Can anyone help diagnose a Stihl brushcutter problem please?

[Alpinejim]

Hi, sorry if this post is in the wrong place, I'm new.

 

I have a Stihl FS450 brushcutter which suddenly stopped one day like it had run out of fuel.

I checked fuel and filled it but it would 't start.

Next I checked for a spark. There was one, although it looked a little weak to me, certainly not like I've seen on the car. Still, since there was a spark, I continued to suspect a fuel problem.

I changed the in-tank filter, the fuel line from tank to carb, the air filter and the spark plug. No start.

I checked the coil and lead and got continuity through to the spark plug cap - although it's hard to know exactly what's going on inside the coil but I checked it from the small metal tag at the back which has a wire connected.

 

Back to fuelling and I rebuilt the carb with new parts, no start.

I changed the complete carb, no start.

Now back to the spark, maybe it was weak. I changed the coil and set the gap to the flywheel at 12 thou, still no start.

I removed and cleaned the little valve that helps starting in case it was stuck causing low compression, all seems fine there.

 

I'm now at a loss to understand what to do next. I can only think that the spark is weak for some reason and not strong enough to ignite the fuel mix.

I suspected a short in the cable that goes to the handle on/off switch but, if that was so, there would be no spark at all.

The piston moves freely. Could there be a problem with the flywheel magnet? Could it degrade to the point that the spark becomes weak? I wouldn't have thought so.

 

Any help will be most appreciated.

[Breezeblock]

disconnect the wire that goes to the kill switch first was it a cheap chinese coil that you fitted or a genuine stihl one put some neat petrol in the carb venturi see if it farts if it doesn't i would still suspect the coil

[Alpinejim]

No, it was a genuine Stihl coil that I put in.

I'll disconnect the wire for the kill circuit and see what happens.

[humpo48]

Make sure the spark plug cap is fitted onto the lead correctly. It's just in place by a pin maybe loose?

[harrythecat178]

Try to start on carb clean, if it doesnt , check the flywheel key is not broken. That would knock timing out and so not start whilst still giving a spark

[M@X]

HI Jim

 

I appreciate this is a very old thread but i have exactly the same problem. Did you ever get this running? 

 

Thanks Max

[AngrySquirrel]

Wasn't there a thing with these jumping the key on the flywheel? @spudulike should be the Daddy here!

[Paddy1000111]

Apart from the above the main thing I haven't seen here is compression? Brush cutters go from freezing cold to flat out without the proper care all the time. Scored cylinders are pretty common.

[spudulike]

41 minutes ago, Paddy1000111 said:

Apart from the above the main thing I haven't seen here is compression? Brush cutters go from freezing cold to flat out without the proper care all the time. Scored cylinders are pretty common.

I was thinking just the same, pop the muffler off and look at the state of the piston. If you have a compression gauge, it should read 150-170psi.

Not sure on the flywheel, it is possible the key has sheared but the first thing I check on pretty much all non running engines is firstly the compression, the spark then the fuel. If it isn't any of those, you just check the spark and fuel systems in more depth!

[adw]

Its seized for sure.