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How to Adjust Your 2-Cycle Carburetor


If you've bought a new 2-cycle engine in the last few years, you'll notice that something's changed. Where older carburetors had a pair of slotted head adjustment screws, the newer ones have weird screws, recessed deep inside the carburetor's casting. Why? Because the EPA thinks you're too stupid to know how to adjust a carburetor. It's one in a long line of rules and regulations ostensibly designed to reduce pollution while having exactly the opposite effect. Carburetors are rarely adjusted properly from the factory, and need readjusted periodically as parts like the metering diaphragm age. The average joe will not spend the money to have it adjusted at the shop; they'll run it in whatever condition it's in, regardless of how much fuel it's using, until it quits working.  Then he'll buy a new one. Keep in mind that the process of manufacturing, assembling, and transporting a new tool produces loads of pollution, probably a lot more than running the tool for all of its useful lifetime.

A Walbro WA-199 carburetor from 1989

A Walbro WT-872 carburetor from 2014

That being said, you can adjust them, it just takes a little more work. Tools for doing this are available online, but I'm not about to pay $20 for a screwdriver I can only use for one thing. A better solution is to permanently fix it by making the adjustment screws into regular screws. Basically, all you need is a rotary tool like a Dremel with a cutting disc. Just make a lateral cut across both screws. Now you have slotted adjustment screws and there's not a damn thing the EPA can do about it.

Safety note: The casting is aluminum, but the screws are steel and will shed sparks. Don't do this close to open containers of gasoline. If you're doing this on a used carburetor, flush all of its internal passages with a low flammability solvent (kerosene, mineral spirits) followed by compressed air, then let it air-dry for a few days.

Resetting the Carburetor to its "Default" Settings

In case you've had the carburetor apart or it's grossly out of adjustment, a good place to start is with the "default" settings. To do this, tighten both screws all of the way in (don't apply a lot of torque, just snug it)  then back them out 1½ full turns. At this point, the engine should start and run, although it may not run well. If you can't start it at this point, try turning the screws ¼ of a turn at a time in either direction until something happens. If that doesn't work the issue is likely something else, like a clogged jet or weak spark.

Adjusting the Idle (Low-Speed) Circuit

Carburetor adjustmentThe idle mixture adjustment screw is the one marked by an L on the casting of the carburetor. Turn the screw very slowly counterclockwise while listening to the sound of the engine. Keep turning until the engine speed starts to drop, then make a note of this point. Now start turning the screw clockwise, the engine should accelerate up to a peak, then its speed should start to drop again, make a note of this position. Now just set the screw in the center, between those two positions.

Adjusting the High Speed Circuit

For this one, we want to go ahead and repeat the procedure we used on the first screw, only with the engine at full throttle: slowly adjust it counterclockwise until the engine speed drops, then clockwise, and center the screw between those two points.

Fine Tuning

It should run OK now, but it's probably a good idea to go ahead and fine tune it. Keep in mind that turning the screws counterclockwise richens the mixture (more fuel to air) and turning it clockwise leans the mixture (more air to fuel). An engine that runs too rich will incompletely burn the excess fuel, leading to increased plug fouling and more smoke in the exhaust, while an engine that runs too lean will run way hotter. Either way, it's not producing the most power. It's always better to err on the rich side with any engine, doubly so with a two-cycle engine since lean fuel also means lean lubrication.

To achieve this optimal slightly rich mixture, you need to turn the high speed screw counterclockwise until the engine starts to run roughly at full throttle. With the engine still at full throttle, turn it clockwise just enough for the engine to run smoothly.


Idle the engine for a few seconds, then give it full throttle. The engine speed should ramp up to full speed within a few seconds. If it hesitates, turn the idle speed adjustment screw counterclockwise, no more than the width of the adjuster screw's slot at a time, until you get a smooth acceleration.


Run the engine at full throttle for a few seconds, then let go of the throttle. The engine speed should return to a smooth idle in two seconds or so. If it's slow to return to idle or idles erratically, adjust the idle speed adjustment scew, no more than the width of the adjuster screw's slot at a time, until you get a smooth deceleration.
Double Check the Idle Speed

All that's left is to set the idle speed, which is easy. Turn the screw clockwise to speed up the idle, and counterclockwise to slow it down. If your equipment has a clutch that engages automatically when the engine speeds up, you'll want to adjust the idle speed just below the threshold of where the clutch engages. If not, you'll want to adjust it to the lowest speed that the engine runs smoothly.

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