Stihl MS400 won't start

[Diulin]

My Stihl MS 400 was the most reliable saw I've ever owned until today when it died and wouldn't restart. Grounding the plug against the engine and pulling the cord, I can't get a spark, though the spark plug looks good and I touched it up with some 600 sandpaper. There's some smell of gas, but not much. Nothing burns when I hold a lighter to the spark plug hole and pull the cord. (The flame just gets sucked in.) Air filter is clean, and the gas is new (always run on non-ethanol gas).

So I wonder if I'm dealing with an issue with the M-tronic carburator, since I gather this carburator controls the spark as well. I don't think it's a problem with the kill switch, because that at least looks to be in good condition.

Any suggestions would be greatly appreciated.

Thanks

 

[Dacha]

Unless u have lots of experience, which i do not, i would take it to stihl diagnostics computer. Which may help...

How long have you had the saw for, how many working hours approx ?

On non mtronic, i would change spark plug, check if it gives spark, check if fuel supply, if need clean carb or swap, if have spare, inspect cylinder and piston.

And most importantly, where CAD comes handy, work with backup saw, so i don't have to rush with any diagnostics work. I.e. do repair work during winter, or on a rainy day.

 

[JJoe_257]

My Stihl MS 400 was the most reliable saw I've ever owned until today when it died and wouldn't restart. Grounding the plug against the engine and pulling the cord, I can't get a spark, though the spark plug looks good and I touched it up with some 600 sandpaper. There's some smell of gas, but not much. Nothing burns when I hold a lighter to the spark plug hole and pull the cord. (The flame just gets sucked in.) Air filter is clean, and the gas is new (always run on non-ethanol gas).

So I wonder if I'm dealing with an issue with the M-tronic carburator, since I gather this carburator controls the spark as well. I don't think it's a problem with the kill switch, because that at least looks to be in good condition.

Any suggestions would be greatly appreciated.

Thanks

Sorry to hear about your 400.. but first things first, don't mess with a lighter near the spark plug hole, that's a good way to get burned or worse!
Since you're not getting any spark at all, I suspect the issue is almost certainly electrical and not the carb.
I'd rule out the simple things first and check if there might be a wire shorting in your kill switch.
Disconnect the kill switch wire at the coil and then check for spark.
If you get a spark, then you know the issue is with the switch or the wiring..
If you still don't get anything, the problem is almost definitely the ignition coil.
It's common for this part to fail, especially after a lot of use.
You could also try testing the coil with a multimeter if you have one, but most of the time, the simplest way is to rule out everything else and then replace the coil.

 

[Astro Lad]

When an MS 400 suddenly loses spark, it's rarely the plug, it's M-Tronic's Achilles' heel. Stihl fanboys won't say it, but once the module fries, you're replacing electronics, not sanding electrodes

 

[STIHLTECH]

OH MY GOD_ IT'S MTRONIC!
You are diagnosing spark like any other saw with an electronic ignition.
Try a NEW sparkplug. Then pull the switch wiring off the coil and try it.
No spark at this point will be a module.

 

[Vashon]

When an MS 400 suddenly loses spark, it's rarely the plug, it's M-Tronic's Achilles' heel. Stihl fanboys won't say it, but once the module fries, you're replacing electronics, not sanding electrodes

Interesting, my dealer talked me out of the MTronic system when I bought my 261 a couple of years ago. I’ve never had any issues with the saw.

 

[STIHLTECH]

Modules will fail occasionally. Don't matter Mtronic or not. Don't matter what make.
The most important thing to do is to diagnose as above.

Truthfully, the Mtronic has just as good a reputation as any other system when working in a shop.
I woild be looking at saw age and ask Stihl if there is a warranty. There may be under emissions.