Problem with Stihl BR 400 blower

[trooper1954]

Hi all,
I have a 30 year old blower that has served me well and given me no problems until now. The problem is it keeps breaking woodruff keys....I have replaced three myself, and each time the key breaks after a very short while, and the blower is a bear to pull start. I finally took it to a shop...they replaced the key and as soon as I got it home it broke> When I contacted them they told me they wouldn't work on it again!
Can someone tell me if this is more than likely a crank shaft issue, and if so, do I have to replace the crank shaft or should I start with changing the bearings first? Any help to determine the cause of this would be appreciated...with the plug removed it turns over fine on the pull cord when there's no compression. I don't want to throw it in the trash as it's in good shape and has never given me problems before. Where to start??
Thanks.

 

[StarTech]

First are you torquing the nut to 18.5 ft/lbs?

Second the high compression could carbon build-up in the cylinder.

 

[trooper1954]

Thx Startech....No....just as tight as I could by hand. How to determine if there's carbon build up in the cylinder? It didn't smoke at all when running

 

[StarTech]

Usually to see in the cylinder without disassembly takes a borescope or endoscope but from experience Stihl are bad about running rich which leads to carbon build-up.

To torque the nut you need a piston stop and a torque wrench. Very to torque by hand without the piston stop.

 

[Hammermechanicman]

Check flywheel for crack around crankshaft hole. Look very close especially on the bottom side.
Check crankshaft taper for scoring or any damage.
If both good install flywheel with new key and seat flywheel by using a socket over the crank on the flywheel and use a hammer to give it a few good hits to seat flywheel on the taper then torque down the nut.

 

[trooper1954]

Thx hammermechanicman...When it was in the shop they replaced the flywheel, and there doesnt appear to be any damage to the crank shaft taper

 

[Hammermechanicman]

Check the fan and make sure it is tight on the shaft.

 

[StarTech]

Check the fan and make sure it is tight on the shaft.

Fan wheel is mounted to the flywheel. This why it critical to properly torque the flywheel retaining nut to spec. The three spline screw IS-M5x16 fan wheel screws are torque to 5.9 ft/lbs per the service manual.

 

[slomo]

Thx hammermechanicman...When it was in the shop they replaced the flywheel, and there doesnt appear to be any damage to the crank shaft taper

Was the correct part ordered for the flywheel? OEM part or Chinese?

If the tapers match up and are clean, proper torque is the last item to set.

 

[trooper1954]

Thx slomo...will check all torque settings and see what happens. I just assumed that having come out of the shop all these things would have been done