OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Trimmers and Brushcutters Two Stroke Trimmers & Brushcutters Stihl FS36 dies at low RPM after warming up...?

Dear Gents,

My (formerly) trusty-ol� Stihl FS36 has recently developed the habit of dying at low RPM after several minutes of operation (which presumably indicates that the problem is somehow temperature-related�).

The unit is quite happy after first start-up, and I don�t sit on �choke� for any more than a couple of seconds, even though I�ve got both the high and low needles set to fairly lean mixtures. I don�t normally run it at any more than mid-range RPM, so even after the ten minutes or so at which the problem starts to occur, the motor still wouldn�t be terribly warm (especially in this weather�) Once the problem does start, the unit will still run strongly if I maintain high RPM, but if I let the revs drop too much, it will either immediately die, or start �hunting� around a bit before eventually dying anyway. Restarting never seems to be terribly difficult, even when the problem is in full swing, but if I don't give it full throttle at such times, it just dies again...

My own thoughts are that it�s no longer possible for the problem to be caused by something like gummed-up carburettor jets, because my first reaction was to pull the carby apart and to clean it to within an inch of its life (including the little gauze filter-screen), to no avail. For what it's worth, the carb itself is a Walbro WT160B, and I replaced both the pump and the metering diaghragms only a couple of years ago.

As regards the possibility that the breather-valve for the fuel-tank is clogged up, I�ve also given that a clean, as well as removed the fuel-cap momentarily when the problem has started to occur, with no improvement resulting.

Might any of you happen to know whether there are any known temperature/age-related Ignition Module faults that are endemic to this model, or have any of you had a similar problem that has gone away after they tried replacing/swapping the Ignition Module? Going by the following (unfortunately inconclusive) thread, I don't think my unit's problem is an isolated one:

https://www.outdoorking-forum.com.au/forum/ubbthreads.php/topics/712/Stihl_FS_36.html

Many Thanks in advance,
Moby Mick.

Moby, I doubt it is an ignition problem because it wouldn't restart, just the fact that it will only run at high revs indicates a fuel issue and I'm thinking air leak once things start to expand

Hello Norm, and thanks very much for replying

My thinking with regard to the possibility of the issue being an ignition problem was that perhaps the module was cooling down slightly each time the unit clonked out, or maybe that the RPM being induced by a vigorous tug on the starter cord was higher than the threshold amount needed to still generate current by the module when warm and faulty. I hear where you're coming from, though, and it's difficult to disagree with your rationale.

So, you're basically thinking that the mix might be ending up too lean at warm low RPM. Might you have any specific scenarios in mind wrt the "fuel leak" idea? It's difficult to think of any parts on the motor that wouldn't have a tendency to instead tighten up when the unit got warm...

Best Wishes for now,
Moby Mick.

Hi Mick, a vigorous tug on the starter cord would not spin the motor any faster than the motor would be spinning at idle so I would rule that line of thought out, my thinking is something is expanding slightly in the carby body, just enough to allow a slight air leak, the fact it will run with revs makes this sound like a possible scenario. Always so much easier if you have parts to swap around to test

Roger on those carb-centred thoughts Norm. It would surely have to be a casting crack or something like failed brazing for it to repeat itself both before and after I dismantled the carb to clean it. The trouble with buying another FS36 2nd-hand for swap-over purposes is that the owner might be selling it due to the exact same problem as what I'm having with mine...

I might also try some further googling of the problem, but from the Walbro point of view as opposed to the Stihl.

Many Thanks again,
Moby Mick.

gday seen thise so many times sounds like a blocked muffler screen give it a clean the way u go

Most times a plugged spark arrestor will give allow you to idle but will not allow you to throttle up.

When carburetor was rebuilt was the metering height set or did you just replace it with the one in the kit. The ones in the kit are not set correctly. If your metering is too low ie too lean then the carburetor will lean out.

A cool engine usually has surplus fuel mix in the crankcase which vaporizes as the engine heats up until it fully was up then the engine leans out.

Also Stihls seems to like to be ran a little on the rich. So when tuning tune idle mix first to best speed and increase slightly rich. After that tune the high side for spec max rpm leaning to the rich side if over speeding.

Good morning Wiix. Thanks for the suggestion, but it so happens that I already took the spark-arrestor off about 10 years ago. I suppose the muffler could be somewhat blocked in it's place, but the unit still really likes to rev when given its head, and a lean-ish "Hi" screw setting. If I think I've got enough MAPP gas left in the blowtorch, I'll contemplate a de-coking...

Hello AVB across many miles of ocean! My rebuild kit was the abbreviated version, with only the diaphragms and gaskets in it. The original Inlet Needle Lever was therefore carried over, and I consequently never bothered to adjust it. But I like your train of thought regarding the gradual evaporation of the residual fuel in the crankcase from cold. Fits in nicely as at least one piece of the puzzle. I guess I've got nothing to lose by giving the Lever a slight bend upwards to enrich proceedings, but the thing is - I've already got both mix-screws set to slightly leaner-than-factory settings. I'll see if I can get around to it this afternoon. Many Thanks for the suggestions!

Best Wishes to both of you,
Moby Mick.

Why on earth are you setting them lean ?
They are set too lean to start with in order to comply with EPA regulations
Adjust rich till they 8 stroke, lean till they miss then slightly on the rich side of 1/2 way between.
If you are using the trimmer at any speed under flat chat then you are killing the engine and loading up the drive system

Hello there Rovert, and many thanks for the reply.

I've set those mix-screws to lean in order to eliminate the exhaust smoke that would otherwise arise due to the higher-than-normal amount of oil that I add to the petrol. I like to use a slightly oilier fuel mix so that I can be confident that every drop of fuel that's running through the crankcase is nice and slippery for the sake of the moving parts down there. I figure that any increases to engine temperatures that might result from running things a touch lean will be mitigated by the fact that I never run the unit flat out anyway.

As for killing the unit by using it at part-throttle, I'd say she's not doing too badly regardless, given that she's now going on for 20 years old, and has had to look after an average of 5 suburban-sized lawns over all of that time. The only thing I've ever had to replace that might be due to low-speed use has been the clutch drum. But I'd sooner fork out for a new clutch drum every 20 years or so than to end up butchering the edges like a lot of the contractors that I see working in our street. Our edges are manicured - not gouged. Besides that, I easily get 4 lawns per tank out of her at the speeds I edge at, and I'm fairly certain that I used to get 5 back when we had 5 lawns to look after. So the fuel and oil we save would easily pay for a new clutch every decade or so...

Look, you've got me there with the term "8 stroke"(?). Is it a "burble"-type of thing? You'll have to elaborate a little so that I'm clued up in the event that I play around with the Intlet Needle Lever, and consequently need to give her a re-tune.

Best Wishes,
Moby Mick.

Mick.

First of all the power head is designed for a particular volume of air to be passing over the cooling fins.
Running even a little slower means a lot less cooling is is an expidential type of curve.
Running slower does generate less net heat but it also ends up in different places so your crankcase runs a lot hotter than it is designed to.

The jets in your carb are of a fixed size and designed to flow a particular amount of fuel into the engine as a fuel / oil mix.
Adding more oil means you are getting less fuel than the engine is designed to work with and that is before you you start playing with the jets.

So all you are going to achieve is a lot of carbon build up on the piston & head which nearly always ends up breaking loose at the exhaust port , getting stuck on the ring and scoring the bore.

If the smoke bothers you then switch to a synthetic low smoke oil at the same mix ratio as the machine instruction tell you to run.

You really are your own worst enemy.

Running a thicker mix does absolutely no benefit to your engine in any way shape or form.
I run all my blue smoke stuff at 50:1 and my FS 35 is better than 30 years old and never missed a beat,
Only thin replaced on the engine has been the fuel filter & lines and mine does about 20 hours work a week, every week.
The Chainsaw is the same age and again working flawlessly.
Never touched the carbs on either other to adjust for unleaded and of course change the filter, very regularly.

8 stroking is really a 4 stroke engine term.
It means missing due to richness generally accompanied with some black smoke.

I really do not know why you are here as you obviously think you know more about how to care for and use your trimmer than the engineers who designed it and I shall sign off and leave you to your fate.

Just one last thing, all the lawn care people I know use an Atom to do the edges except 1 who use a tilt a cut because they do actually cut dead strait edges.
Good luck.
* stroking

Ahhh... Ummm... Okaaaay....... Thanks Rovert!

Well, any other suggestions folks, before I ceremonially fall on my sword?

Best Wishes,
Moby Mick.

Mick,

Some members can be real downers. Just keep trying as that how I have learned most of what I know about 2 cycles. As 8 Stroking I have never heard that term and I have working equipment for over 30 yrs. The term I have heard especially when it comes to chainsaws is 4 stroking.

Rovert reminds me of the only barred member of a forum I manage here that was finally barred for insulting members especially our elderly members. Moderators wanted to give more rope to hang himself and he did after 6 months of warnings when he jumped on one of the moderators. If I had my way he would have been gone after the 3rd warning.

Hello again AVB, and Hearty Thanks for the words of encouragement!

Yes, I have to say - I was feeling a bit like a little doggy there for a while, who wasn�t quite sure whether he was being lightly smacked, or just vigorously patted� Ha ha ha! Anyway, he might be a disciple of shock tactics. If I was a bit younger, they might have done me some good!

For what it�s worth, I can sympathise with the �maximum throttle� train of thought to a certain degree, but only so far. For example, mower engines with alloy blocks, but without cast-iron liners, might theoretically suffer slightly extra wear when loaded up at any speed other than full throttle (which is still only 3000rpm or so) due to the vibration caused by their relatively longer strokes. But as for grass-trimmers, with their relatively shorter strokes, a constant 10,000 rpm seems like complete overkill in light of the load in question, as well as a waste of fuel. It�s also virtually impossible to neatly bevel your edges at that speed, so I�d have to start �trenching� them all (which certainly isn�t as pretty). But anyway, I digress�

I didn�t end up giving the Inlet Needle Lever a tweak yesterday after all, but I�ve dialled in another 1/8th turn or so of richness to both mix-screws as a first step, and she�s now burbling more at both idle and full throttle. I wont really know the full outcome until I have to do all of the lawns all again. But if she still wants to play hard-to-get when that happens, well - it will definitely be a case of carby out, lever adjusted, and muffler de-coked. If that doesn�t work, it could well be time for a new FS38 (because the "36" also needs a new Rope Rotor...)

Very Best Wishes across the Moat to you AVB!
Moby Mick.

Well I was not going to come back, and I am really not trying to be abusive either.
However if that trimmer came into my workshop then the first thing I would do is get you to start it and verify what you are telling me actually happens.
Then I would toss the fuel in the tank, mix 1 litre of fresh 40:1, run your fuel out then replace it and try again.
This is because bad / stale fuel accounts for around 75% of all the jobs that come in the door.
Next I look up the Stihl book, check the factory settings and readjust your trimmer to how the factory says it should be.
From there is gets a fine tune using a tacho.
Next it is out in the yard to find somewhere that needs some trimming, usually I would run the entire tank through to be sure there was no other problems then call you back to come & pick it up, relieve you of $ 45 and get on with something else.

I would be fairly sure most on this list would do something similar.

OTOH if a customer came in and told me they were running 30:1 because they have decided the engine needs more oil than the factory engineers, then they had adjusted the carb by the amount the exhaust smoked rather than how well the engine burns the fuel . Followed by they only use it at 1/2 speed for whatever reason they think is justified (a position which by the way it could be running on either or both the low & high jets )
I would advise that it would be better set back to what the factory recommended.
The the custommer tells me his setting are fine because it has been working that way for a long time, my answer would be that the cummulative affect of the misuse and mal adjustments is what the trimmer is suffering from and if you want to continue doing the same you are wasting both of our times.
Now if you can fault this logic I will be amazed.
If I adjust the top speed to 13500rpm, the trimmer will not blow up today, or tomorrow but some time the rod will let go because I have the engine running 1500 rpm too fast. Now it might run for 20 years like this but eventually it will throw the rod.
If I run the trimmer too slow then the gas velocities are too low and because the timing is fixed the spark comes at the wrong time so the heat developement & dispersal is wrong and eventually this will cause grief.
Usually this will be excessive wear on the thrust faces of the piston &/or the bore and carbon build up around the exhaust port because the gas is going past too slow , the port is too hot and the exhaust contains too much oil.

If your wife put 1 grain of arsenic in your morning tea, it will not kill you today, or tomorrow ,or next week, or next month or next year but eventually it will kill you, saying she has been doing it for 20 years and you seemed to be working fine will not get her off the murder charge.
If you preffer an absurdly humourous simile .

I don't lie to my customers nor molly coddle or flatter them and they seem to appreciate strait talk as I have 3 week waiting time at the moment and this is the off season.

As before, good bye and good luck, I only haunt this forum for brief periods every few months.

Sincere thanks for an interesting reply, Rovert, with some decent general suggestions regarding potentially stale fuel, lean mixture settings, and part-throttle-openings. Unfortunately they don�t quite seal the deal for me due to the number of years that the problem has taken to surface, the ease with which the machine still starts, and how reliably she still runs up until the point that the problem intervenes. Add to that the fact that I�ve already given the carburettor a thorough cleaning, to no avail.

That�s not to say that one of those factors you mentioned is not still the underlying cause, but unfortunately your suggestions remain too broad and unsubstantiated at this point. The issue is apparently not a common one, but the link to the other thread that I provided in my first post illustrates that it is not an isolated one either.

With regard to your comments that I should be running the unit at full throttle for heat-dissipation reasons, please note that I had a look through the Owner�s Instruction Manual, and there�s definitely nothing anywhere in there about needing to operate the unit at full throttle - even for just part of the time.

I did however discover the following paragraph amongst the General Notes on Operation:
�After long periods of full-throttle operation, allow engine to run for a while at idling speed so that the heat in the engine can be dissipated by flow of cooling air. This protects engine-mounted components (ignition, carburettor) from thermal overload.�
This is not an instruction to run the unit at full throttle, but rather an explanation by Stihl that idling the machine cools it better than does full-throttle operation.

I�ve got enough of the current batch of fuel left to run it again up to a decent temperature on the new mix-screw settings, and I will definitely run some fresh fuel through the unit after that if the problem persists in spite of the new settings, before pulling the pin on it. Hopefully all will be good again by then. For what it's worth, the fuel was bought from a very-high volume outlet.

Best Wishes,
Moby Mick.