Join Date: Jun 2009
Location: Baton Rouge, LA
Assuming your engine has the Hi and Lo needles, the Hi one will be further from the engine, and the Lo will be closest to the engine. Most any Walbro carb'ed engine will start and run (very richly, though) with both screws between 1.5 and 2 turns out from gently closed. Once she fires, she should run with the choke open, but the engine will be down on power and smoke a lot. Time to start tuning.
First, hold the throttle wide open, and slowly close the Hi needle (further from the engine). The engine will smooth out and speed up. Note where the engine begins to sound smooth, and continue closing the needle (called leaning) until the engine begins to stumble and get rough again. Note this position, and then set the Hi needle to a little to the rich side of the middle of the 2 points you noted..... Hope that's clear...
Now, go to idle. Let the engine idle a little bit (30 seconds or so). Open the throttle and see how it performs. Probably it'll stumble, sputter, smoke and maybe quit. If so, lean the Lo needle a bit (closest to the engine) and try again. If it simply quits immediately, the idle is lean and you'd need to open the Lo needle slightly. Repeat this until the engine accelerates from an idle cleanly to full throttle.
Because the idle jet has some effect on the main jet, after you get the idle working right, just revisit the Hi needle as described above to fine tune it. You want it set a bit towards the rich side of the 2 stumble points for best performance and engine life.
A quick caution for you... When the engine is being tuned, don't tune it so it's always running completely flawlessly smooth. It's safer if you rich it just slightly, so once in a while it burbles slightly. That's a sign of a slightly rich mixture, and since our oil is in the gas, it helps ensure adequate oil. Just the slightest, most occasional burble when the engine's under low load (not cutting) is great. Once line meets grass, the load goes up and the engine will produce best power.