Lawn Tractor Engine Over-Revving

[Mark Sladden]

I have a 2 year old Sears Craftsman 25hp Lawn Tractor fitted with a Briggs &
Stratton Intek V-win engine (Model 445700-0129). Recently the engine will
Rev at full spate if the throttle is anywhere above the idle position (when
I say full spate I mean it Revs way over where it used to even when it was
on full-throttle). If there were a red-line and a tachometer the needle
would be off the scale.

Any ideas as to what is causing this, the throttle cable and mechanism isn't
jammed. Could this be an air leak at the carb? Engine? Something wrong with
the Electronic Ignition?

Thanks for any help

Regards Mark

[Andrew Duane USG]

Likely the governor. Different engines use different designs. On small
lawnmowers and such it is often just a small plastic wind blade that
the engine speed pushes air across, pushing it out. In my case, mice
often packed bedding material in there, wedging the blade and overrevving.

--

Andrew L. Duane (JOT-7) du...@zk3.dec.com
HP/Compaq Corporation Andrew...@compaq.com
110 Spit Brook Road Andrew...@hp.com
M/S ZKO3-3/U14
Nashua, NH 03062-2698 (603)-884-1294

[GTO69RA4]

On most small engine throttles your lever pulls on a spring which is countered
by the governor, which in turn move the actual throttle butterfly. I would
guess that the governor link is broken, or the internal parts are.

GTO(John)

[Bob M.]

Very likely the governor, but unfortunately the mechanism is extremely
complicated on the V-twin, certainly as compared to ALL other B&S engines (I
had one on a generator).

With the engine off, and the throttle at idle, the throttle linkage should
keep the butterfly closed. At faster speeds, it tries to open the butterfly.
The governor tends to close it, using some very strong springs if needed, to
keep it from over-revving. The top speed on most gasoline engines is under
3400 rpm (generators are the primary exception). You might be able to see
the throttle linkages depending on how the motor is mounted. But even with
the manual, it's still difficult to figure out what to adjust.

On the horizontal shaft model, the governor shaft rises up from the rear of
the engine as a 1/4 inch shaft. A link attaches to that. It could have
slipped, loosened, or come off entirely. There's a definite procedure for
adjusting the entire mechanism, best done by following the procedure in a
B&S engine manual. They sell one just for the V-twin engines, or you can
find manuals and/or CDs with manuals on eBay. If you want to fix it yourself
(assuming it's not a governor part that's broken inside the engine), the
manuals are essential, and very inexpensive.

Bob M.
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"GTO69RA4" <gto6...@aol.com> wrote in message
news:20030409171559...@mb-fs.aol.com...

[Mark Sladden]

Thank you all for your responses. I looked at the tractor again last night,
I can see all of linkage clearly (I removed the hood and guard plate). It
doesn't appear that the governor shaft is slipping on the linkage (I
tightened it just to be sure and I can see the end of the shaft turning).
The linkage also appears to be working correctly.

If I start the engine and push governor linkage back to where it sits if the
engine is idling I can control the over revving but as soon as I allow the
throttle spring to do it's work it's back to over revving.

Two things surprised me. The first is that the governor shaft has very
little travel, it only moves somewhere between an 1/8th and a 1/4 of a turn,
that 'seems' like very little movement. The second is that the governor
does not seem to pull back against the spring which is pulling it open
(there is a small plate in the linkage that will push the governor back when
the engine is at idle but there is no real 'pull' coming from the governor
itself). I don't see anywhere that a spring may have broken on the linkage
itself which would be pulling the governor closed (If anyone knows that
there 'should' be one on the linkage please let me know), so I'm guessing
the governor is broken inside the engine.

I am gutted this thing is 2 years old and I'm just not up to splitting an
engine like this so it looks likes I'll have to seek out a service center or
local engine shop. $$$$$ Ouch!

Thank you once again for all of the responses, I at least managed to learn
something.

Regards Mark

[Bob M.]

OK. Without an engine model number, I'm going by memory here.

The governor on the V-Twin engine operates with weights on or near the
camshaft inside the engine. The faster the engine goes, the more the weights
spin outwards. The governor shaft is connected to move as the weights move
in or out. There will be very little shart rotation, probably less than 1/8
of a turn. There shouldn't be any binding on the shaft and you might be able
to turn it fully by hand.

In general, with the engine stopped, the governor would attempt to let the
engine go to full speed, so it might open the throttle plate all the way. As
you found, you can push against the linkage and slow the engine up. The
governor shaft should try to do that for you, and in fact is the only
component that is trying to slow things down. The rest of the throttle
linkage will try to speed things up via springs.

One way that these are often adjusted is to watch the way things move as the
speed changes, then with the engine off, you would loosen the linkage clamp,
rotate the governor shaft in the direction that would give you full speed,
move the linkage in the same direction (full open throttle), then tighten
the clamp. The V-Twin makes it difficult because of all the secondary
linkages.

Again, depending on how soon you want to get it fixed, the B&S manual will
give you all the info you'll need, specific to your engine.

Hope this helps. I can give you more specific info if you provide the full
model and type numbers from the engine.
Bob M.
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"Mark Sladden" <Mark.S...@teknospamx.com> wrote in message
news:Vqila.15$gc2...@paloalto-snr1.gtei.net...

[Mark Sladden]

Bob,

Thank you for your help again. I tried your suggestion ("loosen the linkage

clamp, rotate the governor shaft in the direction that would give you full
speed,

move the linkage in the same direction (full open throttle), then tighten")
but I still can't prevent the over-revving. I have booked it into a local
repair shop, I am now fairly confident that it is not a linkage problem but
an internal governor problem.

The model BTW is 445777 type 0129

Thank again everyone, this was a first trip to this newsgroup and I can see
myself returning regularly, what a helpful place!

Regards Mark

[Bob M.]

I found a parts breakdown for that engine series. Seems there is a pair of
plastic gears inside that spin the governor parts. Should be an easy job to
replace them once you get to them, and it probably requires pulling the
motor and replacing some seals and gaskets in the process.

However, the bigger question is why it broke, if that's where the problem
is? Hopefully the repair shop knows what they're doing and finds something
much simpler, and charges you appropriately. Let us know what they
eventually find wrong.

If you would like, I can send you that parts breakdown as a PDF file (about
800 kb). Let me know what e-mail address you would like it sent to if I
can't figure it out from below.

Bob M.
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"Mark Sladden" <Mark.S...@teknospamx.com> wrote in message

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