Stihl HS85 carb adjustment

13 Nov 2008
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United Kingdom
I assume the carb is the original so some wear is expected.

I adjusted the carb what I thought was correct a month ago & did a hedge no problem.

Today I started it & waiting a minute or two for it to warm up.
I noticed that when I touched the throttle it was boggy & mostly died.
Playing with the throttle is little until it sounded ok I then gave it nearly full revs & started to cut the hedge. Hedge was only say 30 ft long & trimmer must of stopped / stalled / died say 3 time.

So I’ve adjusted it again like this.
L screw in until seated then 1 turn back out.

At this point I thought it was going to stall so now over to the LA screw of which i turned clockwise until the blades started to move then half a turn anti-clockwise.

I then took this short video clip to show you the result.

A bit boggy I think still so what’s you advise for which screw to now turn & which way.

Blades look fine when running so not bent etc & I’ve also lubricated them with spray oil. Gearbox grease replaced with the proper expensive Stihl stuff.

Video link here.


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I can’t access your video I’m afraid but I think the H (high speed) screw is maybe a fraction rich so turn it in a fraction and try again.
The L screw is for low speed and tickover running and has little affect on the full speed setting.
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There isn’t a H screw.

There’s only L and LA
The manual shows L = 1
The adjust LA screw for blades to move them back off half a turn.
OK, so the L screw is the only one that you can juggle with which will alter the mixture......LA is the mechanical throttle stop screw.
Make sure the air cleaner is clean too, and not too much oil in there.
John :)
Check your carb doesn't have a hidden screw which adjusts the mixture in the centre of the throttle pivot.
also, if you have excessive fuel spraying out of the carb with filter removed the exhaust/port may be restricted.
@Burnerman i attach the info from the manual for carb adjustment.
Not being rude but wondering if your advice for the LA screw is from knowledge of other carbs / tools ?

This model has two air filters & both were replaced last month.
I didnt put any oil on them though ?

One is a rough like scouring pad then the other is a thin foam type.
Rough filter is put on first followed by the foam filter then the plastic cap which holds it all in place.


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I'm pretty sure that LA is a mechanical adjustment but without a full picture of the carb I can't be sure.....I can't see why there would be two mixture screws for low speed running?
Anyway, the instructions are comprehensive so twiddling screw L is all you can do, but only by a tiny fraction of a turn.....less than 1/8th turn.
The air filter system is correct for that machine......the foam piece goes to the outer and the inner thicker one needs to be clean. You don't oil these - two stroke machines have a little fuel back spray due to their design so you can expect the filters to be slightly oily.
If you still get no joy then it would seem a carb strip down is inevitable once more - together with a diaphragm replacement.
John :)
So is my current issue due to the mixture being to weak or to rich ?

Then do i screw in or out ?

The million dollar question, as the symptoms can be remarkably similar!
As the high speed running mixture is fixed, flat out running won't be affected at all.
I'd start by turning the L screw in 1/8th of a turn and see if the machine picks up better - this makes the mixture weaker
Then try turning the same screw out 1/8th of a turn - this makes the mixture richer.
John :)

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