You take the muffler off the make sure the exhaust port is not clogged?

After checking that, mix 2oz of SeaFoam to one gallon of gas, run a tankful to clean the carb and see whether that will help.

Write back after trying these.

Try getting a fresh batch of gas again. Sometimes even a "fresh" from the pump batch is marginal.

It won't be the exhaust port if he is able to run it for 5 minutes before it starts to die (I assume it runs under load in those first five minutes). If exhaust is clogged he would not be able to run load at all if it started and it would be obvious from the get go, not after five minutes. I would guess a fuel problem or an air leak. Chances are your spark is working but you can confirm that if it is checked shortly after it stops running.

I would compression test (probably good since you are getting it started). More importantly I'd do a pressure/vac test. I'd also do a good carb rebuild. The parts are easy to get and not too expensive. They are really not that difficult. But I understand if you don't have time or the desire to do that. A pressure/vac test is more involved in terms of tools. The only other option I could suggest would be to find a local guy that is good with small engines.

Yungman....exhaust didnt seemed blocked. I couldnt find SeaFoam locally but I tried another ether based fuel line cleaner (Start Your Engine) that appears to be similar. It ran fine for about 5 minutes, the conked out again. THen it wouldnt even idle well after that. I've messed around with the fuel mixture screws and the low altitude screws so much now Im not sure how to get them "correct". They are marked as 1 full turn on the low speed screw and 3/4 turn on high speed. The Low Altitude screw is adjusted until the head doesnt turn.

masiman...I found someone locally who said he could look at it tomorrow. Dont know if he knows his stuff, but hopefully so.

Frustrating for a pretty new machine....I found the receipt when I was looking for the manual and I bought it last May so it is barely 13 months old. This is the 2nd Stihl tool that has given me problems. They had to replace the clutch in my chainsaw after 3 months. Just waiting for my blower to die next....:(

Don't rule out carb issue, My Honda behave like that, it ran find until a few minutes into trimming then it started to bog when applying throttle. It was a blocked carb. I actually found metal fragments that plug the jets. I cleared it and it work.

SeaFoam is good for cleaning carb. You have to run a tankful of the mix to tell the difference. I hate to have you go open the carb at this point.

As for vacuum leak, you can always spray a little soap water at the seams and run the engine to see any bubbles come out. For a one year old trimmer, my bet is dirty carb. Even if you empty your tank, there are still gas in the carb and the primer bulb. they do go bad. best is to use Stabil. When I put on long term storage, after empty the tank, I position the primer bulb to face up and prime out all the gas. re-empty the tank again. then I close the choke, hold the throttle WOT and start it to run it dry.

I won't spend any money until I run a tankful of SeaFoam mix. Takes one tank to clean it. It is not going to do magic.

And the winner is.... DIRTY CARB!!!! ok, no surpise I guess. He tore it down while I was there, cleaned everything, blew it out. Found a bit of sludge in a small screen fuel filter nside the carb body and figured that was it. Took it home and ran it for almost 3.5 hours yesterday.

I guess I've learned my lesson about refeuling around clippings and the TRUE definition of "fresh fuel"

Glad you fixed it. Clipping usually don't get into the carb because it is too big to pass through the gas filter. It is either inside originally and work it's way to the critical passage like in my case or it is gum formed from stale gas. Use Stabil and try my way of winterizing next time.

Junk it and get an Echo or Tanaka.

I was having the same "won't start/hard to start" problem and replaced the carb on my FS55R and it still won't run. It barely sputters but won't run.

It is about 3 years old. I pulled my old Echo curved shank off the wall, which I hadn't run since the Stihl arrived and it runs. Don't know what to do with the Stihl...it's good when it runs. What do you suggest?

Hey bro..11.99 for a new carb. I dont even bother any more.

https://m.ebay.com/itm/Carburetor-For-STIHL-Zama-FS38-FS45-FS46-FS55-FS55R-Carb-4140-120-0619-/191556257558?hash=item2c99a48716%3Ag%3AYC0AAOSwdpxUWOgo&_trkparms=pageci%253A7f123bfe-9c1f-11e7-98dc-74dbd180a0d1%257Cparentrq%253A92faf87515e0a9cb47529f2dffe95a15%257Ciid%253A1

Thanks! Maybe your new carb is better quality than the HOOAI I just put on! Could be. https://www.amazon.com/gp/product/B01MUK0O78/ref=oh_aui_detailpage_o03_s00?ie=UTF8&psc=1

What is better Poweka, Zama, or FitBest?

Zama

They all use cardboard seals which is the problem. I actually researched the polymer components and found statements by the makers re alcohol not effecting their product. Its a game of dollars.

Zama have EPDM !

no. sorry. kiss my ring. the seal is made of wood, but the important parts, yes I could say that.

All EPDM . Kiss Away . Also Walbro Commercial grade carbs on Husky , Dolmar and some older Stihl Saws .

Not those cheesy gaskets I get, but you are specifying commercial grade. I have not had to rebuild my stihl ts400, but neither have I built my bg85 blower. My big Stihl back pack blower was rebuilt, the pole saw, the little chainsaw etc. Even the stihl auger. My kits have a pressed gasket that matches the valving gasket and I swear that is the weak link in the design.

Most definitely Commercial Grade Only Bro . I agree , the cheapo homeowner grade kits are not worth the postage lol . EPDM or the previous Viton kits are the only way to go . Having been rebuilding carbs on various saws . blowers , trimmers , snowmobiles etc for over 25 yrs , u get what you pay for . $3.00 kits do not cut it . I pay $12.00 to $13.00 but they outlast the saw these days . Back in the day tillotson and keihen were the norm unfortunately offshore products have flooded the market . Sometimes you can find retro Walbros carbs on web with viton seals and gaskets for $6.00 to $8.00 , I buy them up immediately when I find them lol .

Problem solved. Clogged muffler screen.