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Hello,

I have a stihl fs400. I can not adust the L/H on carb so that the head stops rotating.. What could it be? Adjusting L?H does have a minor effect on revs.

Hope somebody can advise.

Thanks

Carburetors with H, L and LA screws

Cutting tool rotates when engine is idling: Turn the idle speed screw (LA) counterclockwise until the cutting tool stops rotating then turn the screw about another half to one full turn in the same direction from that position.

Carburetors with LD screw

Carefully screw the idle speed screw (LD) down onto its seat counterclockwise (left-hand thread).
Then open it two full turns clockwise.
Adjusting Idle Speed: Carry out standard setting.: Start and warm up the engine.

Engine stops while idling: Turn the idle speed screw (LD) clockwise until the engine runs smoothly the cutting tool must not rotate.
Cutting tool rotates when engine is idling: Turn the idle speed screw (LD) counterclockwise until the cutting tool stops rotating then turn the
screw about another one full turn in the same direction from that position.
Erratic idling behavior, poor acceleration
: Turn the idle speed screw (LD) slowly counterclockwise no more than one half turn. General Information The carburetor comes from the factory with a standard setting.
This setting provides an optimum fuel-air mixture under most operating conditions. With this carburetor it is only possible to adjust the high speed screw within fine limits.

Standard Setting
: Shut off the engine.
: Mount approved cutting tool.
: Check the air filter and replace if necessary.
: Check adjustment of the throttle cable and readjust if necessary see "Adjusting the Throttle Cable".
: Turn high speed screw (H) counterclockwise (max. 3/4 turn) as far as stop.
: Carefully screw the low speed screw (L) down onto its seat. Then
open it one turn counterclockwise. : Start the engine and run it until it is warm.
: Adjust idle speed with the idle speed screw (LA) so that the cutting tool does not rotate.

Hi, Thanks.. I'd got that from the manual also. It has only the one screw.. Turning it doesn't make sufficient difference it still keeps spinning away.. I was wondering if it could be something to do with the clutch.. I opened it up and all appeared fine.. very minor surface rust but all pretty free.

See if you can back off the throttle cable a bit.

Hi thanks. If/ when i do the throttle trigger does n't pop back properly. I've now disconnected the cable to rule out that possibilty. The screw is fully in its seat and still it spins.

If you are sure that there is no idle speed screw, try turning in or out the L screw again. say 1/4 turn a time.

You do know that carbs with LA screw is (left-hand thread).

The idle screw is fully in its seat. If I unscrew it revs increase ans do rpm of head. The screw wont screw in any further.

Is this the setup that you have..? and how fast would you say its running.

nope. Just 1 screw. the others are capped off or maybe special tool.

Got to be this one then, check to see if end of idle valve screw and seat are not damage, and have you cleaned out the carb at any time.
I'll be off air till tomorrow, so will check your reply then.

Hi yes. thats the one..

Sorry.. I'd missed the question on how fast its running. Although my response is pretty useless as I have not an idea on how to quantify. Although if carefully done I can stop the head from turning with my hand but that resistance puts strain on engine/ can be heard. However now with that screw fully in its prone to stall and then I cant start it without widning it back out.

New carb. well was new 1 year ago but it's just been left on the shelf till a month ago or so when I started putting her back together. not been used other than testing.