Stihl backpack blower not starting

[xjpelham]

My 17 year old Still BR340 blower has me stumped. Always worked perfectly although it’s been getting harder to start over the past couple seasons and finally gave up the ghost this year. I went through the checklist, fresh gas mix, good compression (130 psi), good spark, new fuel filter, air filter and spark plug. Spark arrester clean as a whistle. Cleaned out and rebuilt the carburetor twice. Still nothing. I even went ahead and got a new ignition coil and an aftermarket carburetor. Getting fuel into chamber as spark plug gets wet. No luck starting though. Choke on, 2 or three pulls, choke off and it still doesn’t want to pop off. Thumb on/off switch working fine according to my multimeter.

Any suggestions are welcome as I don’t know where to go from here.

Thanks

 

[StarTech]

You may have a sheared flywheel key. But getting to it to even check is a major undertaking as the fan side must be disassemble.

 

[Fish]

You might pull the muffler and check for excess carbon blocking the exhaust.

 

[Fish]

Also, you might have it badly flooded. Dump out all of the fuel and keep trying to start it with the tank empty, it may just start up after a while and burn off that excess fuel.

 

[StarTech]

Good points especially on being flooded.

 

[xjpelham]

Thank you StarTech, you were exactly right about the flywheel key.

It took me a while but I finally got it all apart. Looks like the woodruff key is sheared off and now flush with the crankshaft. Would you know how I can get this little piece out without damaging the crankshaft? I'm guessing I can still use the old flywheel as it doesn't look damaged.

Thanks again for your quick reply. It really helped me diagnose the problem.

Great forum.

 

[StarTech]

Usually you can take a pin punch and hammer and tap it down on end and the other will roll up so you can pull it on those Woodruff keys. They are moon shaped keys. Just remember when install the flywheel to torque the M8-1 nut to 18.5 ft/lbs. Service manual has it at 25Nm (18.439 ft/lbs).

 

[xjpelham]

You are right again StarTech. I used a centerpunch and a few taps later the broken key is out. A local dealer has one in stock. A $1.00 part!

Thanks for the torque specs. Very helpful.

 

[slomo]

I was rooting for some dead engine case seals. Glad to hear it's running again.

slomo

 

[bertsmobile1]

Before you put the key in , take a fine file to the edges of the key slot
Usually a sheared key will raise a little burr that will prevent the taper locking so the next time you try & start it the key will shear again.
It is also prudent to do the same with the flywheel.
Don't over do it you are not trying to cut a champhur on the edges, just knock off the burs.
One step further is to use some Brasso as a lapping paste & lap the flywheel to the shaft