OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Trimmers and Brushcutters Two Stroke Trimmers & Brushcutters Stihl stopped working, trying to fix...

Hey all,

I have a STIHL KM56RC trimmer that was running fine, then decided to not start/run anymore, so I took it a part, gave the carb a good clean with parts cleaner spray, inspected the gaskets/diaphragm, all seemed good, but I still couldn't get it to start properly... it would only start with the choke on (half I think, it was the first click.. will check tomorrow).
Checked online, a few videos of possible fixes, so I tried them, no fix. I ended up taking it apart again, gave it another good clean today, made sure parts cleaner was making it through the hole, so i could see that it wasn't blocked. I used fresh fuel, checked gasket/diaphragm again, seemed fine.
Wound the High/Low screws all the way in, as well as the idle, tried again, still not quite right. It start with the choke after a few pulls, but dies without it.

I started it again with choke, and went full-throttle, then adjusted the high screw, still wasn't quite right when I let go, but eventually, it would run without choke, or throttle.. but eventually cut out. I tried again, and tried adjusting the low screw, for some reason it makes little difference, I could probably unscrew the whole screw and it wouldn't help... so I'm thinking, it's possibly to do with the low rpm screw section.. but I remember cleaning it until the cleaner came out of the other end, so I'm not sure why it would be blocking up?

Aside from cleaning again, which I can't see making a huge issue, what else would cause it to not run correctly? to me, everything seemed fine when I cleaned it

iirc too, if I backed off the idle screw from being all the way in, it would die.

Another thing I noticed, there seemed to be oil coming out of the exhaust port, or at least near that area (see pics)... not sure if this is normal for 2strokes, or possibly an issue ?

[edit] I went to do a compression test, but my tester seems to be busted. I think I need to replace some o-rings, then hopefully it'll work again.

Any suggestions?

Have you checked the muffler spark arrester for clogging?

As above typically is just a blocked muffler, however could be a few things..

From stihl themselves the proper way to check the carburettor is to pressure test it, mainly by pressurising the fuel inlet line and making sure it holds. Have you put a kit in it?

There are a few circuits in the carb that you can't check or clean without removing welsh plugs, but carb issues like that are usually only a problem if it hasn't run in a long time

Also check the fuel lines and filter, a tiny hole in the fuel line or blocked filter will do exactly this

Not sure why you tried it with the screws all the way in? that will stop all fuel from going through the carburettor. 1 1/2 turns out is usually where i would start

A compression test won't tell you very much other than if the rings are ok, and I am assuming you have done the visual check through the exhaust port..

Only other real test for a 2 stroke is a crankcase pressure test to check the crank seals are holding, Unlikely being quite new but not impossible

Hi Widget,

One suggestion before go any further is to replace the fuel filter in the tank and test again.

A mate ok mine had terrible problems with a Stihl chainsaw, he pulled the carby apart several times before taking it to a Stihl shop. They knew straight away what the problem was it had a pinhole in the carby feed line and said it was a common problem with them. To me that plug looks super lean so could be the fuel supply problem

Didn't think to check, as it ran, and it seemed the exhaust was functioning. I'll take it off and give it a good clean. Should I just clean the arrester, or try clean all the inside of the muffler ?



I haven't. Did consider picking one up years back, but forgot about it. I think I saw one that supposedly does vacuum testing, and pressure testing, on ebay, will have to have a look.


Without knowing the condition of the carb, would it be worth removing the welsh plugs, and cleaning, and replacing them, or not worth it ?


I had a look at the filter, it looked really clean, almost new. I'll take a pic and post, but in the meantime, before buying a new one, would it be worth taking it out, cleaning with parts cleaner, and maybe pushing a little air through it to see if it isn't blocked ?


Sorry, I meant all the way in to a minimum, but not fully closed. This is the video I followed when I did it..https://www.youtube.com/watch?v=gmhxTvGRtCg



Would that only affect fuel delivery at lower RPMs ? When I hold down the accelerator trigger to max revs, doesn't seem to cut out.


Thanks for all the feedback


Q: Is there any difference in using a parts cleaner vs using a specific carburettor cleaner ?
This is what I use, https://www.supercheapauto.com.au/p/sca-sca-brake-and-parts-cleaner---400g/0240.html#vid=EpzUqZuHNVTOR1UXVpMMXA%3D%3D&q=parts+cleaner&lang=en_AU&start=6

Should I be using something like this https://www.supercheapauto.com.au/p/sca-sca-throttle-body-and-carby-cleaner---400g/0238.html ?


Thanks again.

Widget,

A piece of advice here on these carburetors. Get it ultrasonic cleaned, regular carburetor cleaners can damage the delicate rubber check valves inside the carburetor; plus using an ultrasonic cleaner you don't need to remove the welsh plugs normally though I do remove the mixture screws.

A couple setting that comes to mind. One is the metering lever height, it is flush with casing. Use a straight to set this. Second the initial mixture settings is usually 1-1/2 turns ccw from lightly seated position. This is the Zama will start settings. You fine tune from there.

As for the compression test mention. It only tell only the condition of the cylinder, piston, and rings but will not tell if you have a crankcase air leak. This is what the pressure/vac tests are for. As pressure testing the carburetor, the test will only show if the fuel pump side is sealed and that the metering valve is leaking or not. You can not test the metering side.

One note if the metering diaphragm is stiffen it will cause problems is it will move as intended controlling fuel flow.

Added Note: If when doing compression test. You do it with the throttle held open. If you readings are @ 100 psi or less than you have a severely worn cylinder or stuck rings. From my experience it takes at least 100 psi for a 2 cycle to run correctly

[update] - still issues, new issues...

Regarding the fuel lines: I removed the lines, cleaned and inspected them. I couldn't see any holes or tears, they all looks pretty good. The green fuel intake line, I plugged one end, and put the air gun nozzle on the other, stuck it under water, didn't see any bubbles. The test was a little tricky to do, as I'm not sure how the line is made, but pressure seemed to build in sections of the hose causing it to bubble line blowing up a balloon.. so the test may have been a dud, and not accurate. I checked the hose again after that, still seemed fine. Do you reckon I should change it anyway ?

Regarding the Fuel filter: Couldn't find spare (good) filter on hand, but I removed the one in the Stihl, it looks pretty good and clean. I put a bit of compressed air into it, and I could feel it coming out of the filter-mesh. Change anyway ? (see attached pic)

Regarding the spark arrester, I removed the mesh, and it looked fine, not blockages, looked clean (see attached picture). However, I have noticed that running the trimmer without the muffler, seems to run, not well, but doesn't cut out when I off the choke... however, putting the muffler on, it cuts out. I blew air through the muffler and felt it exit, so I'm not sure why it would do that.


Yes!, I've been definitely looking into getting an US cleaner for a while now . I'm not sure if it's big enough, but I have been keeping an eye out for a small cheap unit that's used for jewlery etc.. but feel it may not be deep enough for trimmer carbs. The other I'm looking at is maybe a larger 6L (give or take), but they're much more costly, so have to save.


Will have a look, though do they always have to be flush? I serviced a Weed Eater the other day, cleaned the carb, and had a look at the lever height.. it's much lower than the casing, but it starts first pull and runs alright. I'll have another look at it hopefully tomorrow.

As for the compression test mention. It only tell only the condition of the cylinder, piston, and rings but will not tell if you have a crankcase air leak. This is what the pressure/vac tests are for. As pressure testing the carburetor, the test will only show if the fuel pump side is sealed and that the metering valve is leaking or not. You can not test the metering side.
If when doing compression test. You do it with the throttle held open. If you readings are @ 100 psi or less than you have a severely worn cylinder or stuck rings. From my experience it takes at least 100 psi for a 2 cycle to run correctly[/quote]

Thanks for the tip . I managed to do a compression, holding the throttle open.. it read 95psi. The piston is a little rough (as I'll mention below)


So after inspecting the hoses and spark arrester, I looked into the exhaust port, and there is scoring. I decided to just take the head off and see how bad. I'd say it would need replacement, but is this likely to be the/an issue ?

I pulled it a part, cleaned it, but it back together, and got it to start. I could only get it start by using the choke, but when it started, i flicked the choke off and it stayed on (without the muffler, as mentioned above). I tried messing around with the Hi/Low/idle, but having issues there. The low I could get down, but not too low otherwise it'll cough and end up stalling. I couldn't start it again, even when hot, unless I use the choke... but once started, I turn choke off, and it seems to idle fine.
Trying to adjust the Hi screw does nothing now, even turning all the way in it revs so loud/fast (see video).. I can't change that. Now, I noticed there was red gasket material on the lower part of the engine.. I didn't replace that, though should... could having a broken seal there cause a significant increase is RPM ?

[EDIT] Note, after putting it all back together, I noticed the oil/fuel leaking out didn't occur. Looking at the gasket, I'm not sure if it spat out the exhaust due to the scored/stuck piston ring, or whether there was a leak from there. When I removed the head to reveal the piston, I also removed the ring, and cleaned it up, then put it back on and made sure it had movement... perhaps doing that caused it to seal the cylinder better, preventing excess oil coming through the rings ??


Attached are some pics, and links to some video... at this stage I'm ready to shelve the trimmer, as I can't figure out why it's doing it, though could be the piston.. will have to look into whether it's worth buying a replacement.

Higher revving video -
Idle/stall video -

A couple comments here.

First that Weedeater brand probably had a Walbro WA or WT carburetor and yes they are set differently.

YUCK...As to the piston and cylinder condition. Looks like was ran too lean on oil or straight gassed.

I usually just kill them off when they are this bad but costs may be different in you neck of the woods.

And you had it running with that piston, hard to believe. I have found that with some of the Victas with pistons/rings like that the have compression but they will not start, can't understand how this one ran. New piston and rings and a hone and it might run again, no idea as to what is available from Stihl

Cylinder plating too thin to hone plus Stihl has no oversize pistons and rings available; just the standard size stuff. Done tried honing with a flex hone and plating was gone before the cylinder cleaned up. Pricing of new parts is steep here.

Stihl recently forced Zama to stop selling repair kits for the Stihl only carburetors here. I now paying as much as 4X for the same kits and it gets worst as none of the local dealers even stock the kits so I am sock an additional 20 usd shipping fee.