Posted 07-08-2002 at 18:30:26
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First thing is to make sure it has a good spark plug in it. If in doubt, spend the 5 bucks on a new one.
The "generic" setup for most any carb is to lightly seat (full in) the mixture screws (low speed and high speed) and then back them out 1 and 1/2 turns. That'll give you a setting that will allow you to get it started. Then follow these steps:
1. Adjust the low speed mixture screw for highest idle.
2. Adjust the idle speed screw (on the linkage) for specified idle. On a string trimmer, that's fast enough to keep the engine from stalling, but not so fast as to engage the clutch and have the string head turning.
(You may have to work back and forth between 1 & 2 a couple of times)
3. Adjust the high speed jet for clean running at operating speed, and also for clean "throttle up". It should rev cleanly from idle to full speed. If it stumbles, readjust the high speed jet and let it idle down, then rev it up again.
It's been a while since I've done it, and I can't remember off the top of my head if the low speed adjustment affects throttle-up or not, but it may. Also, if you can't get it to throttle up cleanly, you might try tweaking the idle up a bit.
When you have a clean idle, it throttles up smoothly, and runs cleanly at WOT (wide open throttle) you're all set.
BIG HINT: Most poorly running string trimmers (or any other two-stroke engine) are due to clogged mufflers. You can disassemble the muffler and there is a spark arrester screen inside that gets plugged with carbon. Scrape it clean, burn off the excess with a torch, or soak it in solvent, and scrape the mung off the inside of the muffler. It's nearly miraculous how that simple bit of maintenance improves the running qualities.
I've also seen mud daubers build nests in 'em...