction?
Remove the Muffler and Start the Unit. If the Operates Correctly, then the Spark Arrestor Screen is Clogged and the Muffler/Screen Requires Replacing.
Make Sure the Exhaust Port is Not Restricted Also.
Hope this Helps.

Good Luck

Respectfully

John

I had this same problem, would run a couple of seconds, stop, and you couldn't start it again.
Had good gas/oil mix 50:1 and 25cc Homelite. Had replaced the fuel filter, gas, spark plug, and took carb apart and cleaned with carb cleaner and primer bulb worked fine. None of that fixed it. What did? Take the muffler off and see if it will start and run with the muffler off. If it does, chances are the muffler is clogged. Also check the exhaust ports and clean them out if plugged with black oil carbon residue.
My muffler was clogged up, took it off, cleaned out the exhaust ports, there was some oil that was in the muffler(oil that didn't burn during combustion). It now starts and runs like a champ on first or second pull.

Check the muffler for obstructions. Remove the muffler and check the exhaust port sometimes they are near plugged. If so remove all the crud with the cylinder in up position to prevent getting carbon in cylinder. Trimm to adjust Turn both L and H set screws clockwise until they bottom the back each one out 1 and 1/4 turn. This should allow you to start the trimmer. Then rev to max rpm and adj. the H screw to max rpm. Then set the idle with the L screw with good idle without string head turning. Then when throttled it should do so without hesitation. Once this is set you may have to fine trim the H screw for max rpm again.