If your engine is relatively new (made within the last 10 years or so) and
has only one speed, then there may be no adjustments (like the one discussed
in the section: Cleaning Craftsman (Tecumseh)
carburetors. It will either work or it won't - in which case it needs
cleaning or parts replaced. Or your overhaul was less than 100% effective.
You can tell if your carburetor is of this type as there will be no adjusting
screws on the carburetor. For Craftsman types, there will be a solid hex nut
on the bottom holding the float bowl in place. There may or may not be a
|NotTaR of small Gasoline Engines and Rotary Lawn Mowers : Carburetor adjustments
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For adjustable types, it is best to refer to your engine manual. However, here
is the general procedure. Some of the specific numbers may differ for your
In all cases, before touching any adjustments, make sure your air filter is
in place, and clean (or new). Fill the fuel tank about half full with fresh
There are three adjustments on a typical carburetor:
- Main mixture - Bottom of Craftsman (Tecumseh) float carburetors.
- Idle mixture - Side into body of Craftsman (Tecumseh) float carburetors.
- Idle speed - Sets relaxed position of throttle plate.
Initially, carefully and gently turn the two mixture controls in until they
Note: "In" means clockwise (the way you would tighten a normal screw) and
"out" means counterclockwise (the way you would loosen a normal screw).
CAUTION: do not force them - you are not trying to tighten anything - as you
will damage the needles and seats which will require replacement of the
needles or entire carburetor. Then back them out 1 to 1-1/2 turns. Set the
idle speed screw 1 to 2 turns beyond where it contacts the throttle plate.
Refer to your engine manual for specific recommendations! These settings
should allow the engine to start and run, though perhaps not entirely smoothly
or with great enthusiasm.
- Start the engine and allow it to reach normal operating temperature - a
couple of minutes. Make sure any choke is off once it is started and
running stably. This will also flush any old deteriorated gasoline from the
- With the engine throttle control set for the maximum recommended rpm, very
slowly rotate the main mixture screw counterclockwise (loosen) until the
speed begins to drop off due to too rich a mixture. Then, rotate the screw
very slowly clockwise (tighten) until the engine begins to cut out. Very
slowly means a fraction of a turn at a time - then wait a few seconds for
the adjustment to have an effect. Note the number of turns between these
two positions and set the screw in the middle of this range.
- Repeat this procedure with the engine throttle control set to the idle
or slow speed position but using the idle mixture screw instead.
- If there is a high speed adjustment - possibly on the throttle control
itself or the throttle control bracket, it is best to set it using a
tachometer. However, it is possible to do a very good job by comparing
the speed by ear to an identical type engine that is set correctly. See
the section: Setting engine speed.
WARNING: if in doubt, set it low. It is better to end up with a scraggly
lawn than bodily injury or a blown engine! Note that by ear, 2 stroke
will always sound faster than 4 stroke engines for the same output
speed because they have twice as many explosions per rotation of the
- Set the idle speed adjustment just high enough that the engine idles
smoothly and isn't about to cut out. A tachometer can be used to set
it to specifications but there is no risk is just adjusting it to idle
- Test the engine under load. It should respond to load pickup immediately.
An engine that dies is set too lean. An engine that runs roughly when
picking up load is set too rich. Make a small adjustment (i.e., 1/8th
turn) and test again.
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