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Basic rpm question
#1
J
JBtoro
J
JBtoro
Bought a used Toro Personal Pace with B&S 6.75 with auto choke. It also has the blade brake. Essentially it runs cosistent, starts okay, no sputtering, etc. I set the rpms under no-load with a Tiny Tach at 2900 +/- 100 per Toro's web site and a B&S email response (FWIW, others here and elsewhere say it should be 3200). When I engage the blade the rpm's drop a few hundred an then go down to 2500-2600 when mowing through medium high grass. The engine sounds anemic at those speeds. Thus, to get the engine to spin at 2900 under mowing load, I have to bend the governor tang so that the no-load is around 3400-3600 which is considerably over the spec that the manufactures gave me (and maybe damaging to the engine?). Question is this: Are the rpms supposed to decline by a few hundred under load or is the throttle, for example, supposed to open more to compensate for the load and thus maintain the rpm that was set under no-load? Thanks from a new member.
#2
I
ILENGINE
I
ILENGINE
Three things here. You will not damage the engine running at 3400-3600 rpm because the engine is designed to operate at that speed. The blade manufactures have determined that the maximum safe speed for the blade is having a tip speed of 19000 foot per minute. So the longer the blade the slower the engine needs to turn to avoid exceeding that safety zone. What you are experiencing with the rpm drop is called governor droop. It is normal and some engines droop more than others. Generators are normally set to operate at 3750 to compensate for the 150 rpm, but some of the electronic governor systems may only have a 50 rpm drop.
Pushmower engines normally have a droop around 250-300
Pushmower engines normally have a droop around 250-300
#3
J
JBtoro
ILEngine, thanks so much. I have searched and searched and could not find any explanation of the rpm drop-off; aka "drrop," Neither Toro nor Briggs would even discuss it ("Take it to a dealer".) Regarding the 3400-3600 speed, why do you think Toro & Briggs are specifying 2900 +/-? Is it for some perceived or maybe gov't mandated safety reason regarding blade tip speed? Here is the e-mail response I got from Briggs, ""Thank you for your inquiry. The ENG NO-LOAD SPEED is 2900 +/-100 RPM. This engine is a fixed speed engine."
J
JBtoro
Three things here. You will not damage the engine running at 3400-3600 rpm because the engine is designed to operate at that speed. The blade manufactures have determined that the maximum safe speed for the blade is having a tip speed of 19000 foot per minute. So the longer the blade the slower the engine needs to turn to avoid exceeding that safety zone. What you are experiencing with the rpm drop is called governor droop. It is normal and some engines droop more than others. Generators are normally set to operate at 3750 to compensate for the 150 rpm, but some of the electronic governor systems may only have a 50 rpm drop.
Pushmower engines normally have a droop around 250-300
ILEngine, thanks so much. I have searched and searched and could not find any explanation of the rpm drop-off; aka "drrop," Neither Toro nor Briggs would even discuss it ("Take it to a dealer".) Regarding the 3400-3600 speed, why do you think Toro & Briggs are specifying 2900 +/-? Is it for some perceived or maybe gov't mandated safety reason regarding blade tip speed? Here is the e-mail response I got from Briggs, ""Thank you for your inquiry. The ENG NO-LOAD SPEED is 2900 +/-100 RPM. This engine is a fixed speed engine."
#4
I
ILENGINE
It comes from say Toro calling up Briggs and said we need to purchase a train car of 4.75 lb/ft engines with this crankshaft length and diameter to use with a 22 inch blade and Briggs then sets the rpm's to match the safety speed for that blade before shipping the engines to Toro which they then attach to their deck when they assembly the mower and don't have to start and set the governed speed themselves.
I
ILENGINE
ILEngine, thanks so much. I have searched and searched and could not find any explanation of the rpm drop-off; aka "drrop," Neither Toro nor Briggs would even discuss it ("Take it to a dealer".) Regarding the 3400-3600 speed, why do you think Toro & Briggs are specifying 2900 +/-? Is it for some perceived or maybe gov't mandated safety reason regarding blade tip speed? Here is the e-mail response I got from Briggs, ""Thank you for your inquiry. The ENG NO-LOAD SPEED is 2900 +/-100 RPM. This engine is a fixed speed engine."
It comes from say Toro calling up Briggs and said we need to purchase a train car of 4.75 lb/ft engines with this crankshaft length and diameter to use with a 22 inch blade and Briggs then sets the rpm's to match the safety speed for that blade before shipping the engines to Toro which they then attach to their deck when they assembly the mower and don't have to start and set the governed speed themselves.
#5
B
bertsmobile1
B
bertsmobile1
Ilengine has it in a nut shell.
When the only fully locally made push mower were researching noise reduction they found a lot came from the blades so they put a larger diameter blade carrier on it and dropped the engine speed from 3700 +/- 200 rpm to 3200 +/- 100 rpm .This knocked 9dBa off the noise level and the dB scale is logerithmic so that is nearly a 1/3 reduction.
However they forgot to update the engine handbook that came with the mower so every one was sending them in for repair because they were not going fast enough.
When the only fully locally made push mower were researching noise reduction they found a lot came from the blades so they put a larger diameter blade carrier on it and dropped the engine speed from 3700 +/- 200 rpm to 3200 +/- 100 rpm .This knocked 9dBa off the noise level and the dB scale is logerithmic so that is nearly a 1/3 reduction.
However they forgot to update the engine handbook that came with the mower so every one was sending them in for repair because they were not going fast enough.
#6
J
JBtoro
Thanks, mate. I received more good info in 2 replies here than I was able to get from hours of searching and wasted phone calls to Toro & Briggs. Wish I had known about this great site from the get-go.
J
JBtoro
Ilengine has it in a nut shell.
When the only fully locally made push mower were researching noise reduction they found a lot came from the blades so they put a larger diameter blade carrier on it and dropped the engine speed from 3700 +/- 200 rpm to 3200 +/- 100 rpm .This knocked 9dBa off the noise level and the dB scale is logerithmic so that is nearly a 1/3 reduction.
However they forgot to update the engine handbook that came with the mower so every one was sending them in for repair because they were not going fast enough.
Thanks, mate. I received more good info in 2 replies here than I was able to get from hours of searching and wasted phone calls to Toro & Briggs. Wish I had known about this great site from the get-go.
#7
J
JBtoro
J
JBtoro
Back again and more confused than before. I decided to set the no-load speed high enough by bending the tang so that the engine would "droop" down to about 3200 rpm. Maybe that idea is reasonable in theory but here's what actually happened.
First I warmed up to NOT. Set the no-load to about 3480 =/-100, sounded good there. Engaged the blade, rpm dropped to about 3180. So far so good. Engaged Personal Pace and began moving forward, rpm dropped to 2720 +/- 100. Sounded "just okay" but kind of wimpy. Pulled back on (disengaged) Personal Pace but kept blade engaged, rpm returned to 3100-3200 range. Disengaged the blade, back to 3400-3500 range.
Thus, I surmise that there are 3 factors affecting the initial no-load rpms of a Toro Personal Pace mower (or other mower brands with an on-the-go variable speed feature): 1) blade engagement, 2) Personal Pace engagement of the rear drive wheels, and grass thickness (I couldn't really test this because my grass is not very high right now). Of these 3, PPace was the most significant.
Any thoughts, guys? Should a feature like PPace cause such a drag? Does that feature need to be adjusted? It seems to work good in as far as propelling goes.
First I warmed up to NOT. Set the no-load to about 3480 =/-100, sounded good there. Engaged the blade, rpm dropped to about 3180. So far so good. Engaged Personal Pace and began moving forward, rpm dropped to 2720 +/- 100. Sounded "just okay" but kind of wimpy. Pulled back on (disengaged) Personal Pace but kept blade engaged, rpm returned to 3100-3200 range. Disengaged the blade, back to 3400-3500 range.
Thus, I surmise that there are 3 factors affecting the initial no-load rpms of a Toro Personal Pace mower (or other mower brands with an on-the-go variable speed feature): 1) blade engagement, 2) Personal Pace engagement of the rear drive wheels, and grass thickness (I couldn't really test this because my grass is not very high right now). Of these 3, PPace was the most significant.
Any thoughts, guys? Should a feature like PPace cause such a drag? Does that feature need to be adjusted? It seems to work good in as far as propelling goes.
#8
B
bertsmobile1
B
bertsmobile1
With drops like that either you have serious problems with that engine reducing the power output.
Or here is too much load being applied to the engine
or the govenor is not working.
When you engege the blades and the race have some one look close at the carb to verify that the govenor is allowing the throttle to open further when the engine slows down
Or here is too much load being applied to the engine
or the govenor is not working.
When you engege the blades and the race have some one look close at the carb to verify that the govenor is allowing the throttle to open further when the engine slows down
#9
J
JBtoro
This Briggs engine has the auto choke feature, which as I understand it, has a thermostat that heats up & moves (opens) a vane that is connected to the throttle plate. Thus, when the engine is at NOT, the throttle should be open all the way. That's what has me puzzled, I believe this engine is designed to be wide open or fully closed; there are to throttle adjustments like on my 15.5 Briggs lawn tractor engine.
J
JBtoro
When you engege the blades and the race have some one look close at the carb to verify that the govenor is allowing the throttle to open further when the engine slows down
This Briggs engine has the auto choke feature, which as I understand it, has a thermostat that heats up & moves (opens) a vane that is connected to the throttle plate. Thus, when the engine is at NOT, the throttle should be open all the way. That's what has me puzzled, I believe this engine is designed to be wide open or fully closed; there are to throttle adjustments like on my 15.5 Briggs lawn tractor engine.
#10
B
bertsmobile1
B
bertsmobile1
The throttle and the choke are 2 different things.
choke is the butterfly at the front of the carb
The throttle is the butterfly at the back of the carb ( nearer to the engine )
choke is the butterfly at the front of the carb
The throttle is the butterfly at the back of the carb ( nearer to the engine )
#11
J
JBtoro
Yes, thanks for the clarification; I knew that but got my terms twisted. But my confusion persists. There is no throttle control on this mower. Doesn't that mean it is always at full throttle? The only way to adjust engine speed that I know of is by adjusting the length of the governor spring (bending the tang), which I have done. But, it drops from that adjustment under blade engagement and then more after PPace is engaged.
Update: My neighbor has a newer Toro PPace. His rpm can't be measured no-load because he doesn't have blade brake. Thus, he must engage the blades prior to pulling the starter rope. With my tach, his measured 2880 with blade engagement and 2880 after PPace; no noticeable change in engine sound.. Something (carb maybe; more gas?) kicked in to hold his rpm speed rock steady even after adding the load from rear wheel propulsion. But what?
J
JBtoro
The throttle and the choke are 2 different things.
choke is the butterfly at the front of the carb
The throttle is the butterfly at the back of the carb ( nearer to the engine )
Yes, thanks for the clarification; I knew that but got my terms twisted. But my confusion persists. There is no throttle control on this mower. Doesn't that mean it is always at full throttle? The only way to adjust engine speed that I know of is by adjusting the length of the governor spring (bending the tang), which I have done. But, it drops from that adjustment under blade engagement and then more after PPace is engaged.
Update: My neighbor has a newer Toro PPace. His rpm can't be measured no-load because he doesn't have blade brake. Thus, he must engage the blades prior to pulling the starter rope. With my tach, his measured 2880 with blade engagement and 2880 after PPace; no noticeable change in engine sound.. Something (carb maybe; more gas?) kicked in to hold his rpm speed rock steady even after adding the load from rear wheel propulsion. But what?