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Stihl HL94 moving out of tune

Started by Renardiere, November 01, 2020, 05:04:41 PM

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Hello all, pleased to be a new member of this forum.

 I'm having a persistent problem with a customers HL94. It seems to go out of tune after an hour or twos work. I've been all through this machine including full carb strip and clean yet the problem recurs.
 I've kept the machine back at home to use myself and see if I can reproduce the problem. So, after a couple of hours hard use this morning with out any problems sure enough it started bogging down when asked to rev. Small tweak of the high circuit screw and everything is great again.
  I've come to conclusion that the problem is just down to the H screw moving with vibration. There are no springs under the screw heads in these carbs so I'm wondering what might be a sensible approach to reducing the tendency to move with vibration.
 Has anyone had this issue with these before and any suggestions how to solve?



Hi Renardiere, and welcome to TGMF.

Can't say I've had this exact problem with the HL94, but they can go out of tune quite easily I have found.  Does yours have the mixture adjusters one on top, and one on the lower side of the carb? The one on top in the middle of the throttle shaft can be very loose and can move, so might be worth checking that one.   


Hi Mick, thanks for the welcome.

 Yes, low circuit screw is on top, runs thru the throttle spindle. Interestingly it does not seem to be a problem, more the H screw.
 I'm thinking about trying some ultra low strength Loctite but uncertain whether a petrol soaked environment is going to let it work.
 I know the old motorcyclists trick was to squash the needle threads a bit but that goes against the grain for me.

 Thanks for confirming that they are known for going out of tune.


As a test, to verify if it's that screw moving and causing the problem, wonder if it's possible to push a bit of rubber pipe down between the high speed screw and the recess, just to stop it moving.


Good plan!  I'll give it a try.

Many thanks


Please let us know if you find anything, it's always handy info to have.  :tup:


Ok, update. I've taken Mick's suggestion to use rubber tube and developed it. Using some Delrin I've turned up a couple of plugs which are a friction fit on the heads of the mixture screws and same, tight fit on the ID of the sleeves that surround the screws.
 I've run a half tank of fuel through the clippers just now and it seems to be working well. I'll get them back to my customer and hope that he agrees that these have solved the problem.

 Thanks for the suggestion Mick.




Fingers crossed then, I hope that cures the issue. 

Nice work on those Delrin plugs, now if Stihl could do similar.  :idea: