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Walbro carb tuning on 2 stroke trimmer

Posted by ceesneezy (My Page) on
Fri, Jun 11, 10 at 15:44

I am new to carburetor tuning. When tuning a carb on a two-stroke machine such as a trimmer, what does it mean to adjust throttle or air mixture screw until it "four cycles"? And what does this sound like? See the paragraph below which I found online about Walbro carb tuning on 2 strokes. Also does the same method apply to tuning a 4 stroke engine such as your typical lawn mower? Thanks

"Set the lowend & highend needles to about 1 3/4 to 2 turns each. Choke the carb or prime it, until the carb is wet. Fire up the engine and let it warm up. Let's set the topend first since it's the easier of the two. Go to full throttle. Adjust the topend needle for peak RPM. Leave it wide open for about a minute to see if it changes any. Should the engine go lean, open the lowend needle slightly, if this dosen't work... you will have to adjust the needle valve inside the carb.( I will explain this later) If the topend runs OK, then slowly pull the throttle down until the engine begins to "four cycle" hold the throttle there. Adjust the lowend needle until the "four cycling" barely stops. Now lower the throttle more until it "four cycles" again, and adjust the lowend again. Keep doing this until you reach full idle. Now, from full idle begin to throttle up until the engine starts to bog or hesitate. Open up the lowend needle just enough to eliminate the bog or hesitation.

When this is done right, you will be able to set the throttle in any position and it won't four cycle, plus you will be able to transition from idle to full power without any hesitation at all. Now, for easy starting it's best to have the lowend a little rich and it will four-stroke a little."


Follow-Up Postings:

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RE: Walbro carb tuning on 2 stroke trimmer

These are commonly set a touch rich to protect the engine. Just beyond the screaming 2 cycle setting, some refer to this as "4 stroking" Your typical lawnmower is non adjustable, but when they were -had generally been set to high/ stable speed with smooth throttle response.


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RE: Walbro carb tuning on 2 stroke trimmer

what is meant by 4cycling


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RE: Walbro carb tuning on 2 stroke trimmer

When applied to a 2 stroke engine, it describes an intermittent misfire caused by the mixture being rich. A rich mixture at idle helps with throttle response as small carburetors usually don't have an accelerator pump and the mixture can go too lean when accelerating. This condition is called "bog" . As in "My trimmer bogs out when I pull the trigger".

Another term to describe the same thing is "gurgle" or "burbble".


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RE: Walbro carb tuning on 2 stroke trimmer

I am a real old dude and 4 cycling means it is missing a stroke. Instead of power each time it gets power every other time.
Running, but running sloppy.


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