# Stihl FS90 only starts and runs when primer is pushed in.



## tpence2177

So I inherited a fairly low hours Stihl FS90 from my dad that he had accidentally dropped off a trailer going down the road. Dad couldn't get it started after a new carb and coil so he just got a new one drained the gas and set it aside for several years. I was wanting a dedicated brush blade trimmer to go with my FS80 so I didn't have to swap them out all the time. Got it to my house and got it apart and I found out why it wouldn't start. The rocker arm had come off the exhaust valve. Got that fixed. Primed it up and it flooded really bad the first time I tried to start it (acted like the tank wasn't venting because gas would pour out of the mouth of the carb even with just turning the trimmer over a couple of times, as well as I could hear a hiss when I opened the gas tank.) so I pulled the plug over night and loosened the gas cap. Next day I got it to crank but it only stays running if I press the primer bulb. Trimmer runs great as long as I feed it gas with it. Does it sound like the fuel lines are on backwards? That was my next adventure considering so far this thing has cost me nothing. Piston and cylinder looks great from what I can tell. Don't have a way to measure compression right now though so I am unsure of that. I would think it would be ok though. Primer fills up just fine with no staying deflated or anything. Thanks everyone for the help!


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## Conquistador3

If you need to keep pumping fuel into it, it's likely the carburetor's fuel pump is dying or already dead: it's parts # 15 and 18 in this diagram http://www.zamacarb.com/carburetors/309


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## tpence2177

Ok so likely dad just got a bad carb? If so that's an easy fix. It had only had fuel in it once before and then was drained so I was thinking the carb was still good. I'll check into that thanks!


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## Conquistador3

Not necessarily: since you said the brushcutter sat unused "for several years", the membranes may have simply hardened up and/or cracked due to age.


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## tpence2177

Gotcha thanks I'll order a new carb 


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## Bedford T

Have you pressure tested the fuel circuit? If it's not delivering fuel only when you do it manually it could be outside the carb and that seems feasible since it took a big tumble. I don't have a IPL so not sure how impulse plays in the matter but you might check that


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## tpence2177

I'll have to try and find an Ipl and look. Thought about changing out all the fuel lines before I do anything since dad uses ethanol fuel, even though he drained it I'm sure there was still some in the lines. Going to do that and order the orange cap to hopefully fix the gas tank vent problem. Need to watch a few videos and make sure that the lines are hooked up correctly. My dad has been a mechanic for almost 50 years but I don't know if he marked them before he swapped carbs. I'll do all that and then if it doesn't help change carbs. If I remember correctly it only set around 2 years empty. If none of those fixes work I'm sure I'll have more questions lol. 


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## lone wolf

tpence2177 said:


> I'll have to try and find an Ipl and look. Thought about changing out all the fuel lines before I do anything since dad uses ethanol fuel, even though he drained it I'm sure there was still some in the lines. Going to do that and order the orange cap to hopefully fix the gas tank vent problem. Need to watch a few videos and make sure that the lines are hooked up correctly. My dad has been a mechanic for almost 50 years but I don't know if he marked them before he swapped carbs. I'll do all that and then if it doesn't help change carbs. If I remember correctly it only set around 2 years empty. If none of those fixes work I'm sure I'll have more questions lol.
> 
> 
> Sent from my iPhone using Tapatalk Pro


Ever fix it?


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## tpence2177

Got a new tank vent that I haven't installed. It started the other day but flooded out. Just had the plug out since Friday. Going to try and change the vent out in the next few days and try it again. Thanks for checking in! I'll report back soon 


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## lone wolf

tpence2177 said:


> Got a new tank vent that I haven't installed. It started the other day but flooded out. Just had the plug out since Friday. Going to try and change the vent out in the next few days and try it again. Thanks for checking in! I'll report back soon
> 
> 
> Sent from my iPhone using Tapatalk Pro


Keeps flooding ,I bet the needle is sticking or the lever is not set right. Take it apart and look.


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## lone wolf

http://www.zamacarb.com/assets/images/techsupport/AdjMeteringHeight.pdf

Basically that lever should be parallel to the body is all. Make sure nothing is stuck where the needle seats.


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## tpence2177

Awesome thanks! I'll check it out in the next few days!


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## lone wolf

tpence2177 said:


> Awesome thanks! I'll check it out in the next few days!
> 
> 
> Sent from my iPhone using Tapatalk Pro


Don't lose the little spring. Use grease on the spring to hold it in place when you put it back. You can also pressure test the carb at the fuel inlet to see if it holding 7 lbs is good with no drop.


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## tpence2177

Need to update this thread. Got it back running great today. Don't have an area or the time to tear into the carb so I put a new stihl carb on it today. Took 10 minutes and it was back running strong. Thanks lone wolf for all the help! I'll tear into that other carb to learn how to fix them. 


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## lone wolf

tpence2177 said:


> Need to update this thread. Got it back running great today. Don't have an area or the time to tear into the carb so I put a new stihl carb on it today. Took 10 minutes and it was back running strong. Thanks lone wolf for all the help! I'll tear into that other carb to learn how to fix them.
> 
> 
> Sent from my iPhone using Tapatalk Pro



Good deal. How much was the carb 45.oo?


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## tpence2177

$50 plus tax so like $55. I could've gotten a Chinese one for $10 but I needed it up running soon and today was a good day to tinker so I went for it. Grass is already growing here in Alabama. 


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## lone wolf

tpence2177 said:


> $50 plus tax so like $55. I could've gotten a Chinese one for $10 but I needed it up running soon and today was a good day to tinker so I went for it. Grass is already growing here in Alabama.
> 
> 
> Sent from my iPhone using Tapatalk Pro


OEM is mostly better.


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## tpence2177

That's what I was thinking. I had nothing in the weed eater and I successfully got it to run on prime several times so I knew it was good. Didn't mind spending that to have an update from my fs80 which is going to have the grass blade on it permanently now 


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