Ok....
Normally our pumps have a physical governed limit of roughly 2500-2550
RPM. That is the max upper limit that the engine will turn over under
wide open throttle. This means that in "N" or "P" that with the
throttle pedal on the floor, the engine will rev to 2500 or so and
that's it, no matter how long you hold the pedal down, it won't climb
any higher.
This limit is set using the "high idle" screw or governor screw. Under
normal conditions the governed speed is preceeded by the defueling
stage. By that I mean that around 2350-2400 RPM the injection pump
starts to cut back the fuel volume or begins "defueling" as it's called.
This defueling happens due to one of two reason, or both reasons:They
are > the engine RPM reaches a point were the governor conterweights
open up and force the pump to start decreasing fuel amount..
and/or....The amount of Load on the engine decreases enough that the
pump doesn't use/need as much throttle pedal to maintain speed.
One or both can kick in and cause the pump to defuel. Either way, the
max speed is still set by the physical "high idle" screw setting, but
the defuel point varies depending on the above factors. The high idle
screw can be backed out to allow for a max RPM of as much as 3300-3500
RPM. Obviously that's a bit too high but it makes a point here....If
you set your high idle or governed speed to, say....3000 RPM, the two
defuel factors still exist. If the RPM climbs too high OR the load
decreases OR both, the pump begins to defuel. In very general terms the
defuel point usually starts roughly 200-300 RPM before max governed
speed/RPM.
Now, the governor spring really does a couple of things, but primarily it does the following....
Lets use the 3000 RPM limit or governed speed again, as an example.
Under normal conditions you would start to see defueling at about 2700
RPM lets say.....With the governor spring in place that onset of defuel
does not happen like normal. It holds the same amount of fueling you
are demanding right up to the governed speed, then it finally looses
the battle between the RPM and Load governor circuits and begins to
defuel. With the governor spring in place and with that type of
behaviour the pump can actually allow or permit an RPM "flare" above
the 3000 RPM limit. This happens because instead of the fueling level
starting to ease back prior to your governed limit (3000
RPM) the heavier fueling continues right up to or slightly beyond at
physical governed limit of 3000 RPM. This can result in an RPM flare
above 3000. I have seen a flare as high as 3700 RPM before the RPM/Load
governor systems pull the fuel back drastically enough to stop the
flare. It will finally settle back to the
3000 RPM level once this happens..... so.....In a nutshell you should
do some RPM testing first. With the engine warmed up, and good oil
pressure, slowly but firmly press the pedal to the floor. You would do
this in Neutral or Park of course. This is the actual proper method of
checking your governed speed as per the manual.
Make note of your top/max RPM. Ease out of it and let the idle come
back down,... then repeat. You could have a consistent high idle of
roughly 2500-2500 RPM. If that is the case the governor spring will
allow heavy fueling (depending on the amount of gas pedal) right up to
that RPM. If you back the governor scew out two to three turns and
repeat you will see you governed speed (high idle) rise higher. Repeat
until it's where you want it before installing the governor spring. I'd
suggest 2900 RPM to start with and you can go from there depending on
how the engine behaves after the install.
