OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Trimmers and Brushcutters Two Stroke Trimmers & Brushcutters Stihl FS86 surges after overhaul

Just put a new kit in the carby, new air filter, new plug. Gave it an overhaul because it died and wouldn't start. Before it died it had to run on 3/4 choke to get any high speed. Now I can get it to start and idle OK but on high speed it surges all the time and eventually stops. Starts again straight up but does the same. Playing with the choke doesn't help. The whole kit was used - needle, mesh screen, diaphragm and gaskets. Where do I start on the troubleshooting trail?

If it is cycling in speed at constant throttle, it may be running lean at high speed. Have you tried turning the high speed mixture needle anticlockwise a bit?

Also, is that carburetor a Walbro WA or a Walbro WT?

Yes Grumpy have tried the screw - and its a WT

If no extra fuel can be obtained by choking it, it sounds as if there is a problem in the fuel pump. I suggest having a look at the pump diaphragm (not the metering diaphragm), to make sure it wasn't damaged by installing it. I've done that myself, as recently as the day before yesterday.

Did you replace the in tank fuel filter?

Good thinking, Joe. Since it had the same fault before the carburetor overhaul (which included the fuel pump diaphragm replacement), a clogged in-tank filter is the best overall fit to the symptoms. I should have read the case-history more carefully.

Hmmmm - fuel filter - that may answer the next problem. Turned out I had the wrong "diaphragm" installed. I used an identical piece that didn't have flaps in the holes - duh! . Started up OK so put it away till last weekend when I got stuck into some serious trimming. Ran great for an hour then gradually lost power - kept adjusting till it eventually died and now won't start. So guess the fuel filter needs a look see.

It shouldn't have run at all without flap valves in the fuel pump - must have been stretching the pump diaphragm considerably to make it act like flaps. You will have to replace the diaphragm with the correct one of course - let's hope that fixes it, but there may have been damage to the piston and bore due to sustained running at full load with lean mixture.

The symptoms all along were a good match for fuel supply insufficiency, which is usually caused by intake fuel filter, fuel pump mesh screen, or fuel pump diaphragm, in pretty much that order of probability. Less likely, but sometimes happens, it could be low metering lever setting, crud in the passage from fuel pump to metering needle (only possible if it had been running without the screen on that passage), or sticking of a frayed viton section of the metering needle. At this point you seem to have dealt with most of that except for the intake fuel filter, as Joe pointed out. However, until you deal with the filter and replace the pump diaphragm, you probably won't know whether you've overheated the piston and ruined the engine. If you want to, you could remove the muffler and take a look at the side of the piston, through the exhaust port. If there is any scoring, or serious discolouring, there may be a problem. If it is smooth and shiny, you should be good to go when you finish fixing the fuel issue.

Didn't run without the right diaphragm - just that when I selected the part from the universal kit of parts I selected on material (semi transparent with white grid) and shape - didn't look at the holes. After changing to the correct one it ran fine till it slowly died - have new filters on the way. If that doesn't work I'll pull the exhaust off and check the piston - in the mean time the grass is growing

While the grass is growing ,it would be a good idea to not only check the value of your piston as this is a quick and simple procedure and while you have the muffler off give it a good clean and take out as much carbon as you can. I like to soak the muffler in a bucket overnight and then blow out with the compressor and also check that the muffler screen is clear. Also good to clean the exhaust port clear of carbon by scraping it carefully and making sure no carbon goes back in the engine. Also check your spark plug as sometimes it's the simple things can trip you. Good luck matey.

New fuel filter in - haven't taken the muffler off yet as I have miss-placed the star head bit that fits the bolts. Don't know if it is connected to the problem or not but since I fitted a new fuel tank a few months back every time I hang it up after use with fuel in the tank within a week it is empty and oil all over the shaft. Thought it was just syphoning out and tried arranging the filter in the tank higher than the carby but didn't seem to help. Now after just trying to start it it was sitting on the garage floor and fuel was dripping out of the carby - tilted it over a bit and it poured about 5ml out, sat it back level and after a while it started to drip again. The carburettor is higher than the tank so why is it leaking? It was the correct replacement tank and cap. (thanks roebuck - it has a new plug)

Found a star drive on the end of a spark plug tool - couldn't undo the muffler bolts and twisted the star bit in the process. Ran a compression check and recorded 100psi several times so that looks good. There is spark