Many inexpensive mowers don't even have a speed adjustment screw. Spring tension and the linkage to the governor set speed - period. Note that actual speed is rarely that critical for engine driven equipment as long as it is within safe limits. See the comments below on string trimmer speed settings. However, what options do you have where there is a need to set the precise engine speed?

• Use a tachometer designed for small engines. If you do a lot of small engine work, this may be a worthwhile investment. For most of us, it would gather dust.

• Use a tachometer designed for automotive engines. You may already have one of these if you do your own tune-ups. Sure, right, that went out of style about the same time as emissions controls! OK, maybe your father did his own tune-ups :-).

  Since an automobile engine spark plug fires on every other revolution of the crankshaft rather than every revolution as with most single cylinder 2 and 4 stroke engines, it will probably be necessary to multiply the reading by a factor of 2. (Even though there is a power stroke every other revolution for the 4 stroke engine, the ignition system is usually active on every revolution. However, there are a few exceptions to this rule.)

• If you have electronic test equipment such as almost any oscilloscope or frequency counter, it is a simple matter to couple its input to the spark plug wire on its insulation (not to the plug itself!). Then, the speed is equal to the pulse rate for most single cylinder 2 stroke and 4 stroke engines.

• Compare it by ear to another engine of the same type (2 stroke or 4 stroke) that is correctly set (i.e., you didn't muck with it!). This is actually a remarkably accurate way of setting the speed.

For equipment where a shaft with a known speed ratio to the engine crankshaft is available (i.e., a power take-off or trimmer head), an optical stroboscope of one form or another may be used. You will just need to paint or tape some stripes on the rotating part to put under strobe illumination:

• A fluorescent lamp powered by a magnetic ballast (not an electronic ballast) flickers at 120 Hz (in the U.S., 100 Hz in countries with 50 Hz power) and may be all you need to accurately set speed.

For example, for something like a string trimmer which has a direct coupled hub, strobe disks with 2, and 3, and 4 radial lines will appear stationary under fluorescent illumination for 3,600, 2,400, and 1,800 RPM respectively.

There can be ambiguity but if you are already in the ball park, this sort of approach may be all you need.

• Use a stroboscope which is calibrated in RPM or Hz. You may be able to borrow one from a high school physics lab or Disco!

(From: Philippe Habib ([email protected]).)

Go to a hobby shop that sells radio controlled airplanes. Plunk down $30 or so and get an optical tach. Paint 2 strips on the (crankshaft) hub of your equipment to simulate a propeller, and you're done.

(From: J. Matthew Good ([email protected]).)

You shouldn't need a tachometer on a trimmer. Two stroke engines in the size and power range of line trimmers can't overspeed as they don't have the power with a line head installed. Just set the carburetor up so that it 'two-fours' at wide open throttle and you should be all set. If it needed to be adjusted with a tachometer, it would have some kind of governor on it.

(From: Mowerman ([email protected]).)

B&S engines have a spring in the governor arrangement. You want to change tension on the spring to change the speed. (This is basically true of most other small engines as well but the details will differ.) The spring is attached to a metal tung (strip) in the linkage at front of motor, this is made of a tinny metal so you can easy bend this tung. To lower speed you want to lessen the tension on the spring. You can do this while the motor is running at top speed but it would be safer to do the bending while mower is not running. By pushing in tung you will lessen top speed gently as it doesn't take much to alter that speed. Often this tung gets knocked in while mowing around bushes or other protruding material and "hey presto" your engine is only idling. It is a design problem that briggs should be working on, however I love B&S engines with their simplicity and ease of maintenance.

Adjusting the carburetor on a 2 stroke engine

This is generic advice but probably a good place to start. It assumes that there are 2 adjusting screws - idle and high speed mixture.

(From: [email protected]).

I would start with them both backed out from the closed position to 2 turns open, for starters. One should be idle mix and the other should be high speed. Crank the motor, get it warmed up. You may have to fiddle with the throttle and or choke until its warmed up. Then slowly turn in the low speed jet, until it starts to die, then back it out another 1/4 to 1/2 turn. Then hold the throttle wide open, and slowly turn in the high speed, until it really starts to smooth out and rev high, and start screaming, then back this out until it starts to run rough or slow down, and then turn it back in to midway between these two positions. Keep playing with the low speed needle until you get rapid immediate response from the throttle, and good idle with the idle adjustment. Then play with the high speed needle, but always back it out from the setting about 1/4 turn or so, from where it runs the smoothest. This setting of backing it out will allow more fuel when under a load, and keep your 2 cycle motor from running too lean. Put a load on it and just tweak the settings just a hair at a time. If you're in the ball park it won't take much adjustment either way to make a difference. Better on the rich side than lean side. Also make sure your oil / gas is mixed at the proper ratio. 99% of 2 cycle motors are adjusted the same, no mater what brand they are. Most will start and run with both screws open 2 turns initially. Just don't close the high speed off any more than necessary no matter how well it runs.