Sthil FS 55 RC brush cutter runs but has no power ?????????????

[patanga]

It's all back together and the carbon has been removed (fired) from the muffler and the spark arrestor. While the runing has improved somewhat it still won't rev out cleanly and the throttle response remains poor. Along with that, the only way I can get it to idle on its own is to screw the idle adjuster in to force a result. I've tried tuning the L&H mixture adjusters as best I can using one turn out on both as a base setting. I've also tried swapping the Sthil carb with the aftermarket unit and while the new carb seems to deliver a marginally improved result it hasn't made much of an improvement. In terms of the gains made; At least the motor now produces enough power to turn the shaft and the cable head but not in a way that it could do any work. To my amazment none of the Sthil agents carried gaskets for the muffler and or the carb base and so it was hi temp silicon for the former and a reuse of the carb gasket. I don't think the carb gasket would cause this symptom but I'm not sure what else could be going on. Thoughts anyone?

 

[StarTech]

The initial H screw setting is 1-1/2 turns. As L it initial setting is 1 turn and using a tach set the LA to 2800. From there is a matter slight adjustments to get the best throttle acceleration. After final tuning then reset idle speed back to 2800 rpms. These carbs can be picky on the adjustment so patience is key.

 

[bertsmobile1]

Start the trimer & let it warm up .
Hold the throttle wide open then turn the H needle VERY SLOWLY in each direction till engine starts to faulter then return it to the 1/2 way point between the 2 faulter points .
release the trigger then do the same with the L needle
Adust the throttle stop
Then go back and tweak the H and L needles again.
Some times you need to do this a dozen or more times before you get it spot on .
While the needles are marked L & H , the L will supply fuel over the entire throttle range so it does affect the full throttle response just it is much lesser than the H needle .
The same applies to the low speed idle, if the throttle stop is wound up too far .
And I can not empathise the word SLOWLY enough.
These engines are tiny and when the fuel is compressed in the crankcase some of the fuel and oil will condense in the crankcase and it will take a second or two come into balance for that speed.
The adjusting tools all have square handles and I cut a notch on one edge so I can accurately count the turns .

 

[StarTech]

You do know that L affects the H adjustment, that why L is adjusted first.

 

[bertsmobile1]

You do know that L affects the H adjustment, that why L is adjusted first.

Only if you have the throttle stop down far enough not to be drawing fuel through the main jet .
We might need to agree to dissagree on this point.'
No matter which jet you start with you always have to go back for a second time at least to get it spot on .
The further out the H needle is the lower signal it will respond to .
It is not uncommon to get a unit in with the L fully shut off and idling on the H with the throttle stop open way too far .
And funny enough they always say it is hard to start from cold And usually they have not touched the carb.
Most get done in 2 hits except the EPA compliant ones where the thread pitch is so fine that rich faulter & lean faulter are better than 2 turns apart.

 

[patanga]

O.K. thanks. I'll give that a go and report back.

 

[StarTech]

Maybe this will clear things up.

Zama Carburetor Systems