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Mower engine speed adjustment

Posted by williaea pa (My Page) on
Wed, Apr 4, 07 at 15:07

Hi,

I was wondering what engine speed I should be running my Briggs and Stratton Quantum 12H802 engine at? Right now it's running at about 3000 RPM which seemed a bit slow to me - it struck me that 3600 RPM was the old standard. Two questions. What speed should I run it at for best mulching performance. How do I go about changing the speed (i.e. where is the governor on this engine).

Thanks

Eric


Follow-Up Postings:

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RE: Mower engine speed adjustment

  • Posted by canguy British Columbia (My Page) on
    Wed, Apr 4, 07 at 15:19

That is close to the correct speed for a 21" mower. This is due to goobmint imposed blade tip speed regulations. The engine will not run properly at 3600 due to the carburetor, not enough fuel and air.


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RE: Mower engine speed adjustment

That's the deal - just go with it.


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RE: Mower engine speed adjustment

Thanks guys. I knew there was a reason I didn't like the government! So they are actually limiting the amount of fuel that is getting through the carbs these days? When did these regulations take effect (i.e. how far out of date am I?)


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RE: Mower engine speed adjustment

I do not know when the regulation that governs lawn mower engine speed went into effect, but it has nothing to do with regulating gas flow. It is, rather, to regulate the tip speed of the cutting blade for safety reasons. The rpm is supposed to be limited to somewhere between 3000 and 3200 rpm. The big controversy has arisen because most engine manufacturers advertise a horsepower reading reached at 3600 rpm, a level never attainable in actual practice. Most walk-behind lawn mower manufacturers have now quit advertising a horsepower rating, opting instead to give a maximum torque reading.


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RE: Mower engine speed adjustment

"So they are actually limiting the amount of fuel that is getting through the carbs these days?"

Actually, it's the amount of air:-). If you can't get enough oxygen, extra fuel doesn't do any good.
Tip speed is limited to something like 17,000 FPM. That means a 21" blade is limited to about 3100 RPM. Now if you have an 18" blade, you can run at 3600 RPM.
I've heard that commercial mowers have a higher limit.

In one sense, you are correct about limiting the amount of fuel. It's called smog compliant. The manfs. tend to run the engines too lean thanks to the govt. That's also the reason a lot of the newer mowers are either full speed or off. No inefficient, polluting idle circuit on the carb.
So much for "warming up" an engine.


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RE: Mower engine speed adjustment

Actually, it's 19000fpm. And the reason is to limit damage done by rocks that might get thrown from under the deck. So a 21" mower could run at 3454rpm and be legal. Mowers are logically set a little lower than this to account for governor "overrun" during real-life operation. But I'd say 3000rpm is too far below the limit and really hurts performance, especially for mulching. Going to 3200rpm or 3300rpm doesn't sound like a big increase, but when you consider that power increases with the square of speed, that 300rpm speed increase (10%) is really a 21% increase in power applied to the grass. That makes a big difference in the air that's being moved under the deck. So _if_ you're competent at resetting the governor, you could bump that up a little more and be safe and legal.

PS - All the bogus Briggs power ratings were so high they could never be attained at _any_ rpm by the real engines. They have been hittin the crack pipe hard over there for a while now. The Japanese engines are a lot closer to reality.

PPS - Pasted in below, for your reading pleasure, is the full ANSI federal regulations for mowers. Have a blast!
----------------------------
e) Power lawnmowers--(1) General requirements. (i) Power lawnmowers
of the walk-behind, riding-rotary, and reel power lawnmowers designed
for sale to the general public shall meet the design specifications in
``American National Standard Safety Specifications for Power
Lawnmowers'' ANSI B71.1-X1968, which is incorporated by reference as
specified in Sec. 1910.6. These specifications do not apply to a walk-
behind mower which has been converted to a riding mower by the addition
of a sulky. Also, these specifications do not apply to flail mowers,
sicklebar mowers, or mowers designed for commercial use.
(ii) All power-driven chains, belts, and gears shall be so
positioned or otherwise guarded to prevent the operator's accidental
contact therewith, during normal starting, mounting, and operation of
the machine.
(iii) A shutoff device shall be provided to stop operation of the
motor or engine. This device shall require manual and intentional
reactivation to restart the motor or engine.
(iv) All positions of the operating controls shall be clearly
identified.
(v) The words, ``Caution. Be sure the operating control(s) is in
neutral before starting the engine,'' or similar wording shall be
clearly visible at an engine starting control point on self-propelled
mowers.
(2) Walk-behind and riding rotary mowers. (i) The mower blade shall
be enclosed except on the bottom and the enclosure shall extend to or
below the lowest cutting point of the blade in the lowest blade
position.
(ii) Guards which must be removed to install a catcher assembly
shall comply with the following:
(a) Warning instructions shall be affixed to the mower near the
opening stating that the mower shall not be used without either the
catcher assembly or the guard in place.

[[Page 668]]

(b) The catcher assembly or the guard shall be shipped and sold as
part of the mower.
(c) The instruction manual shall state that the mower shall not be
used without either the catcher assembly or the guard in place.
(d) The catcher assembly, when properly and completely installed,
shall not create a condition which violates the limits given for the
guarded opening.
(iii) Openings in the blade enclosure, intended for the discharge of
grass, shall be limited to a maximum vertical angle of the opening of
30[deg]. Measurements shall be taken from the lowest blade position.
(iv) The total effective opening area of the grass discharge
opening(s) shall not exceed 1,000 square degrees on units having a width
of cut less than 27\1/2\ inches, or 2,000 square degrees on units having
a width of cut 27\1/2\ inches or over.
(v) The word ``Caution.'' or stronger wording, shall be placed on
the mower at or near each discharge opening.
(vi) [Reserved]
(vii) Blade(s) shall stop rotating from the manufacturer's specified
maximum speed within 15 seconds after declutching, or shutting off
power.
(viii) In a multipiece blade, the means of fastening the cutting
members to the body of the blade or disc shall be so designed that they
will not become worn to a hazardous condition before the cutting members
themselves are worn beyond use.
(ix) The maximum tip speed of any blade shall be 19,000 feet per
minute.
(3) Walk-behind rotary mowers. (i) The horizontal angle of the
opening(s) in the blade enclosure, intended for the discharge of grass,
shall not contact the operator area.
(ii) There shall be one of the following at all openings in the
blade enclosure intended for the discharge of grass:
(a) A minimum unobstructed horizontal distance of 3 inches from the
end of the discharge chute to the blade tip circle.
(b) A rigid bar fastened across the discharge opening, secured to
prevent removal without the use of tools. The bottom of the bar shall be
no higher than the bottom edge of the blade enclosure.
(iii) The highest point(s) of the front of the blade enclosure,
except discharge openings, shall be such that any line extending a
maximum of 15[deg] downward from the horizontal toward the blade shaft
axis (axes) shall not intersect the horizontal plane within the blade
tip circle. The highest point(s) on the blade enclosure front, except
discharge-openings, shall not exceed 1\1/4\ inches above the lowest
cutting point of the blade in the lowest blade position. Mowers with a
swingover handle are to be considered as having no front in the blade
enclosure and therefore shall comply with paragraph (e)(2)(i) of this
section.
(iv) The mower handle shall be fastened to the mower so as to
prevent loss of control by unintentional uncoupling while in operation.
(v) A positive upstop or latch shall be provided for the mower
handle in the normal operating position(s). The upstop shall not be
subject to unintentional disengagement during normal operation of the
mower. The upstop or latch shall not allow the center or the handle
grips to come closer than 17 inches horizontally behind the closest path
of the mower blade(s) unless manually disengaged.
(vi) A swing-over handle, which complies with the above
requirements, will be permitted.
(vii) Wheel drive disengaging controls, except deadman controls,
shall move opposite to the direction of the vehicle motion in order to
disengage the drive. Deadman controls shall automatically interrupt
power to a drive when the operator's actuating force is removed, and may
operate in any direction to disengage the drive.
(4) Riding rotary mowers. (i) The highest point(s) of all openings
in the blade enclosure, front shall be limited by a vertical angle of
opening of 15[deg] and a maximum distance of 1\1/4\ inches above the
lowest cutting point of the blade in the lowest blade position.
(ii) Opening(s) shall be placed so that grass or debris will not
discharge directly toward any part of an operator seated in a normal
operator position.

[[Page 669]]

(iii) There shall be one of the following at all openings in the
blade enclosure intended for the discharge of grass:
(a) A minimum unobstructed horizontal distance of 6 inches from the
end of the discharge chute to the blade tip circle.
(b) A rigid bar fastened across the discharge opening, secured to
prevent removal without the use of tools. The bottom of the bar shall be
no higher than the bottom edge of the blade enclosure.
(iv) Mowers shall be provided with stops to prevent jackknifing or
locking of the steering mechanism.
(v) Vehicle stopping means shall be provided.
(vi) Hand-operated wheel drive disengaging controls shall move
opposite to the direction of vehicle motion in order to disengage the
drive. Foot-operated wheel drive disengaging controls shall be depressed
to disengage the drive. Deadman controls, both hand and foot operated,
shall automatically interrupt power to a drive when the operator's
actuating force is removed, and may operate in any direction to
disengage the drive.

[39 FR 23502, June 27, 1974, as amended at 43 FR 49750, Oct. 24, 1978;
49 FR 5323, Feb. 10, 1984; 50 FR 4649, Feb. 1, 1985; 61 FR 9240, Mar. 7,
1996]


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RE: Mower engine speed adjustment

Well, I'll be...it IS 3456 RPM (21"). How did 3200 get 'institutionalized' here? I know we've all been 'parroting' it for years. So, to the original post, yes, 3000 is a little slow, but still in the operational ballpark. Most lawn mower governors are adjustable enough to pick up a few hundred RPM. Go to B&S and see if you can download an engine manual. I guess I'll have to get out the fancy laser tach I bought and check out my mowers with an eye toward setting them a little higher. My 10550 is already high, at least it sounds like it after I adjusted the governor for smoother running - I put four clicks on it which should be about 250 additional RPM. This tach, available at Harbor Freight, requires a reflective tape spot be applied to a rotating surface. All Hondas with GCV should be checked because they really seem to run slow. All Duraforce engines seem to be extremely slow from the factory.


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RE: Mower engine speed adjustment

Oh, I guarantee that his motor can be adjusted to run faster than 3456rpm. Just don't ask me how I know...


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RE: Mower engine speed adjustment

Matt,

So it's your mower that's making the screaming sound I hear coming from the direction of Houston? :-)

I'll play with the governor once it gets up into the 50's again in PA. We are supposed to have snow flurries tomorrow!

Eric


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RE: Mower engine speed adjustment

The screaming is coming from Greater St. Louis now. I'll have to change that.


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RE: Mower engine speed adjustment

I have a 4hp 22" Briggs and Stratton lawn mower. It runs good and has had proper maintenance. Economically it is good using about 4 cups of fuel in an hour. But at 4hp it is not the best for mulching. So how can I legally increase the torque of the engine?


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RE: Mower engine speed adjustment

Is it old enough to have a mixture adjustment? If not, all you can reasonably do is speed up the engine which will make more power. If you can slightly enrich the mixture it will also help make more power, but if you don't have a needle valve you'd have to enlarge the main jet.


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RE: Mower engine speed adjustment

I'd like to speed up my new Lawn-Boy commercial, but the max speed of the self-propel is good as-is. If I make the engine run faster, I'll have to feather the Variable Speed bail all the time. So, ground speed on self-propelled mowers figures into the engine RPM deal.


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RE: Mower engine speed adjustment

My dad has a 2002 Craftsman 22" mower/mulcher, and to my ears it sounded slower than the 2900 rpm minimum called for in the owner's manual. When measured with TinyTach the reading came back as 2750 rpm.

Maximum blade tip speed of 3299 rpm would be translated as 2804 rpm at 85% efficiency, 2969 rpm at 90% efficiency and 3134 rpm at 95% efficiency. Which of the three would work best?

Thank you in advance,


Ben Edge


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RE: Mower engine speed adjustment

Relating % of max blade speed to efficiency??? I don't think so- Efficiency of WHAT?

Use the max speed. The faster the engine is running, the more kinetic energy you have stored in the rotating parts (handy when heading through that real thick but short patch of grass where you accidentally spilled the fertilizer) plus the engine is putting out more HP than at the lower speeds. The faster blade is simply going to cut/much better.


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RE: Mower engine speed adjustment

My point is that, with respect to keeping the maximum blade tip speed at 19,000 fpm, which is not to be exceeded.

However, there is a recommendation I saw some time ago that the minimum blade tip speed of a lawn mower should be no lower than 14,000 fpm.


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RE: Mower engine speed adjustment

Just to toss in something that everyone knows and is never overlooked - a sharp blade helps.


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RE: Mower engine speed adjustment

'Just to toss in something that everyone knows and is never overlooked - a sharp blade helps'

Very funny! Even those who will check the oil every year or so and maybe even change it maybe once in the life of the mower (a short life) practically never take the blade off and sharpen/replace it. I have seen plenty of dangerously thin/cracked blades. That's one good thing about many aftermarket 'Gator'-type blades - they're really thick and usually outlast the mower they're on. I have five blades that I rotate on my two Lawn-Boy 'Insight' mowers that take the same blade. I keep three sharp and just take the dull one off and replace it, minimizing down-time. Then I have to remember to sharpen the take-off and balance it when I get around to it.


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RE: Mower engine speed adjustment

For those of you who want to know how fast to run an engine in relation to the length of the blade, allow me to post these calculations:

ANSI B71.1-1964 and earlier standards:
For 18" blade: 4456 (4250) rpm max
19" blade: 4222 (4000) rpm max
20" blade: 4011 (3800) rpm max
21" blade: 3820 (3600) rpm max
22" blade: 3646 (3450) rpm max

ANSI B71.1-1968 standard and revisions thereafter:
For 18" blade: 4032 (3800) rpm max
19" blade: 3820 (3600) rpm max
20" blade: 3629 (3400) rpm max
21" blade: 3456 (3250) rpm max
22" blade: 3299 (3100) rpm max

Maximum speed ranges which have at least a 200 rpm subtraction from the real limit, as represented in parentheses, may be used with respect to fluctuations in readings on your tachometers.

Maximum recommended operating speeds may vary by engine model. Many Tecumseh 4-cycle rotary mower engines have a maximum recommended speed of 3200 rpm, whereas maximum torque may occur either at this point or at either 2600 or 2900 rpm.

~Ben


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