Small gas engines such as those found on the Ryobi String Trimmer are in constant need of maintenance and adjustment to work properly. If the engine won't start, first make sure that the power switch is set in the "on" position. Check the fuel tank to make sure it's not empty or low. Unscrew the spark plug from the port in the side of the engine and look at the leads at the end. If they have a black deposit on them, the spark plug needs to be replaced. Smell the spark plug. If it smells overpoweringly of fuel, then the engine has been flooded. Set the trimmer aside for half an hour and try again. If the engine still won't start then the carburettor needs to be cleaned and adjusted. If you're suitably experienced with engines it is possible to do this by yourself, but it is strongly suggested that you leave this to a professional.
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Engine Lacks Power and Dies Frequently
Begin by checking the air filter. Most Ryobis keep the air filter in a plastic housing with an easily opened clasp. Pop the clasp and look the filter over. If you see any colouring aside from plain paper white the filter needs to be cleaned or replaced. Should this not fix the problem, remove the engine's spark plug and look at the gap. Sometimes a little grease will cover over the gap between the two leads on the plug, preventing it from sparking reliably. Use a nail file to scrape away any deposits and ensure that the two leads are not touching. Look at the muffler. If it is blackened or any ash falls from it when you touch it, there's likely to be much more carbon build-up on the inside. It should be disassembled and scraped clean. Should the engine still lack power then either the internal compression of the engine is low or the carburettor needs to be adjusted. Both of these things should be seen to by a licensed repair person.
Runs Hot or Overheats
There are a number of reasons why a trimmer would run hot or overheat. The first is that the fuel mixture could be off. Remember that these trimmers use a specific ratio of unleaded gasoline to oil. If this is wrong then the engine could heat up due to friction, or it could start burning a little too much oil depending on the ratio used. Also you could be using the wrong type of spark plug. A spark plug that is too powerful could cause the engine to race and overheat itself. The only other reason for a trimmer to overheat is that it is unable to cool itself properly. This would be caused by a build-up of carbon within the muffler. The muffler should never be touched while it is still hot. Once it has cooled off, it should be disassembled and scrubbed clean.