Outdoor Power Equipment and Small Engines - Mcculloch trimmer looses power when hot

Hi,
I'm hoping someone can shed some light on a problem I have. I recently purchased a Mcculloch FR44 trimmer. It's about three years old.
I can start it and it runs fine for about 30 mins (idling and on full throttle) then it starts to play up. It still idles fine, but when I try to run it on full throttle it starts loosing power and runs erratically. It won't maintain a constant speed.

Any thoughts or suggestions are greatly appreciated.

Try loosening the gas cap when it misbehaves and see if that helps. If not, I'd suspect you have an air leak into the crankcase through any number of possible leak areas where it is assembled.

Thanks,

Tried that - it made a small difference - if anything it idles a bit quicker with the cap unscrewed.
The problem seems to be constant, rather than just when it's hot. Could it be a fuel blockage where it's not getting enough gas at full throttle, but enough to keep it going at lower revs?? I cleaned the tank out (although it was not particularly dirty), and the fuel line into the carb is clean.....Could the carb need cleaning?

Thanks again.

If it's running like it should for 30 minutes, then the carb should be fine. A plugged up carb won't run good to start with. For one to run for 30 minutes and then mess up, I'd say it's either vacuum developing in the fuel tank due to a bad gas tank vent, or as the engine parts warm up and expand, an air leak opens, leaning out the fuel mixture until it dies.

I mistakenly thought it was an issue when it got hot, but when I started it up cold the other day the problem was at the start, so that lead me to believe it is a fuel problem.
From cold, when I open the throttle it will rev up to full speed but then slow down to a little faster than idle, then rev up again to full speed, then slow down, and on and on....does this sound like a plugged carb?
Thanks,

It's a lean mixture problem. An air leak can do that, but so can a clogged carb, fuel filter, loose cylinder, bad fuel lines or a cracked primer bulb. If the problem isn't evident after checking these things visually, I suggest replacing the fuel lines (check the filter in the process), and if it still does it, open the carb up and clean/recondition it.

Thanks Cheese.
The carb was blocked so that seems to have sorted it.

Thanks again.
James

Great! Thanks for the update!