Small Engines (Lawn Mowers, etc.): Small Engine Troubleshooting- Weedeater, crankcase pressure, engine crankcase

Question
QUESTION: I have a small engine question. I have a Weedeater XT200 that starts but then runs erratically and dies after a short time. I have changed the spark plug, cleaned the air filter, installed new fuel lines, replaced the carburetor with a brand new carb, a new gasket between carb and engine, and still no success. The only additional things I can think of that I did not attempt is replacing the fuel pickup inside the tank, or actual engine work. Can you help me with further troubleshooting tips. Thanks

ANSWER: That's a pretty old trimmer.  Have you checked the engine compression with a compression gauge?  It should have at least 100 psi to run properly.

Check the muffler and exhaust ports.  Carbon deposits build up and over time can restrict exhaust flow causing erratic operation.  The last item to check is crankcase pressure.  Leaks were rare on this model but just make sure the cylinder to engine crankcase bolts are tight.

Did you adjust the carb correctly?

Let me know if this helps.
Eric

---------- FOLLOW-UP ----------

QUESTION: Thanks Eric. I have adjusted the carb although it's difficult to know if the adjustment is correct when the trimmer often doesn't run long enough to see if the adjustments are making a difference. You are correct the trimmer is getting old, but it worked fine until about 2 months ago when it started this problem. I am determined to fix it. I will check the muffler and exhaust ports next. If they have carbon deposits, what is the best way to clean them?

Answer
Use a small flat blade screwdriver and scrape out the deposits.  The muffler doesn't need to be perfectly clean, just make sure it's not plugged up.

Do you have a compression or a friend who might have one?  You can work all you want on the trimmer but if the compression is low you will never fix it without an engiine overhaul.

Eric