Trying to get an old 2-cycle Yardman snowthrower running decently.
Looking at the engine there are two screws that actually go into the
Both screws were actually out from 3-1/2 to 4-1/2 turns!! Seems excessive.
It was running ..but crappy.
Don't know what the screws are, or their approximate correct settings. One
screw is on the left nearest the piston. Other on the right nearest the carb
intake. But both only about 1/2 inch apart.
There is another screw above these that appears to be a mechanical idle
speed screw or something else. It goes completely through a narrow aluminum
boss, but I can't even tell it does anything (has a spring around it, but it
doesn't directly go into the caruretor so don't believe it is mixture
related. And I assume a 2-cycle thrower supposed to run at near max rpm
Turned both screws out about 2 to 2-1/2 turns from seat ....smoothness
increased, and so did speed, ..but I don't want to adjust the wrong screw
and run the engine too lean.
Any ideas on the approximate general 2-cycle settings?
If you look closley you may find a letter H and L stamped on the carb housing.
In any case turn both screws in all the way back out each 1 1/2 turns. Start
the engine and let warm up a bit if you can keep it running. Once warmed up set
the trottle at full speed. If you can't find any letters on the housing try
turning the smaller of the two in until the engine faulters a bit, then turn
out until engine peaks. If your getting no response it is the low speed
adustment screw. Once you have the high speed adjustment turned out till it is
peak rpms open it another 1/2 turn. The reason for this is you don't want it
running to lean as the oil in the fuel is the only lubrication it gets. Once
the high speed is set lower the throttle down to idle and adust the other screw
untill the idle is the smoothest. If the idle speed is too high adjust your
mechanical stop screw to raise or lower it.
Typically one is the low speed mixture adjustment and the other is the high
speed mixture adjustment. The third screw, with the spring on it, is most
likely the idle speed adjustment. If there's no manual throttle control on
the unit, you'll have to hold the throttle mechanism against the idle stop,
then adjust the proper mixture screw for maximum RPM. Usually only one
mixture screw will have an effect at idle, so that will identify the idle
mixture screw. If I were to take a guess, it would be the one furthest from
the engine and nearest the air intake. After that, let the throttle go to
its governed speed setting, or pop it open yourself, and adjust the high
speed mixture screw so the engine accelerates promptly. You may have to
repeat these adjustments as they could interact somewhat.
You might want to remove each mixture screw, make sure it's tip is clean,
and reinstall, at some point in the process, after you're comfortable with
the adjustment process. The screws should be a lot closer to the typical
1-1/2 turns out than they are right now.
low speed is nearest the engine, high speed furthest. hold the
throttle closed against the other screw and adjust the low speed, then
let speed come up and adjust the high side. set the high side so it
smooths out under a load, the best time to set them is when there is
snow to load it. if it has rubber paddles tip it forward to load it.
you want a slight burble to it when running with no load. be
carefull, the old techumseh 2-strokes had a plastic screw on the low
speed side and were very prone to snapping off when adjusting!!
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