PRODUCT BULLETIN – MECHANICAL GOVERNORS

Unnecessary product returns are costly to your company as well as to Pierce.  Many of these returns indicate possible assembly and adjustment errors by customer personnel.

When installing and adjusting a mechanical governor, two points of instruction are much more essential than they seem on the outset:

1.     The length of the linkage that connects the governor output lever to the carburetor lever can ONLY be determined AFTER the governor is COMPLETELY installed in place on the engine.  Here are some linkage-setting instructions common to most mechanical governor installations:

a.      Without the engine running, adjust the governor to apply some tension on the governor spring.  This forces the governor output lever to the “wide open throttle” position.  That’s the only position noteable on the governor; closed-throttle position is not discernable on the governor.  That’s why it is necessary to set rod length – in the next two steps – at wide open throttle.

b.     Attach the throttle linkage to either the governor or the carburetor lever first.

c.      THEN, adjust the length of the linkage so that, when it is attached at the other end, the carburetor lever is also held at the “wide open throttle” position; just slightly off the carburetor’s wide open stop (up to 1/16 inch) is preferable.

d.     Attach the linkage securely, remove the governor spring tension, move the linkage by hand, and be sure it moves freely through the full travel of the carburetor lever WITHOUT RESISTANCE.  No other engine components can touch this connecting linkage - wiring, hoses, etc.

                        Just “short-cutting” this one critical adjustment - with “preset” throttle linkages

or careless adjustments or lack of observation - have generated warranty claim

comments such as: 

                        “Governor will not pick up load”       “Won’t rev up”

                        “Governor over-speeds”                     “Would not respond”

                        “Cannot set governor speed”              “Not controlling speed”

2.     The “bumper screw” is used ONLY to eliminate a surge or “hunting” at maximum no-load speed.  Before starting the engine, make sure the bumper screw thread is engaged only 3 or 4 turns (approx 1/8 inch) into the governor housing.

      IF, after your normal startup and speed adjustment, you encounter a no-load surge, THEN the

      bumper screw can be turned slowly - while the engine is running - in  a clockwise direction, until

      the surge JUST stops.  The bumper screw MUST NOT be turned in far enough to RAISE the

      no-load speed.

                        An incorrect adjustment here can generate warranty claim comments like:

                        “Engine surges when load is removed”

                        “Engine runs away or over-speeds when load is removed.   

Something else to watch for: A mechanical governor has limited travel to move the throttle rod and carburetor lever.  In operation, the governor output (rocker) shaft has 15° of movement.  That means the linear travel available to move the throttle rod is only ¼ inch per inch of lever length, at best. (i.e., a 4-inch-long lever will move the rod one inch).  The movement required to move the carburetor from fully-open throttle to closed throttle MUST NOT EXCEED the available governor lever movement.  It’s best to plan on using no more that 7/8 of the calculated useable governor lever travel.