Stihl fs85 engine

[mistyman]

Having a problem with an fs85 stihl weed wacker, that started to backfire and now will not start. Have changed piston and barrel, and flywheel, has strong spark, but just will not fire

 

[hlw49]

Air leak?

 

[mistyman]

Air leak?

Dunno where. Barrel, piston and rings are brand new, as is manifold gasket. Going to change the bottom end in the morning, in case the big end has any play, but we're just taking shots in the dark, now

 

[hlw49]

Seals around the crank shaft and any mating surface.

 

[mistyman]

Yes, found that, when i filled the crankcase with gas, a major leak through the seals. Being experienced in 4 stroke, I never realised that the crankcase integrity mattered so much on a 2 stroke. Live and learn, thanks