Carburetor adjustments

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 engine, however.

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 gasoline.

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 just seat.

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 carburetor!

• 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 crankshaft!

• 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 smoothly.

• 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.