My stihl FS 85 starts and idles fine however when revved up it dies and sometimes stalls it runs irratite, i have replaced the diaphram, got new fuel and checked spark plug and any air pick up line leaks, fuel pick up filter is ok , this machine has only done approx 2 hours since new. I had left old fuel in hence why i replaced the diaphram.any suggestions.

I want to post on here, because I just solved a similar problem with my fs76 trimmer. I have used this a lot and then started losing power when under a load, I rebuilt the carburetor several times, thinking, I bought a cheap kit or something, didn't solve my problem, it would start strong, but then start to lose power, but making adjustments on the carburetor would allow me to continue at not full power. I also changed out the spark plug, and fuel filter, no improvement. I decided to try a different carburetor, and put one on from an echo, using the echo linkage and modifying the casing, not changing out the manifold, same problem(I was supprised the carburetor worked on this though, not all the holes to the manifold matched.) I figured this thing must have bad compression, and checked the compression, 120 cold, 94 hot . . .not the problem. Before checking the compression, I checked the spark arrestor, no problem there, took off the muffler . . .this had lots of carbon buildup, I chipped all of it away. Checking the specs for this model, I changed my gas to 50:1 as I had been using 40:1, spark gap good, cleaned spark plug, cleaned contacts on the fly wheel. Starting without muffler, starts right off, idles good, cuts good for about 3 minutes full power, then loses power. I started thinking maybe a air leak at manifold. I noticed that if I pushed my leg into the air filter cover pushing the carb into the manifold I had full throttle without power loss. So, I replaced the manifold to block gasket with an old Ryobi engine gasket cutting to what I need. . . same problem, so I tilt the trimmer to where the carb is pointing to my face, and I have full throttle I level it out runs good for about a minute and I lose power, I tilt it again and power restored. The problem was the gas cap!!!! The Gas cap, this thing had stumped me for several months, I cut at full power, high grass for 45 minutes, when I would lose power I would open the gas cap and then close it and power would be restored, I eventually didn't have to open It up any more.

Hi Trevor
Here are a few things to check.

Sounds like you have dirt/debris in your caburetor jets,fuel passages or a diaphgram that is becoming non-flexible.
Check/Clean/Replace your Air Filter, a dirty air filter can make your engine run to rich with fuel.
Also be sure to check and clean your spark arrestor in the exaust,if you have one installed on your engine.
Note: ALWAYS USE COMPRESSED AIR TO CLEAN YOUR JETS AND PASSAGES, VERY IMPORTANT.
Note:Before you disassemble the carburetor:
Write down on a piece of paper or take a picture of how the linkage attaches to your carburetor for later reference when you go to reinstall your carburetor.
Mark each piece with a awl, or some kind of instrument that will make an alignment scratch before you disassemble the carburetor into separate pieces.
That way you will know which way it goes back together when you reassemble it.
Sometimes you can get by with priming the carburetor or by using a very small shot of starting fluid and letting it run a few times like that, and it will flush the gunk out of the jets,but most of the time you will need to rebuild the carburetor.
Be sure to check your fuel tank for water and dirt/debris, if there is water/debris then you need to clean your tank.
Check you fuel line condition after a while they will degrade and need replacment.
If you need to replace your fuel line, cut the end of the fuel line to an angle for easy insertation into the fuel tank, see attached photo.
Check/Clean/Replace your fuel filter if you have one, normally they are located in the fuel tank of chainsaws and weedeaters.
When you remove your fuel lines from the carburetor be sure to make a drawing to how the lines are connected to the carburetor or take a pictue for later reference.
Normally the big line will be the line the fuel filter is connected to inside of the tank.The smaller of the two lines is the return to the fuel tank from the carburetor after it is pumped thru the carburetor by the primer bulb.
Make sure you are using fresh fuel...and oil if your using a two cycle chainsaw or weedeater with the oil to the right mixture...too much oil as it can cause hard starting and excessive smoking.
If the chainsaw/trimmer is over a couple of years old, then I recommend that you buy and install a new carburetor repair kit,because the diaphragm will get hard and that will cause it to be hard to crank.
The diaphgram may look good and flexible, but it can be deceiving and not act as a fuel pump as it should because it has become too hard and will cause hard starting,start and run and shut off, etc.
When you clean your carburetor, I recommend that you use a laquer thinner type cleaner to clean and dissolve the laquer build-up in the float and needle jet passages.
Be sure to remove all plastic and rubber parts before using the laquer thinner because it can dissolve the plastic parts and render them unuseable.
Be sure to use COMPRESSED AIR to blow out all the fuel and air passages.The higher air pressure is needed to blow some of the trash/debris from the fuel or air passages.
Be careful when blowing out the passages, because there are sometimes small rubber type seats in the bottom of some of the passages.
Keep in mind that the float (if you have one) for the carburetor must be level when you go to reassemble the carburetor or follow the instructions you get with the carburetor kit, or you could also ask the parts man that you get your kit from.
When you clean your carburetor and remove the jet screws, you will first need to lightly seat the jet screws.
But before you lightly seat the jet screws count the number of turns it takes to seat the jet screws from their original position.
Be sure to mark the turns down on a piece of paper.
That way when you put the jets back in, you know to lightly seat them first and then turn them back out to their original position before you started.
Note:
The little spring inside of the carburetor goes under the float arm.
That is where your fuel inlet needle/float valve is located...on the arm at the end.
Normally there is a small indention in the carburetor base and a small protrusion on the underneath of the float arm where the spring will be in the right postion for installation.
The spring will set in the indention and you will install the float arm with the needle/float valve and float rod into position over top of the spring,you will push down until it is in position and then you can tighten the screw that holds the float arm assembly in position.
Once you have your carburetor cleaned/rebuilt that should solve your problem.
Please take time to rate me
Bud

Fuel line trim

All pretty good answers. The guy with instructions for rebuilding is pretty good. Always slight variations though. Simplest thing, is buy a new carb and install. That way you don't spend untold hours trying to make sure the carb is JUST right. Also check the needle jet closest to starter , this is to set the lean or rich settings. Sometimes is not getting enough gas if this is to tight or even to loose. Proper settings is tighten all way, then turn back out 1 1/2 turns. Good Luck !