Export thread

This brushcutter starts and runs but wont idle and has no power. The only way I can keep it running is either by continually triggering the throttle, or slowly open the throttle to about 1/3 and hold it in place. From there it will run (sort of) but produces no power and will not rev out. The power is insufficient to even engage the clutch.
The motor does have compression and spark.
There was some water and contamination in the fuel tank and so all fuel was replaced with new.
Carb, filters (Fuel & Air) hoses and spark plug have all been replaced with new parts.
Even with those parts renewed the running symptoms remain unchanged.

Ideas from the Sthil brains trust would be appreciated.
Thanks in advance.

New carbs don't usually come ready to run out of the box.
You will have to adjust the carb to were it seems it's running at its best.

And also, your model make and serial numbers would be greatly appreciated.
As for it will help us diagnose your problem better.

Welcome.
First question
1) genuine Stihl parts from a Stihl shop
2) parts from a real mower store or a real on line retailer
3) parts from a dubious supplier as found on Amazon & Ebay .
4) do ou run the recommened fuel : oil ration ar are you inclined to add a little extra oil ?

next, when was the last time you used it ?
Where was it stored between then & now ?
Was it running 100% the last time it was used ?

And a quickie to rule out mud daubers, remove the muffler and see how it runs.
This will be both very noisy and will most likely destroy the exhaust gasket.

Can be the spark arrester on the muffler. It appears to be the screw in type. These are bad to clog with carbon. Also considering the age of these units it is possible that you need a carburetor repair. Considering the recent ridiculous changes at Zama it is just about as cheap to replace the carburetor as it is to get a full carburetor repair kit.

logert gogert said:

And also, your model make and serial numbers would be greatly appreciated.
As for it will help us diagnose your problem better.

Click to expand...

In this case he posted the model number in the header. For Stihl it is about all that is need to look up the parts on the IPLs I have here. Now Stihl is constantly superseding PNs so only a Stihl can give the current PNs numbers for 4140 power plant.

Thanks for the feedback.
The make is a Sthil and model is FS 55 RC as listed in the subject line.

It has a new after market carb but runs exactly the same as it did with the O.E. Sthil carb.

I'm checking it over for a friend. The fuel supplied by the owner is fresh (today) premix.

My guess is that it had been sitting for a while but not more than a winter season. Stored in a garden shed.

I'm curious about the 'spark arrester on the muffler' comment by StarTech????? If they are clogged might that be a possible cause for the poor running a start condition??... If that is so is there a solution for cleaning the part?

Yes as the engine can't breathe properly.

Remove the spark arrester and use a propane torch and burn off the carbon , just don't heat it as it can melt if too hot. If is one I thinking it has it take a 15mm deep well socket. Plus if it is the screw in type orchard wasps can plug them up but it is more likely it is just carbon plugged.

Make sure the carburetor is tuned correctly.
Max. permissible speed (with cutting attachment): 10,000 rpm (If above this tune rich to limit rpms)
Idle speed: 2,800 rpm

Pull the muffler off and look at the piston for scoring. What you describe sounds like low compression.

A 16 mm or 5/8" socket to remove the little tube/screen on the muffler, it is plugged. Burn off the carbon or clean out the tube, and wash the filter in hot soapy water.

Thank you all. After removing the muffler I found no piston scoring so all good there. The spark arrestor was filthy and the muffler was carboned up as well. Muffler has been burnt out and after a light wire buffing the bulk of the grime on the arrestor was removed. I then dipped it in some petrol and let the rest burn off. All is now clean and again once back together I expect it will be able to breath and run once again. Been a long time since I've messed around with old 2 smokes but it's coming back to me. Appreciate the feedback.

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?

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.

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 .

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

StarTech said:

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

Click to expand...

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.

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

Maybe this will clear things up.

Zama Carburetor Systems