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I'm working on a Stihl FS 38 trimmer that a friend said he couldn't get it started. I checked the compression and it had 90#. I then checked the spark and it was kinda eratic so I put a new coil and flywheel on it Finally getting a spark but still won't start. I then put on a new cylinder with new piston and rings now it has 120# of compression. But still won't start; doesn't even try to hit. I then put on a new carburetor; still nothing. It has kinda of backfired out the carburetor a couple times but that is as close as it gets to even making a sound of starting. It won't even hit using starting fluid. I'm stumped; I put everything on it new that I can think of. Somebody give me some ideas please. Thanks in advance to anyone who responds

Check and make sure that the new flywheel's key is not sheared. It is easy to not torque the clutch back on sufficiently.

If the key was fine, put back on the old flywheel and coil. There are a few versions, and that may be an issue.

I'll change out the flywheel and coil and see what that does. Could the oil seals be sucking air ;enough to make it not hit?

Yeah put on the old coil and flywheel, the new style coil requires a new crankshaft too.

Isn't there anyone out there can give me a fix on this FS-38 trimmer? I've tried everything to no avail. Thanks

did you look at the flywheel key like fish said?

goosemasterkl said:

Isn't there anyone out there can give me a fix on this FS-38 trimmer? I've tried everything to no avail. Thanks

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He put a new engine on this thing. Doens't need a key LOL.

slomo

You did check the spark arrester?

These are the times when one might consider an nice 80 volt electric weedwhacker. They do a great job, no more oil and/or gas, and a refill is as close as the next battery.
I'm not there yet, but I'm getting close.

But just how does the batteries recharge and how many do needed during the cutting 3000' ft fence line 4'-10' on each side? I usually go through 1 to 2 gallons fuel mix per cutting along multiple refills of trimmer line.

Fish said:

Yeah put on the old coil and flywheel, the new style coil requires a new crankshaft too.

View attachment 53826

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Fish was this done all of the 4140 engines or just the FS38?

StarTech said:

But just how does the batteries recharge and how many do needed during the cutting 3000' ft fence line 4'-10' on each side? I usually go through 1 to 2 gallons fuel mix per cutting along multiple refills of trimmer line.

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Of course, one size does not fit all.
However, in your case I can imagine small stations with a battery charger and a solar panel on top. Attached to a fence post. Make the station a little bigger and put the whacker line in there, as well. You can walk the whole line without carrying anything with you. free electricity.

The update was a long while back, so it applies to the fs45/fs55. But the newer fs38s are essentially identical, so I was wondering if his new flywheel and coil might be made for the older version. To be honest, the original flywheel and coil likely work just fine.

Ahhh I did have the SB for the HL45 along the other models which how it as listed on my system but it didn't reference the FS38. I make a side note for my records. I haven't seen a FS38 but have the FS45 and FS55.

As said I havent but coil problem on Stihl and that was dog damage. Apparently he really hated the blower; I think was the unit but it been 10 yrs ago and so many machines since then.

Here is a screenshot from the brushcutter menu of the 2012 mediacat. Not sure when they came up with the fs38, but it is the same as the fs45, the only difference I see is the sticker, and a slightly smaller cutting width shield.

Yeah I checked the key and the spark arrestor and both of them are fine. Alimony I do have a new engine but I stock the parts it's just I hate for something like this to puzzle me. I'm bound and determined to get this trimmer running.

So have you tried putting the old coil and flywheel back on?

Yeah; but still no start; not even tries to hit. So frustrating.

Just for the record, if you have it back apart a little, turn the engine to tdc, and see where the magnets are in relation to the coil.

I'll check that tomorrow. Thanks

I had one sent back to me, guy couldn't get it to hit on anything. Turns out that the crank had been pressed together in the incorrect position and the timing was way off, but in your case, I don't know. Also, give us all of the numbers on your coil, and also the flywheel if you have it off.

The trimmer had been running good until he let his brother-in-law use it and he thought he had just flooded it and it hasn't ran since which leaves us to where we're at right now.

Fish here's a picture of the flywheel magnets at TDC.

goosemasterkl said:

Fish here's a picture of the flywheel magnets at TDC.View attachment 54000

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That is an odd looking coil, what are the numbers on it?

Here is a brand new fs38 at TDC.

The position of the magnets looks the same don't they? It's an aftermarket coil; but the old one looks the same way.

goosemasterkl said:

The position of the magnets looks the same don't they? It's an aftermarket coil; but the old one looks the same way.

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How are you trying to get it to start?

Try starting with the kill wires unplugged, and do not choke it or prime it.

Last year guy brought in an FS38 that wouldn't run. Could only get an occasional pop out of it. Drove me nuts. Guy finally said he bought a tuneup kit off ebay. New carb, coil, fuel lines, fuel and air filter. Had him bring me the old coil. Both looked identical except for stihl numbers. Put old coil on it it fired right up. Have to admit the china carb actually worked but coil had spark but timing was all wrong.

Fish; I've tried it with the kill switch wires unhooked; I might not have tried starting it without squeezing the throttle trigger. But I think I have but I'll try it.

get a bar magnet or compass and check the polarity of the magnets
Note they must pass in the right direction or the chip will not trigger at the right time
Same as putting a mower coil on upside down.

bertsmobile1; please try to explain it to a dumb old country boy where I might understand how to do what your suggesting. Thanks for the input.

In Fish's photo you will notice the flywheel is marked N & S
So with a bar magnet the north end will be attracted to the south end of magnet in the flywheel and repelled by the north side of the magnet.
I have seen this once where the magnet in a flywheel was back to front.
A compass needle will point to the north side of the magnet .

When a magnet passes a coil you get a sine wave where the generated voltage goes from 0 to a peak then back through 0 to the inverse peak and back to 0
This can be 0 to +V to 0 to -V to 0 or the other way round - first then +.
However the timing chip is polarity sensitive so if it is for a + then - pulse and it gets a - then + puse it will not fire the coil

bertsmobile1; I appreciate your help but you might as well be trying to teach me algebra or some foreign language. I can't understand what you're trying to tell me. Thanks though; I really appreciate it.

goosemasterkl said:

bertsmobile1; I appreciate your help but you might as well be trying to teach me algebra or some foreign language. I can't understand what you're trying to tell me. Thanks though; I really appreciate it.

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Just hold any magnet firmly, and move it close to one magnet on the flywheel, then up to the other magnet. One time it will attract, the other time it will push away. Then, without changing the postion of the magnet in your hand, do the same thing to the other flywheel.
See if they attract/repel exactly the same, or do the opposite.

Ok I'll do that in the morning. Thanks

bertsmobile1; I couldn't stand it so I went out to the shop and did that test. On the stock flywheel the magnet to the right would attract my magnet and the magnet on the left would repel it. The new flywheel both magnets would attract my magnet. What does this mean; if anything?

I'm still curious about the crank seals, i would pour some brake fluid on them , let it sit for a bit,(it really puffs up old rubber) and try it.

I just put new oil seals on the crankshaft.

It explains why you engine is not working.
The new flywheel is either magnetised wrong or it needs a matching coil
Trying to be as simple as possible
As the magnets passes the coil it generates a voltage that starts from 0 and goes either + or - to a maximum number ( number not important )
As the number gets bigger it causes the chip to close a circuit and the coil fires
If the chip is looking for a - number to fire but sees only a + number it won't fire the coil
Or it fires the coil at the wrong time.
I ran a fleet of SR 500 motorcycles and the magneto came with a matched control box & trigger and these items can not be mixed up or it would just not run correctly .
On the old points briggs engines if you are fitting a modern coil with a chip, you have to turn it upside down .

We now need an expert like Fish or Star to be able to confirm if your magnets are wrong and needs to be returned or what coil is needed to work with it.

bertsmobile1; when I checked those flywheels the stock one was off the engine on my work bench and the new one was on the engine crank; does that affect the test I did? Didn't know if both of them was supposed to be off or on the engine. Thanks

bertsmobile1; how do I go about matching a coil with a flywheel? How do I test the stock/old coil to see if it's bad? Thanks

bertsmobile1 said:

On the old points briggs engines if you are fitting a modern coil with a chip, you have to turn it upside down .

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That would be because the Briggs cast iron crankcase has a positive ground system. But it is worth that the aluminum crankcase are negative ground so if you put the coil on them upside down they will misfire due ignition timing. So the only time you need to flip the coil on an old Briggs engine would when a cast iron crankcase is involved.

It is also worth noting that at one time the cast iron cylinder block engine flywheels could be re-magnetized with the new polarity arraignment. As long the old coils are good it is easier just to install an universal ignitor set for the correct polarity.

goosemasterkl said:

The position of the magnets looks the same don't they? It's an aftermarket coil; but the old one looks the same way.

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I just wonder when people a going to learn, when you order aftermarket parts, there is a very good chance that what you order WILL NOT WORK. The aftermarket markets don't have the all the factory specs on ALL the parts.....there are too many different changes in parts. There can be 10 different parts for the same # piece of equipment and are specific to a certain model & serial #

mower king said:

I just wonder when people a going to learn, when you order aftermarket parts, there is a very good chance that what you order WILL NOT WORK. The aftermarket markets don't have the all the factory specs on ALL the parts.....there are too many different changes in parts. There can be 10 different parts for the same # piece of equipment and are specific to a certain model & serial #

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That is why I posted this update, because there are not only different coils, but also the slot for the flywheel is in different versions of the crankshaft, etc....
Which is why I suggest putting the old coil and flywheel back on...

bertsmobile1; what did that tell you when I tested my flywheels with a magnet?

It told me they were not directly interchangeable unless the matching coil is also fitted.
If the new one is different from the old one then it is not a direct replacement for the old one & will require the matching coil.
The flywheel & coil work as a set .
I am not familiar enough with Stihl products to be any more specific.
As of late stihl have been marking their flywheels N & S to specify what the polarity should be.
I would assume that this was done because they had problems with coil polarity .

StarTech said:

But just how does the batteries recharge and how many do needed during the cutting 3000' ft fence line 4'-10' on each side? I usually go through 1 to 2 gallons fuel mix per cutting along multiple refills of trimmer line.

Click to expand...

That's when I'd break out something to kill the grass along that run. Unless I was in the grass cutting business.

goosemasterkl said:

I'm working on a Stihl FS 38 trimmer that a friend said he couldn't get it started. I checked the compression and it had 90#. I then checked the spark and it was kinda eratic so I put a new coil and flywheel on it Finally getting a spark but still won't start. I then put on a new cylinder with new piston and rings now it has 120# of compression. But still won't start; doesn't even try to hit. I then put on a new carburetor; still nothing. It has kinda of backfired out the carburetor a couple times but that is as close as it gets to even making a sound of starting. It won't even hit using starting fluid. I'm stumped; I put everything on it new that I can think of. Somebody give me some ideas please. Thanks in advance to anyone who responds

Click to expand...

Did you ever figure this out? I'm having same issue.. new carb.. new seals.. spark arestor clean. Acting like timing is of.. flywheel key ok.. tried new ignition..
I work on these daily and almost always have them up and running within 30 min max. I'm stumped...