Stihl HT75 tuning problems

dhes

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Hi All,

I am having problems tuning a HT75 pole pruner. The saw seems to run best with H and L fully in, i.e. no fuel but still not running great. I posted link to video below showing the saw running and trying to adjust the H/L. I started the saw with both L/H fully in and first adjusted the idle screw to keep the engine revs up. You can hear the saw bogging down when I try to open the L or H screws. I notice there is quite a bit of fuel mix coming out of the muffler or is this normal?

Any ideas what I'm doing wrong?

PS: I have cleaned the carb twice and all looks ok. Only thing I didnt do was remove the H/L screws and blow through. The H screw has a cap limited so would be difficult to remove without hacking part of the plastic.
 

Fish

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Sounds like the needle is leaking, or you have lost a welch plug.
Here is a bulletin that discusses carb adj.
 

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Fish

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Looking at your video, I would suggest you remove and inspect the muffler and screen, look for obstruction, and peek at the piston.
Also, when it is back together, while it is running, remove the air filter and see if things change drastically.
 

dhes

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Hey, many thanks for reply. I did notice when putting the carb back together that the needle was not fully going down (if i pushed the top with my finger it would move down slightly). Can I bend the needle holder (not sure correct term) to apply more force from the spring through to the needle?
 

dhes

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Someone before me has removed the screen completely. I have some fine wire mesh that i could cut to fit if necessary. My understanding is it's only there to prevent sparks from flying out and catching fire. I had the muffler of previously but will look again thanks.
 

Fish

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Yeah, mainly peek at the piston for damage, or it could be badly carboned over as well.
 

dhes

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I removed the muffler and checked the piston. Couldn't see any obvious damage. Removed the carb and tried to bend the needle holder by pushing down on needle and spring at same time. I probably should have removed it first and used pliers. Removed the air filter and got the engine running and tuned up. It seemed to be running good as shown in first video. I then put the air filter back on and struggled to get it started. Removed filter again and still struggled to get started. I have an old strimmer engine. i might try and swap over the needle and holder to see if that makes a difference.

20200709_201409_resized.jpg

 

Fish

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Have you tried another spark plug?
Take it off and look at it.
 

dhes

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I replaced the sparg plug. That was the first problem I had to fix. Someone had pulled the spark plug boot off. I put on a new spring connector and boot cover. I struggled to get engine going and realised the spring connector was not making good contact with the wire coming from the magneto. I reconnected it and tested for continuity with multimeter. I can put back in old spark plug to see if it makes a difference. I tested both plugs previously with meter and both showed similar resistance.
 
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