Down low in between the fuel line and the return line barbs you should see a black round piece of plastic. This is a plug that covers the adjustment screw. It has to be removed to access the screw. I used a dry wall screw and pulled them out. These are the worst Zama carbs I have ever worked on. I have a stack of Echo 210 and 2100 weed wackers that need new carbs, but I will not spend 65 bucks these cost. I'm now in the process of changing the intake block and going with a traditional no plastic throated carb with both high and low adjustment screws. Good luck!! I have not had any luck with the rebuild kits I have installed. I think the ramp the plastic throat rides on get worn out and will not let it open up enough.
If I get a chance, I'll take a picture of it.
I have 4- 2100 here in my shop that will not run at WOT. They will all start and idle fine but pull the throttle trigger and they die. I have did every thing that I know to do with the K75 carbs, but nothing I can come up with helps. I'm trying to find the old style intake and Walbro carb that Echo used to put on their trimmers that I never had any problems with. I did part a 210 trimmer out and sold it on Ebay and it did well. I sold everything except the k75 carb, I ended up slamming into the wall of my shop after about 3 hours of trying to fix it. I think the plastic throat and the little ramp knob it rides on get worn and causes the problems and that can't be repaired. I have notice that all mine I'm working on are all about 4 years old. New K75 carbs are about 65 bucks on Ebay
