Stihl 046 Magnum runs perfect then randomly dies

[Timbco84]

I've been fighting this gremlin for a while, figured I would go fishing for some help again. I bought this saw from a guy who bought it new then hardly ever used it. It's safe to say the saw sat for a long time without being ran. It's a 046 magnum in pristine condition...all original unmollested stihl parts through out. Piston/cylinder looks brand new, lots of compression. Here is the problem...

The saw starts great and runs perfect for anywhere from 20 minutes to twenty seconds depending on the mood that it is in. It is properly tuned with a tachometer. Once it decides to act up, when I let off the throttle it will try to die. If I feather the throttle when I feel it start to act up, sometimes I can recover it and keep it running. Sometimes in the cut, you can hear/feel it lean out and know it will try to die when you let out of the throttle.

-I have put the case through the vac test with a mity vac, will hold vacuum indefinilty even while rotating the saw all around.
-As it tries to die it still has spark
-The fuel tank vent works properly and never takes a breath when you pop the cap
-I can spray carb cleaner everywhere while it is idling and idle speed never changes
-It had a slightly wrinkled diaphram in the carb so I put a full stihl carb kit in it
-The carburetor passes the vacuum test
-I replaced the spark plug (correct bosch)
-I replaced the Fuel Filter (stihl)
-I replaced the Fuel Line (stihl)
-The air filter is clean
-I cannot locate a bare wire or electrical related issue anywhere.

I would rather figure this out and learn from it than pay someone to fix it...if there is something I am missing or some sort of related experience someone has please share it. The only next steps I can think of is changing the impulse line and clutch side crank seal just to rule out the rest of the likely stuff.

 

[tickhound93]

Maybe richen up the idle a bit? Have to remember that at wot you are still drawing fuelnfrom the idle circuit. Also double check metering lever height.

 

[Tuplin86]

Man, I have the same problem with my 048av! Mine would start to power out, and most times it would still idle, but touch that throttle, and it was all over. I ended up deciding it was getting to hot, as I was using it in the mini mill. Maybe the coil can't take the heat, maybe three isn't enough air flow in the older design. I ran it at 35:1, and it really did run strong. Too bad really... All tests done and parts replaced like you have done. would hold vac and pressure over night. The saw was great otherwise. A bit low on compression tho at 115.
But it is still a good and heavy bucking saw

By the way, I ordered crank gasket, and both crank seals out of Germany, lol. Didn't end up needing them. I know they can be a bit hard to find. So I have them if any one needs one day.

 

[cus_deluxe]

seems like you have pretty much covered the bases. i would try swapping coils and carbs with known working ones if possible. sometimes there are carb gremlins that are near impossible to diagnose. also, you mentioned vacuum test, but not pressure testing. best to do them both, usually guys pressure test first, so if theres a massive leak, theres less chance of sucking debris in somewhere.

 

[pioneerguy600]

Carb.

 

[Whitty21]

Vapour lock or bad coil are my two bets

 

[alexcagle]

+1 CARB.
Sounds like one that I had, that I was able to figure out. It was the high speed check valve not sealing at idle. Letting air backflow through the circuit sucking the bowl dry.

 

[Full Chisel]

That definitely sounds like a bad carb if you haven't been able to tune it to where it idles and runs in a stable manner. New lines and passing a vac/pressure test pretty much rules out anything else. I suppose a bad coil could be possible but the fact that it dies off throttle makes that unlikely, IMO.

 

[alexcagle]

Vapour lock or bad coil are my two bets

How do you know which spelling to pick from the spellings of vapor, vapeur, or vapour up there in Ontario? ..lol

 

[Whitty21]

How do you know which spelling to pick from the spellings of vapor, vapeur, or vapour up there in Ontario? ..lol

Easy. I always spell it the right way...

 

[Timbco84]

Maybe richen up the idle a bit? Have to remember that at wot you are still drawing fuelnfrom the idle circuit. Also double check metering lever height.

I actually tried that once, kind of masked the problem so to speak. But still noticibly not running correct in the long run. Metering level is set to spec. Thanks

 

[Timbco84]

seems like you have pretty much covered the bases. i would try swapping coils and carbs with known working ones if possible. sometimes there are carb gremlins that are near impossible to diagnose. also, you mentioned vacuum test, but not pressure testing. best to do them both, usually guys pressure test first, so if theres a massive leak, theres less chance of sucking debris in somewhere.

I did actually apply pressure also while testing, the mity vac lets me do both. I tested it both vac and pressure, it passed, I put it back together and ran it, problem still present, took it back apart and retested. Still passed, that was a frustrating day.

I know of a saw a friend has that I can swap parts with, might try swapping his coil, I am reluctant to swap carbs because I rebuilt that saw for him a while back when it blew the top end, and it runs way to good now to mess with. Doubt his carb settings would work on my saw, I'm not that lucky.

 

[Timbco84]

Vapour lock or bad coil are my two bets

I feel like I can rule out vapor lock, tank vent works properly and is clean. I can crack the cap when it acts up, it sucks no air, then restart and problem will still be present. I tried to rule the coil out by placing a light up spark tester inline with the plug, seemed to still have spark while it was trying to die, just slowed with the rpms. Guess I couldn't say 100 percent its not the coil though.

 

[Timbco84]

That definitely sounds like a bad carb if you haven't been able to tune it to where it idles and runs in a stable manner. New lines and passing a vac/pressure test pretty much rules out anything else. I suppose a bad coil could be possible but the fact that it dies off throttle makes that unlikely, IMO.

That's the gremlin...I have it tuned to where it idles and revs out per spec, might run absolutely flawless for 20 minutes, then starts acting up for no reason what so ever. Driving me nuts.

Might just start throwing parts at it like a madman, then I'll have some spares for next time eh?

 

[alexcagle]

I realize you said you pressure/vacuum tested it, but did you do it at the carb mount with a stihl block adapter so it includes the intake boot and impulse hose in the test? If not, it could be the boot opening up a crack. I've seen saws run fine because the vacuum held a split shut and ran fine until the seal got flexed apart.
An easy test for that is to idle the saw while on the ground, and push down and pull up on the handlebar, and make the tank to crankcase connections flex. If the idle changes a lot, or dies, that could be it. It is ok if it changes a bit because you are momentarily changing the pressure for a second.

 

[alexcagle]

It could very well be the coil arcing internally at random. dern gremlins

 

[Chainsaw Jim]

What size is the exhaust hole on your muffler? Is it pencil size or bigger than a dime? If it is the one with the pencil sized hole then open it up to about 16mm.
This is similar to a problem I thought was a carb issue on an 028 super, but it was the small muffler opening choking the saw up.

 

[Stovepipe]

Had an internal fuel tank "floaty" hose on a weed whip. (Bad fuel)rotted a tiny crack in the line inside the draw side of the tank. Just enough air at full load Just enough to lean it out. Then feather it back.

Seemed like the carb. Seemed like..... Vapor. / leak.
It was $1.39 of new fuel line.

Best wishes

 

[fearofpavement]

I spent about 7 hours chasing a very similar gremlin on an MS660. Just got it sorted out yesterday and a different carb is what fixed it. It would tune nice, idle, cut, etc. Then it would just randomly bog down and flood out. I had the carb apart 3 times, put a kit in it, to no avail. What a pain...
Sometimes I could cut a dozen 12" oak cookies and it worked great then would fall on its face.

 

[alexcagle]

Had an internal fuel tank "floaty" hose on a weed whip. (Bad fuel)rotted a tiny crack in the line inside the draw side of the tank. Just enough air at full load Just enough to lean it out. Then feather it back.

Seemed like the carb. Seemed like..... Vapor. / leak.
It was $1.39 of new fuel line.

Best wishes

*** I suspected collapsing fuel line Inside tank,
however, the OP said he replaced the moulded fuel line with a new OEM hose.
But I've seen stihl moulded hoses have constrictions at the tank "bung holio" before, but it's rare.
Sorry; had a Beavis and Butthead flashback.....