# Stihl FS-38 trimmer mystery!!!!!!!!!



## goosemaster (Aug 22, 2020)

I'm working on tihl FS-38 trimmer for a friend he said it wouldn't start. I checked the compression and it was 90#. I checked the spark and it was kind of eratic. I put on a new coil, flywheel, cylinder, piston, and rings, new carburetor, new spark plug and it still doesn't even make a sound of hitting. It made a couple backfires out of the carburetor. It won't even try to hit using starting fluid. I'm stumped; I've put everything I could think of new on it to where it should try to run but nothing. Please someone give me some ideas on what I should do now. Thanks in advance to anyone who responds.


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## VaBchBum (Aug 22, 2020)

I just finished rebuilding an FS-80 I was told it wouldn't start, compression was good spark was decent but I had to take the spark plug out squirt starter fluid into the cylinder to get it to fire. Long story short previous owner hasd messed with the carb settings and seated both adjustment screws. Once I opened them back up it came back to life. Might be worth looking at.


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## trains (Aug 22, 2020)

whats the wiring like ? is it earthing out and thus always in the "off" position regardless of where the on off switch is ?
time to check with multimeter to make sure the wiring from the coil to the switch is not earthing.


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## goosemaster (Aug 22, 2020)

I unhooked the kill switch wires and still nothing. The L and H jet screws are right. Somebody mentioned oil seals may be leaking. Could that cause a no start?


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## Goinwheelin (Aug 22, 2020)

Ignition timing ?


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## goosemaster (Aug 22, 2020)

The flywheel's key is good and seated good on the crankshaft.


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## goosemaster (Aug 22, 2020)

VaBchBum; About how much starting fluid in the spark plug is good? I don't want to blow that new cylinder piston kit up. It's got 120# compression now since I put that new cylinder piston kit on it. Is that a good compression? What about the previous 90# compression I had; is that low?


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## stihl86 (Aug 23, 2020)

The list price on that trimmer is $139.95.


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## VaBchBum (Aug 23, 2020)

I'm definitely not one of the experts, my fs 80 had 110#ish of pressure but I can't tell you what it should be as I didn't research it. I only used 1 or 2 little puffs of starting fluid not any kind of a large amount as like you i was afraid of detonation. I have a husqvarna saw that wont start at all unless I put a puff of fluid into the cylinder first weirdest thing I think the carb needs a full rebuild.


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## buzz sawyer (Aug 23, 2020)

Biggest concern with starting fluid is lack of lubrication.


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## goosemaster (Aug 28, 2020)

Isn't there anyone out there can give me a fix on this FS-38 trimmer? Please reply. Seems like I put every new part on it that I can think of with no avail. Thanks and someone please give me a fix for this. Thanks and PLEASE reply


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## Doorfx (Aug 28, 2020)

Is it in the run position? New plug ?


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## goosemaster (Aug 28, 2020)

I can't believe you ask that question. Yes the switch is in the run position and new plug.


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## 660catman (Aug 28, 2020)

Never ever use starting fluid on a 2 stroke motor. Use a mix of gas and oil to pour in cylinder instead. 
And is the choke actually moving when you move lever??


Retired guy from SE Manitoba


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## goosemaster (Aug 28, 2020)

Yes the choke is working correctly!!!


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## Manic84 (Aug 28, 2020)

You've done work on carburation, the ignition system, spark and timing, replaced the cylinder and piston and plug. I'm assuming the exhaust too.
At this point, you've eliminated all the usual suspects, so I'm somewhat at a loss as to what else could be wrong. (you probably are too)

The only thing I can think of at this point is a primary compression issue. But... you've already done everything possible with the cylinder, so you've probably resealed everything already. Recently there was someone on the forum who had a machine that refused to start. It had great (secondary) compression and no trouble with the spark or fuel. I believe he had a blown out base gasket and that was the whole problem. If you have the ability to, do a leak-down test on the trimmer. This will tell you if there is a massive leak somewhere, or the oil seals are faulty.

My memory is a bit fuzzy, but aren't these stihl trimmers a "triple decker clamshell"? I've worked on other machines that have the same setup. From experience, the base can leak from time to time, and needs to be resealed. I've seen them run weird, but I've never seen it so bad to where it won't start. I'm not saying this is the problem, just a possibility.

Make sure it isn't a simple problem or mistake that tunnel vision and frustration made you overlook. It's a royal pain, but you must check everything you've done and make sure it is correct. Clear your mind and focus... Make a checklist on paper or in your head and go from there.


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## goosemaster (Aug 28, 2020)

Thanks for your input; I just recently put a.new set of oil seals on the crankshaft. The gasket.in between the 2 halves looked good when I put it back together not meaning that something could be wrong with it. I wish I had a way to do the leak down test like you mentioned. Wonder if a Stihl dealer would have that service? Thanks again for your input.


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## Manic84 (Aug 28, 2020)

I'm not really stihl dealer experienced, so I don't know their limits. But a dealer might be able to help you if you ask. 
But if you go and ask a dealer, expect some kind of pitch! 

Now, please humor me with one foolish question: When you took the whole assembly apart, did you reseal the base when you installed the new oil seals?


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## goosemaster (Aug 28, 2020)

I just had to drop the bottom cover off and I recleaned it off and applied new silicone. Then let it set overnight before I tried to start it. I've fooled with small engines for a little while now and I have never had one to stump me like this one.


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## Manic84 (Aug 28, 2020)

This *is* a real head scratcher. If you've reapplied the sealant to the very bottom of the short block assembly, that kinda puts a dent in the leak theory. As that first part is where they tend to leak from... maybe the gasket then? Hard to say without doing a leak down test.

And you used OEM parts for the whole thing, right?


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## goosemaster (Aug 28, 2020)

Yeah I believe I'm going to try and get a leak down test done on it. What gets me about the whole thing is; I've always been able to get one to hit using a small shot of starting fluid. Not this one. Nothing.


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## Toyboy (Aug 31, 2020)

Another dumb question but what about the spark arrester screen. Clean?


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## fowvay (Sep 13, 2020)

If you got no results with ether then you have a compression issue. While you had the engine disassembled I hope you cleaned the ring groove and checked the ring gap. Also, be certain the purge circuit on the carburetor is functioning to ensure fuel is in the carburetor. Please let us know what you determine. I recently brought an FS-38 back from the dead for a neighbor but their only issue was with old fuel and dirty carburetor.


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## lone wolf (Sep 13, 2020)

goosemaster said:


> VaBchBum; About how much starting fluid in the spark plug is good? I don't want to blow that new cylinder piston kit up. It's got 120# compression now since I put that new cylinder piston kit on it. Is that a good compression? What about the previous 90# compression I had; is that low?


China parts or Stihl?


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## stihl86 (Sep 13, 2020)

1) A leakdown test and a Vac/Pressure test are two completely different tests.
IF the dealer will/can do a Vac/pressure test, the intake manifold must be on.
2) Most modern 2 cycles have a variable ign timing circuity. It adds in reduced starting effort. If that portion of the module is defective, and they do fail, you'll never get it stated. It might be sparking at the wrong time. That would explain the backfiring. The timing is separate from the magneto portion, meaning it will show good spark.

I will get you credit for you vigilance in trying to get it running.
You already have, in labor and parts well over what the trimmer is/was worth.
On its best day.


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## Okie294life (Sep 13, 2020)

stihl86 said:


> 1) A leakdown test and a Vac/Pressure test are two completely different tests.
> IF the dealer will/can do a Vac/pressure test, the intake manifold must be on.
> 2) Most modern 2 cycles have a variable ign timing circuity. It adds in reduced starting effort. If that portion of the module is defective, and they do fail, you'll never get it stated. It might be sparking at the wrong time. That would explain the backfiring. The timing is separate from the magneto portion, meaning it will show good spark.
> 
> ...


Did you gap the coil when you put it back on? I usually use two sheets of paper or an envelope if I can find one.


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## Ax-man (Sep 16, 2020)

Maybe your new coil and flywheel aren't compatible and spark is occurring at the wrong part of the cycle. Two things would happen if this is the case , big time flooding or the pull start would get jerked out your hand or be very hard to pull over . Why did you buy a new flywheel??? 

If your machine isn't even try to fire starter fluid isn't going to help it will only make matters worse because it takes longer to fire an engine using starter fluid to clear the cylinder out before it does any good . Two stroke mix is the best for obvious reasons.


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## fowvay (Sep 25, 2020)

Goosemaster, did you ever get this FS-38 running? If so, what was the problem?


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