to Adjust Your 2-Cycle Carburetor
If you've bought a new 2-cycle engine in the last few
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
pollution, probably a lot more than running the tool for all of its
A Walbro WA-199
A Walbro WT-872
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
Safety note: The casting is aluminum, but the screws are
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
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
The 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
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.
It should run OK now, but it's probably a good idea to
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
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
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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