Stihl FS120 starting / running issues.

[ipbr21054]

Last few weeks this FS120 has been a pig to start.
I put on the choke, a few pumps of the fuel bubble & pull it over. Once it tries to start I turn off the choke & start to pull again. After a couple of pulls it’s starts & all fine. This is how I have always done it.

This didn’t work last few times so now I’ve started to look for the answer.

I took the carb off & cleaned it out with carb cleaner. Carb is 1 year old as replaces it myself.
All seemed fine so tried my starting process.
I also fitted a new plug & air filter ambush for the sake of it.

So today I’ve started to pull it over but I see petrol in the floor so assumed I’ve flooded it, not because of my starting method but because on an issue I think ?

With cover & air filter removed I pump the bubble once & see fuel coming out of carb & into floor. It comes out the direction of where the air filter would be. Do we this tell you anything ?

I remove plug & looks dry.
Refit plug & pull it over again & it starts.
I can rev it fine so I leave it to warm up a little.
After say a minute it stalls.
Now I’m expecting it to start no problem seeing it’s been running fine but I can pull it over say 3 times & nothing.

Does this sound like spark or fuel issue.
Can’t see a spark in this daylight but once it starts & runs I assume that side of it must be ok.

 

[Burnerman]

I guess this is a 4 Mix engine?
Pumping the primer bulb should allow thecarb to fill, and excess fuel needs to return to the tank.
Look for displaced fuel tubes coming from and returning to the fuel tank, and if all is well here it’s a carb strip down as the needle valve may be stuck open.
John

 

[ipbr21054]

Hi,
4 mix you say ?

I removed carb again this afternoon.
One end when cover removed i see the needle valve which works alonside the see saw part & spring.
This works fine even when blowing through it & holding it down & trying to blow.

The other end is drilled holes & the gasket is more like a mesh.
I sprayed carb cleaner in all the holes & it comes out the other end.

I started it up & run for a small amount of time before it cut out,3,4,5 pulls but nothing.

Im going to see how to set up carb correctly even though ive done this & used it for at least an hour straight with no issues.
I see a pipe from the tank to primer bulb then primer bulb to carb.
Pulled off at the car & when operating primer bulb fuel is passing through fine.

 

[Burnerman]

The 4 Mix engine is a four stroke unit, made by Stihl. Although it has valves, camshaft and pushrods etc it has a total loss lubrication system and therefore needs oil mixed with the petrol at 50:1 ratio as normal.
This is an attempt to reduce pollution as the engine has half the firing strokes compared with a two stroke engine.
Anyway, if the engine is difficult to restart, try one push on the primer bulb before trying.
If the needle valve is working, fuel won’t pass into the carb Venturi and out of the air cleaner.
I’m not a fan of the diaphragms being reused but I guess you’ll know that by now!
John

 

[ipbr21054]

No it’s 2 stroke strimmer
So no valves

 

[Burnerman]

OK, it's an earlier model then.
The same applies with the starting technique - if the engine is running lean it may need one pump of the primer bulb to get it going again.
Has the engine good compression?
John

 

[ipbr21054]

Compression I would say yes.
I’m sure it’s fuel related.

My starting process mentioned above should be correct as that is what I do on all my stuff.

I don’t think it’s getting enough fuel.
I will see what I can find about setting up carb.

 

[Old Salt]

Take a look at Mick the mowerman site on you tube. He has some videos on strimmers, chainsaws etc. Cleaning carbs and setting up.

 

[ipbr21054]

Ok
Ive since found them.

When you said it’s an early one do they do a 4 stroke strimmer ?
Stihl FS120

 

[ipbr21054]

So I’ve watched a video or two & set the carb up going by the advice watched.

Still a pig.
I see fuel coming out the air filter like before so it’s flooded.
Today after I had it running for a while I switched it off & topped up fuel tank. Pig to start again & im expecting it to start fine seeing it’s fine & been running well but no nothing. Not sure why but free I seen fuel running out the air filter I removed air filter cover followed by the filter itself. One pull & it started ??

So today when I didn’t start like I expected it to I popped off air filter etc. Pulled it over & started.

What does that tell you ?

 

[Old Salt]

New air filter.

 

[ipbr21054]

Isn’t clogged & clean.

It’s doing something with the mixture.