Stihl FS 46 String Trimmer Problem

[Wood Doctor]

I ran into a dandy this time. My friend's Stihl FS 46 engine will not start. In fact, it won't even pop, either on choke or otherwise.
(1) Tried a new carb. Still no pop.
(2) Checked for a spark. It's there.
(3) Plug is new, but I tried another anyway. Still no pop.
(4) Checked fuel lines and replaced fuel filter. Still no pop.
(5) Checked rings and compression. Rings are good and compression is at 130 psi.
(6) Checked for muffler blockage. None exists.
This engine should be running, unless it's a timing issue (spark fires at the wrong point). The spark is damp when removed, so that's my last hope. Is there any way that I can easily check that spark ignition firing? Or, is it possible something else is wrong? I'm at the end of the rope. TIA.

 

[CR888]

U say muffler is 'clear' does it have a spark arrestor & is it clean?

 

[DustGrind]

I ran into a dandy this time. My friend's Stihl FS 46 engine will not start. In fact, it won't even pop, either on choke or otherwise.
(1) Tried a new carb. Still no pop.
(2) Checked for a spark. It's there.
(3) Plug is new, but I tried another anyway. Still no pop.
(4) Checked fuel lines and replaced fuel filter. Still no pop.
(5) Checked rings and compression. Rings are good and compression is at 130 psi.
(6) Checked for muffler blockage. None exists.
This engine should be running, unless it's a timing issue (spark fires at the wrong point). The spark is damp when removed, so that's my last hope. Is there any way that I can easily check that spark ignition firing? Or, is it possible something else is wrong? I'm at the end of the rope. TIA.

Couple things I would do.
First: make sure it's getting fuel by attempting to start, then remove the plug and check for fuel and also change the plug.
If you are getting fuel, you could have flooded the machine.
Second: check to see if you sheared the flywheel key.
Could be firing at the wrong time.
Third: remove the muffler and check for scoring on the piston and make sure your rings are not stuck.

 

[DustGrind]

I ran into a dandy this time. My friend's Stihl FS 46 engine will not start. In fact, it won't even pop, either on choke or otherwise.
(1) Tried a new carb. Still no pop.
(2) Checked for a spark. It's there.
(3) Plug is new, but I tried another anyway. Still no pop.
(4) Checked fuel lines and replaced fuel filter. Still no pop.
(5) Checked rings and compression. Rings are good and compression is at 130 psi.
(6) Checked for muffler blockage. None exists.
This engine should be running, unless it's a timing issue (spark fires at the wrong point). The spark is damp when removed, so that's my last hope. Is there any way that I can easily check that spark ignition firing? Or, is it possible something else is wrong? I'm at the end of the rope. TIA.

Explain damp when removed?
Could be your kill switch also. Unhook it from coil and try.

 

[Wood Doctor]

(1) First, there is no spark screen and yes, it is getting fuel to the combustion chamber. Two different carbs verified that and the plug is damp.
(2) Kill switch is OK. There is no spark win O position, and there is a spark in I position.
(3) Piston rings are OK (Item 5 in post #1).

I think it's a sheared flywheel key and the timing is thus off. Or, perhaps the IM is defective and supplying the spark at the wrong time. More likely it's the flywheel in the wrong position when the IM delivers the spark.

I have never encountered a sheared flywheel key on a string trimmer (chainsaw yes, string trimmer no). Possibly another mechanic removed the flywheel and failed to tighten it enough, so it slipped; but that's just a guess. This engine is about five years old. You have to wonder how the flywheel could have slipped and sheared the key in normal use. The pull cord likely puts more strain on it than anything else could, so maybe that was it.

 

[DustGrind]

(1) First, there is no spark screen and yes, it is getting fuel to the combustion chamber. Two different carbs verified that and the plug is damp.
(2) Kill switch is OK. There is no spark win O position, and there is a spark in I position.
(3) Piston rings are OK (Item 5 in post #1).

I think it's a sheared flywheel key and the timing is thus off. Or, perhaps the IM is defective and supplying the spark at the wrong time. More likely it's the flywheel in the wrong position when the IM delivers the spark.

I have never encountered a sheared flywheel key on a string trimmer (chainsaw yes, string trimmer no). Possibly another mechanic removed the flywheel and failed to tighten it enough, so it slipped; but that's just a guess. This engine is about five years old. You have to wonder how the flywheel could have slipped and sheared the key in normal use. The pull cord likely puts more strain on it than anything else could, so maybe that was it.

Using a metal blade. I did it on a echo, or I think that was the cause.
Also, had a chainsaw that was hard to start. Had great spark. When finally started ( taking forever to start) would run great, but when cold, was a pain to start. But would burp.
Turned out to be a bad coil on it. Changed it (even tho I didn't want to cause it had great spark) started and ran great after the change.
I watching your thread here hoping that you will post what you find the problem was.

 

[DustGrind]

 

[Fish]

Take off just the top cover, and remove the plug, and rotate the flywheel until the piston is tdc, and see where the magnets are.

 

[Wood Doctor]

Take off just the top cover, and remove the plug, and rotate the flywheel until the piston is tdc, and see where the magnets are.

When the piston is at TDC, the magnets are at least 45 degrees away from the coil, about where the carb is. So, when the magnets reach the coil, the piston is already approaching BDC, about 90 degrees from BDC. So, The spark is being fired when the piston is already half way down or more. It's possible that the flywheel timing is off my nearly a half revolution.

So, to fix this engine, the flywheel will have to be pulled and reset. To do that, it appears the engine has to be removed completely from the case. Rats!

 

[Fish]

Ain't too bad, once you have the top off. Just 4 more bolts on the bottom of the block.

 

[Fish]

Getting the clutch off takes a skinny Torx wrench, and I just clamp the clutch in a vise. Give me a pic of your flywheel, and I'll send you a free one.
You just pay a few bucks for the shipping.
Also note the order of the hardware/washers/spacers, so you can put it back the same way.

 

[Fish]

There were a few different flywheels on that series...

 

[backhoelover]

what type plug are you using the newer units take a r type plug. if you dont have a r type plug unit will not start.

 

[DustGrind]

Getting the clutch off takes a skinny Torx wrench, and I just clamp the clutch in a vise. Give me a pic of your flywheel, and I'll send you a free one.
You just pay a few bucks for the shipping.
Also note the order of the hardware/washers/spacers, so you can put it back the same way.

That's awesome.
Was the flywheel the problem?

 

[backhoelover]

fyi flywheel is 62.00 if you go that route

 

[DustGrind]

fyi flywheel is 62.00 if you go that route

And he gets a free on with just paying shipping.
You guys are swell.

 

[Wood Doctor]

Timing is the problem, not the flywheel. It slipped somehow and sheared the drive shaft key. Here is yet another discovery. One of the bolts is missing that holds the engine to the case. Stihl manufacturing doesn't make mistakes like that. So, here is my conclusion:

The owner already had a mechanic look at this trimmer. That mechanic also concluded that the flywheel had slipped and the engine's electronic timing was bad. To fix that, the engine had to be removed entirely from the case. That mechanic started to do this and gave up when he found out what was required. When he re-assembled it, he left off an engine mounting bolt. The owner may have had to pay for his analysis. Then the owner called me in desperation to see what I had to say.

Before I do anything else, I'm calling the owner and telling him the truth. This is at least a six-hour job, and after I'm through, it still may not run, but it might. That's the nature of the business. Sometimes it's a zoo.

 

[DustGrind]

Timing is the problem, not the flywheel. It slipped somehow and sheared the drive shaft key. Here is yet another discovery. One of the bolts is missing that holds the engine to the case. Stihl manufacturing doesn't make mistakes like that. So, here is my conclusion:

The owner already had a mechanic look at this trimmer. That mechanic also concluded that the flywheel had slipped and the engine's electronic timing was bad. To fix that, the engine had to be removed entirely from the case. That mechanic started to do this and gave up when he found out what was required. When he re-assembled it, he left off an engine mounting bolt. The owner may have had to pay for his analysis. Then the owner called me in desperation to see what I had to say.

Before I do anything else, I'm calling the owner and telling him the truth. This is at least a six-hour job, and after I'm through, it still may not run, but it might. That's the nature of the business. Sometimes it's a zoo.

Glad you ( think) you found the problem.
Should run.
6 hour fix, your fast. Took me 6 months to put a puzzle together, I thought that was pretty impressive, being the box said 3-5 years

 

[Wood Doctor]

Glad you (think) you found the problem. Should run. 6-hour fix, your fast. Took me 6 months to put a puzzle together, I thought that was pretty impressive, being the box said 3-5 years

Timing is the problem. There is nothing else out there. Process of elimination. Fish also knows what he is doing.

I'm still amazed that this happened to a string trimmer. I think some mechanic got in there, tried to "hop" it up with a spark advance and left the flywheel too loose. Chances are the shaft key is not even there.

 

[Fish]

Hell, just pop on a new engine then....

http://www.ebay.com/itm/Stihl-FS38-...new-trimmer-/202016024849?hash=item2f0917e911