QuestionThanks, Michael. How do I differentiate the high and low mixture screws? I have no manual on this. It has a Walbro carb. The 2 mixture screws are right by each other. One has a black cap, and the other, a white cap. I haven't been able to find a manual online yet.
Scotty
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Followup To
Question -
Hi Michael,
I have 2 problem string trimmers. I got both of them used, cleaned them up and tried to get them running right, so I don't know their history before me.
1. McCullogh (sp?) trimmer: I had to drain and replace fuel to get this one going. It took a while, but I finally got it to start easily; however, it will not rev up above ~1/3 max speed. The trigger and throttle plate move fine. If I try to throttle up too quickly, it will bog down and die. If I slowly increase throttle, it just won't go any faster, and the sound changes with the throttle wide open (sound through the carb). Oh, I doubt that it has anything to do with it, but I accidentally broke the original spark plug, and used a plug from the Toro trimmer below. It is a different model plug, but I can't see that affecting the engine in this way, especially so dramatically.
2. Toro trimmer: Loud noise like marble(s) shaken in tin can. I can't tell from where it's coming. When I disassembled this to clean (before trying to start it) the carb came off, revealing a big hole into the crankcase below the piston (lower piston and complete rod visible. I didn't see anything amiss, though I wasn't specifically looking for anything since I didn't know about the noise then. The spark plug is OK, and I have seen no other evidence of a foreign body in the piston cylinder.
Thanks in advance,
Scotty
Answer -
Hi Scotty,
On the MacCulloch, I would try adjusting the carb's mixture screws. The high speed mixture screw is initially set at 1 & 1/2 turns out. Then adjust it to give good acceleration and top speed, while not closing the screw more than you need to. The low speed mixture screw is set at 3/4 of a turn out. Adjust that, to give a smooth idle. The 3rd screw, the idle speed screw, is set to give a low idle speed without stalling. Once you get it to run well, the spark plug should be changed to the proper one. Different type plugs may run hotter and cause engine damage during long use.
The Toro sounds like there is internal engine damage. If the sound seems like it is coming from the crankcase you could remove the carb and take a look while cranking the engine over. Further dissassembly may be needed.
Let me know how you make out.
Michael
AnswerThere should be a small "H" and "L" stamped near each screw. If not, you can close each, while counting the turns required to close them. Unless they were set very incorrectly, the high speed screw should be unscrewed further.
Let me know what you find.
Michael
