OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Trimmers and Brushcutters Two Stroke Trimmers & Brushcutters Stihl Brushcutter

Hello all,
I have a Stihl brushcutter which is about 10 years old. It has been a good machine and well maintained. It fires and starts fine but after 5 - 10 minutes of cutting seems to lose power. Any ideas what could be the cause?
Thanks
Kevbar.

Hi kevbar. We will need more information before we can be of much help I think. Can you tell us the model of brushcutter, about how many hours it has run, and post a picture or two please?

Generically, there are two things that will need investigation right at the beginning, before considering more subtle possibilities. The first is how long you have been using your current spark plug. As a simple rule of thumb, the spark plug used in a well-tuned two stroke can last about 15 hours before engine power falls off substantially, but only if the engine is mostly running warm and under load. Otherwise, it may last considerably less than 15 hours. Power loss in the vicinity of 40% is to be expected to arise fairly soon after the 15 hour limit. After that, power will continue to decrease.

The second problem that arises happens over a longer period, perhaps as much as 200 hours' use, if the fuel always contains the right amount of the right oil, but can happen very much sooner if the oil supply is a bit skimpy. That problem is loss of cylinder compression. It is usually mainly due to piston ring wear.

If you give us some more detail, we can talk about how to investigate to find the specific problem.

I would be looking at the following.

1. Replace the fuel filter and fuel line (could collapse or have pin holes).

2. Check Exhaust, if it has a wire mesh screen check that it is not blocked and also check the port for carbon build up.

3. While you have the muffler off check the cylinder for any damage.

These are cheap fixes and only take a few minutes to check. It is a common thing that the fuel filter tends to block up with dirt from the fuel which will cause your issue.

Thanks both,
I stripped it down and the cylinder, piston, rings and muffler look in good order.
It is a Stihl FS85 and has seen a lot of work. I tested it yesterday and noticed that it runs well up to about 30 minutes. Then as you press the throttle the power drops away, also if you raise the cutting head the engine runs well again. In effect if the cutting head is down in cutting position the fault persists, if raised so the cutter is more or less horizontal it runs well. I also changed the S/plug which had a white coating on the electrode. The fuel filter looks in good order but there seems to be a slight leek where the line enters the tank. Also seems to be some 2 stroke oil present on the air filter elements. I'll take some photos today and post, but any ideas in the mean time???