Help Stihl MS460 not starting

[Muddy42]

My Stihl MS460 is giving me grief.  I refurbished this saw two years ago - new lines, carb kit, meteor cylinder, and piston. The saw held pressure and vacuum afterwards. Its only been run on aspen and has been fine until now and I use the saw a lot.  Recently the engine cut out whilst cutting at full rev, making a slight ticking noise as the flywheel slowed down.

 

In terms of diagnosis since then, the ticking was coming from the flywheel being too close to the coil, so I fixed that to a business card distance.  Nothing looked damaged.  After this, I checked for spark and it sparks strongly, meaning you can see the spark in daylight, but I replaced the spark plug anyway.  It is drawing fuel to the cylinder, but still won't fire even with a shot of aspen down the spark plug.  The compression feels solid (I don't have a tester but the saw and 20 inch bar will hang from the pull cord fine).  Also without the decomp valve engaged, it still feels like a 76cc saw to start - hard.  From the muffler side, the rings move freely and the piston looks like new, there is some minor carbon when looking at the piston from above.

 

After all this failed, I returned to the Flywheel and pulled it off and cleaned everything. The flywheel key and race all look fine and square to me.  On reassembly my only minor observation was that the flywheel has a tiny bit of circular play back and forth? Could this be my problem? I don't really know what to look for with timing/flywheel key problems.

 

Thanks in advance for any ideas. 

[pleasant]

The flywheel shouldn't move on its axis. If you have 'play' instead of it being a tight fit, then this can throw the timing out. The timing is set by the keyway in the flywheel/crankshaft and is not adjustable. Seems too much of a coincidence not to suggest where the flywheel has come into contact with the contacts on the module it could have stalled the rotation of the flywheel and damaged the keyway?

[spudulike]

The flywheel moving thing - are you saying that the flywheel has a little play on the key when loose? This is entirely normal but if it is loose when it is torqued down, this is very wrong.

The ticking of the flywheel - if the flywheel has clouted the ignition module then it may have done it some damage. I have known flywheel magnets loose their magnetism by being clouted but if you have a healthy spark, it would look more likely that the spark is possibly happening at the wrong time.

An engine, in basic terms, needs fuel spark and compression - if only it was this simple!! The spark needs to happen at the correct time, the fuel needs to be metered and the compression - on a MS460 - typically 160-170psi.....they are usually pretty healthy on compression.

It is very difficult to see if the machine is sparking at the correct time without using a strobe and you need a running engine to do that.

It is possible something in the carb has failed and it isn't metering the fuel correctly but would expect the saw to flood or not be pulling fuel up correctly but perhaps check the carb over. MS460s do tend to build up a lot of shyte above the metering diaphragm, not sure why but they just do so check this out as well.

 

[spudulike]

Oh.....a little caveat is that when an engine stops when running at full tilt then it can often be a complete failure on the HT side, especially if the saw still has decent compression after failure. You could try removing the kill wire off the coil....just to make sure there isn't an intermittent earthing issue.

[Muddy42]

@pleasant and @spudulike thank you.  Yes the flywheel has minor play in a circular motion when the bolt is tightened down. I would say the movement is less than 1 degree, when a full rotation would be 360 degrees.  Please see below, I'll add some pictures and a video of the motion of the flywheel. On second thoughts maybe the flywheel key is a tiny bit scuffed, how bad does it need to be to upset the timing?

 

I will try disconnecting the HT side. If all else fails I will check the carb again

 

 

 

 

 

[spudulike]

The MS460 poly flywheel has a steel boss so is damn strong and yours looks fine as does the key. It is very possible that you are feeling the play between the rings and the piston and it is quite normal. Pop the muffler off and see if you can see the movement as you rotate the flywheel.

It is likely the ignition unit has taken a clout and damaged it so the saw is sparking with the wrong timing but difficult to tell from afar. It is also possible it has a carb issue - did you check under the metering diaphragm cover.

Edited March 16 by spudulike

[Muddy42]

13 hours ago, spudulike said:

The MS460 poly flywheel has a steel boss so is damn strong and yours looks fine as does the key. It is very possible that you are feeling the play between the rings and the piston and it is quite normal. Pop the muffler off and see if you can see the movement as you rotate the flywheel.

It is likely the ignition unit has taken a clout and damaged it so the saw is sparking with the wrong timing but difficult to tell from afar. It is also possible it has a carb issue - did you check under the metering diaphragm cover.

Thank you. I havnt had a chance to check the carb this weekend but I will.

 

I also have a friend who might have have a spare 460 ignition unit to try.

[Muddy42]

11 hours ago, Muddy42 said:

Thank you. I havnt had a chance to check the carb this weekend but I will.

 

I also have a friend who might have have a spare 460 ignition unit to try.

ive checked under the metering diaphragm - all looked fine - no blockages and fuel getting though to the plug. I have also managed to flood the cylinder. Nor will  it fire with a shot of fuel into the plug.

 

Im off to show it to a friend who ‘knows his saws’ and has a big box of spare ignition modules!

[spudulike]

If you have flooded it, not wishing either of us to suck eggs.....remove the spark plug, turn the ignition OFF, turn the saw upside down and pull it over really hard a few times. This really helps clear puddled fuel and you may see it drip or spatter out of the saw hence the ignition off and YES, I have seen this ignite one one occasion - pretty impressive but not good for hair on arms or eyebrows!!!

I also find heating the plug up with a plumbers torch or gas cooking hob can help with getting the engine to fire but get it in and pull it over quick....no choke but fast idle position.

At least a working module will prove if it is that or not. Has he got a good carb to try? 

[Muddy42]

10 hours ago, spudulike said:

If you have flooded it, not wishing either of us to suck eggs.....remove the spark plug, turn the ignition OFF, turn the saw upside down and pull it over really hard a few times. This really helps clear puddled fuel and you may see it drip or spatter out of the saw hence the ignition off and YES, I have seen this ignite one one occasion - pretty impressive but not good for hair on arms or eyebrows!!!

I also find heating the plug up with a plumbers torch or gas cooking hob can help with getting the engine to fire but get it in and pull it over quick....no choke but fast idle position.

At least a working module will prove if it is that or not. Has he got a good carb to try? 

 

Agree.  Sorry I flooded the saw on purpose - so I know that it was sucking fuel.  Very odd, I will keep trying and provide an update.