Stihl 038 not starting mystery

[Olddevonstihls]

Using my 038 for an hours hard work and it stopped instantly, suggesting no spark.   Removed plug to check and indeed no spark.  Area around the kill switch under the air filter was clogged up and when cleaned out, weak spark appeared, adjusted coil gap and then got a good strong spark.  But… it still won’t start.  Stripped down Bing carb and there was some detritus which is now cleaned out and reassembled.  Still won’t start.  When carb stripped again both chambers are full of fuel. Plug is not wet which suggests fuel/air mix is not making its way from the carb into the cylinder. Gaskets and diaphrams are ok.  Carb is holding pressure.  Saw itself is holding pressure and vacuum.  I can feel compression.  Saw won’t start when fuel trickled in and turned over.  The flywheel is on the woodruff key so the timing should be ok.   Do the flooded chambers indicate I’ve still got a blockage of some sort in the carb?  Can anyone suggest what I need to be looking at here?

[Trailoftears]

Probably no help tbh.But when you take the back cowl/cover off to gain access to the a.filter.I seem to remember a translucent vertical fuel tube on the left as viewed from the back handle-old fashioned tank vent?Is that ok/clogged/disconnected or summat praps 🤔

[spudulike]

If the first chamber (Pump Section) is full of petrol, the carb is pumping fuel up from the tank. This may mean that the impulse line is working fine but a machine can run without it if their is positive pressure in the fuel tank...warm day etc.

Fuel in the second chamber (Metering section) means that the needle valve is working OK and the carb is letting fuel through so it can be drawn up in to carb bore to enter the engine.

You say that the plug isn't getting wet, is this with trying to start the machine with the choke on? With the choke on, giving the saw 10-15 pulls should pretty much flood the machine. 

You say the saw engine has pressure and vacuum - if you have a BIG air leak in the crankcase, it can cause lack of fuel...was it checked with a vacuum and pressure pump etc.

The kill switch description - the lack of contact would stop you from stopping the machine as the system connects the coil to earth to stop it. If you want to bypass the kill system, disconnect the kill wire from the coil and try it again. Closing the gap up to the very minimum, ensuring the two don't hit each other, can help maximising the spark, especially on weak flywheel markets and weak coils.

Basically, you need spark, compression and fuel...compression is usually the first one to check....what is it measuring? I have heard..."Has Compression" too many times to find the piston has seized and all you have is friction and NOT compression - whip off the muffler and take a look.

[Trailoftears]

Another random comment!rip in the intake boot,not that likely tho.Still the rubbers on saws like this are what?35 years old and more.

[Trailoftears]

The sudden death syndrome does sound electrical related.coil/plug/timing etc.Never overlook the obvious tho-new plug would be my first step-its cheap,easy and one box ticked!

[Trailoftears]

Another thought-and this has happened to me many,many times-usually with engines working hot and hard-with some hours on them.Free floating carbon from the piston top finds the spark plug gap and you're on stop.By the time you've taken the plug out-all seems well-good spark- carbon particle has dropped away from plug.I usually turn m/c upside down and try to  pump carbon out of spark plug hole then fit new plug.If no new plug re-gap the old plug as much as you can- that makes it harder for the carbon bit to kill the electrode gap.

[Olddevonstihls]

Lots of thoughts here, thank you.

 

if I understand correctly having fuel in both chambers of the carb is good so I can move on from worrying about that.

 

pulling the cord several times on full choke leaves the plug dry.  Squirting carb cleaner both in the plug hole and through the carb wets the plug but it won’t start it as you’d expect.

 

I haven’t measured compression, but it is enough to push my thumb off, and I have checked the exhaust port: the piston is moving freely and while there is some scoring the rings are intact.  It was pulling like a train before it cut out, and had just felled a 40” wide horse chestnut.

 

Vacuum and pressure tested with testing devices with exhaust and carb blanked off and using adapted spark plug. Crank seals were replaced not long ago with split case and new bearings, plus new boot and impulse tube.

 

plug is new and is specced for the saw.

 

Breather pipe is new and was venting properly until the electrical cut out.

the spark is as blue and strong as I’ve ever seen.

 

so I have compression (and it was running fine before it suddenly stopped), I now have a spark, but despite fuel in the carb chambers it doesn’t seem to be getting fuel into the cylinder, yet even with carb cleaner directly into the cylinder, its not kicking.  All I can think of is the timing has gone out of whack but I don’t see how, and anyway that doesn’t explain the apparent lack of fuel.  Something just isn’t right.  

 

[Trailoftears]

Soo irritating with these 'simple' engines when all should be well-but it aint!Tried pouring a small amount of proper  fuel mix directly into the chamber via plug hole 25/35ml odd?and pulling over with plug swiftly placed back in?If it will fire up briefly at least it then points to fuel starvation?

Other than that is the attatched choke actuating gubbins attached to inner air filter body doing is job when using choke position on switch?

[Mick Jones]

As spud mentioned if you disconnect the stop wire from coil this eliminates any possible earthing fault. Its quite possible you have a spark out side the cylinder but the spark fails under compression. Sounds like its worth trying a replacement coil.

[spudulike]

Yes on the carb sounding OK, it may still have a check valve issue but if you spray a bit of carb cleaner or even WD40 through the carb with the throttle open, it should fire if you pull it over plus it should flood if you keep pulling it over on full choke. If this isn't happening, it sounds like the check valve is jammed shut which is relatively unusual. You could remove the H screw and spray a bit of cleaner down its hole.

On the spark, it sounds OK - you can open the gap right up on a spare plug and see if the spark can jump the gap OK which will test the spark strength.

It does sound carb related from the plug not getting wet but also the stopping instantly also sounds like HT system but the saw should fire if you spray cleaner etc through the carb as previously said...a bit strange.

Try undoing the H screw to 2-3 turns and pull over on full choke and see if the plug gets wet after 15 pulls or see if it even tries to fail. I guess the choke is shutting correctly and the high speed mechanism kicks the throttle valve open as it should.