Thanks for the help.
I've been using Stihl oil at 40:1 as I don't have a user manual and it's
quite old. The newer Stihl machines I know use 50:1 but I also have an old
AV15 chainsaw which uses 25:1.
The other thing that was puzzling is that the HS85 only has a single
'High/Low' mixture adjusting screw - all my others have two separate ones. I
think I need to locate a user manual.

"[H]omer" wrote in message [snip]
Dirty carb or air filter(s) or both.
Bad/wrong oil mix.
Empty the tank and clean the whole thing. See your local garage for the
correct cleaning fluids (soapy water for the air filters). While you're
there get fresh 2-stroke and check the proportions (usually 50:1 - but
you *will* need to know for sure).

Then again, maybe something's just worked loose - again the mech. at the
garage might help.