# Stihl FS300 - idling and overheating problem



## Kinay (May 26, 2017)

Hi everyone!

I'm looking for help with problem that is haunting me for two weeks now. I bought cheaply fs300 trimmer that had idling problem, hoping for simple carb rebuild. But problem is bigger and I'm slowly loosing my mind 

First the symptoms:
- engine starts nice, after 2-3 pulls, but stalls almost immediately but started on choke. Without choke it stats on goes up on revs (as trigger is locked halfway), but after releasing trigger it iddles unstable nad eventualy dies (after 10-15s)

At this point I wanted to test if idle is the only problem, so here what was next:

- if reved up it goes nice, have plenty of power for about 15 minues, then revs are going down and so is power (also if idled it will regain revs slowly, but still to pretty low level)
- if then left to idle it will die, and restarting it is next to imposible for a long time (4 hours waiting and still nie response)

I thought it might be coil when heated up, so i went another day to test spark on hot engine, run it for another 15 minutes just to realize i didnt take spark plug wrench with me. But at home when checking it out i found something bad has happedned. Muffler gasket was melted, and muffler itself has beautifull blue color on it. Also after these two 15 minutes sesions top of the piston is covered in something that looks like burned oil (no carbon deposit).


Went to local stihl distributor, and he told me after pulling stater there is low compression (but couldnt check it as he didn't have the fitting for large spark plug... and i just realized he should have used decomp valve hole), but it's strange as i replaced piston rings, and they have ~.011 end gap, piston and cylider have no scoring on them and rings fit nicely to cylinder walls.

What was done:
- carb rebuild (this is one screw adjusment carb installed, so no H,L adjustment posible), without taking out welch plugs, as im not sure if i can remount them propertly
- new fuel lines and fuel filter
- new rings, cylinder gasket, neddle bearing
- new spark plug
- checked impulse line, inlet rubber seal, searched for cracks

my guesses for now:
- carb is dead and need replacing
- compression is bad as service said, so new piston/cylinder
- some air leak, resulting in lean mixture and overheating
- mix of above...

Any advice on what to look for will be very helpfull. My local Stihl service is not very cooperative, and their only advice so far was to trash it and buy new one.


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## AVB (Nov 28, 2014)

Sounds you're running too lean.

Did you set the new metering lever and install the gaskets in the correct order?

Also if you don't have a copy of the service manual drop me a PM with your email address and I will send you copy.


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## Kinay (May 26, 2017)

Carb got new metering lever, needle, gaskets and diaphragms. Mounting order checked twice with Zama diagrams, and metering lever is set flush with carb body.

I got the service manual and been doing everything as it says.


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## AVB (Nov 28, 2014)

I would do the crankcase pressure/vac tests next to check for leaks as it could be a leaky crankshaft seal depending age of the unit. 

Now 2 cycles of any kind can give you fits to repair especially when you're inexperience working on them. It took me over 2 yrs to get a working knowledge of the in and outs. I still every once awhile have a new symptom thrown my way. Having the test equipment is a must and a good working experience plus having a working as model sometimes to swap parts out from. The reason for the second is to keep from spending a lot funds guessing at the problem. Also if you can keep the spare running can see if the moves from unit to the other one. Usually done as a last resort but necessary sometimes.

Now problems like yours it can be air leaks, failing spark plug, a failing coil, etc. 

I seen you mention you were going to check for spark. Coils can fail in a way that shifts ignition timing and you would still have great spark. I had a Husqvarna 570 that run great at idle and would lose power as the saw heated up. I just this had Husqvarna 390 that would not get above 7000 rpms and it sounded like it heavy 4 cycling. Both were coil problems. Several years ago I had a Homelite that would literally set the air filter on fire by randomly back firing on acceleration; again another failed coil. 

Then this week I has FS240 that was vibrating like it had a bad driveshaft or gearbox on acceleration but smooth out once up to speed and idled well. Turn out to just be a bad spark plug.


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## Kinay (May 26, 2017)

Thx for advice, unit is probably quite old, and was heavly used for sure.

Just checked old piston rings I removed ealier (they were probably never changed), and were extremaly worn out, near the ends their thicknes was below 1mm.

Is there any way to test timing, or should i just replace coil after doing pressure/vac tests?

One more thing, and i took engine apart today again i can see quite a lot of oil in the crankcase but piston itself was not very well lubricated.


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## Kinay (May 26, 2017)

My local Stihl service does not have equipment to do pressure testing on engine, so i will probably have to make testing flange replacement and do it myself.


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## AVB (Nov 28, 2014)

Hmmm, I thought most 2 cycle repair shops would the equipment. Apparently I wrong. 

Now the adapters are not always available but that rubber inner tubes are good for along with a modified spark plug adapter that the test hose can attached to. Or even a stiff piece plastic, basically anything that will seal the ports so you can do the tests.


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## Kinay (May 26, 2017)

I just pryed old seals, and they were garbage, hard as rock, deformed and crakced. Replaced them and tomorow will test if it helped.


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## Kinay (May 26, 2017)

So changing seals did not change anything. My last bet is worn piston/cylinder. Installed new piston rings, and end gap is 0.35mm/.015", compression feels great right after instaling piston with lube, but goes down after engine start (I can pull starter rope without lifting trimmer of ground). Intresting thing is, old rings were extremaly worn, so this engine was probably running with even worse compression.

Before replacing seals engine started without problems, even if it died on choke it could be restarted without it. Now after first start and death it was imposible to make it run again.

In this engine impulse line is just below intake, so i bet with worn piston on the bottom impulse pressure would be greatly reduced. Also i took apart once again fuel pump, and there is no sign of wear in diaphragm, just as it was not moving at all.


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## AVB (Nov 28, 2014)

That's one of the problems of changing just rings on most cycles. The ring is locked into place and wears the same way the cylinder wears. If the cylinder ovals then the ring wears to that oval. Installing a new ring will not correct the cylinder oval and actually can make compression even worse due lack of contact between the ring and the cylinder wall.

If have compression problems I have always change both the piston set and the cylinder.


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## Kinay (May 26, 2017)

AVB said:


> If have compression problems I have always change both the piston set and the cylinder.


Odered it on friday and hoperfully it will be end of my problems. After all there is not much to replace on this engine anymore


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## Kinay (May 26, 2017)

Changed cylinder and piston, and everything is working great. However compression seems to be the same as on the old one, new cylinder got no decomp valve but pulling it is very easy.

I think main problem was worn out piston, especialy bottom part on inlet port side.

One last question, how hot should head gears get? After some use it's quite hot, you can touch it but not hold hand on it. There seems to be good ammout of grease inside.


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## AVB (Nov 28, 2014)

Something I never have checked. I am however planning on picking up one non contact IR temperature meter from HF this weekend as I got a coupon for one in the email. Something I had on the back burner for some time.

Right now I would say if you can touch it; it should be fairly safe. I do air cooled engine get get quite warm at times. Hot enough to burn oil off the cylinder.


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## AVB (Nov 28, 2014)

Upon rereading your post I thinking you're were referring top gear box at the trimmer head end. If so they get quite warm but a person hand temperature being relative it can be normal or problem. I have seen quite warm to the touch myself and still be fine. Note there is a total of 4 ball bearings in the gearbox and any one could be bad; though, you be able feel if one if is bad. 

Initially I thinking you were referring the cylinder head which runs around 250F. 

I did test a FS55 that has a similar gear box with air temp at 75F and it reached 130F after about ten minutes. Now the FS55 don't the torque that the FS300 would have but I would expect similar readings.

Now I do know my can not take the asphalt if the temp is greater than 130F but I can put my palms on it fine.


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## Kinay (May 26, 2017)

Yes I was talking about gearbox at the end  it's getting hot but stable, no matter how long you cut temp stays about the same, so I think its ok.

Engine itself is running cool, no overheating. In fact I was expecting higher temps in normal running (as I said - no experience with 2strokes).


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## AVB (Nov 28, 2014)

If stable then it should be fine. If was a bearing problem the temp would have continue to rise. 

And glad you got it working. 

From here you can build on your knowledge of the 2 cycles. It is like other repairs it one step at a time and you actually learn from your failures and final repairs getting one to work. It took me nearly two years to get fairly good at these and I was working on them weekly. Still learning as I go as I work on more difficult problems repair.

I am now repairing 2 cycles that other shops have given up on as long as I can get the parts.


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