There is no set easy answer. the speed can vary due to different uses, and the same engine used on different equipment can have different speed setting. 2 cycle items can very by as much as a few thousand rpm on different pieces of equipment and will be different depending on brand.
There is no magic number which is why you did not find one.
Small engines are governed to around 3600rpm which is the world standard for small PTO's
Eery country has a maximum blade tip speed limit for mowers and this is reducing all the time .
Micro engines ( chain saws & the like ) have no standard.
Ideally you set the high running speed just past the maximum torque for that particular engine so as it gets loaded up & runs slower there is more power available to do the work at hand.
low speed is set by the emissions permissible by the EPA at idle.
On hand helds it must be low enough to dissengage the clutches and this can be a chicken & egg situation.
#4
sgkent
coming back to my thread. Have two items I will be tuning.
One is a 3.8 HP Tecumseh engine on a 3 blade edger Engine model is probably a Craftsman part # 143.963515. Looks like the peak RPM is 3600 from what I could find here and there. The general Tecumseh manual says to set engine speed down 600 RPM below max so the throttle plate isn't fully open. From what I can tell from general reading, max torque on an engine like this comes about 2600 RPM. So If I set the engine to 3000 RPM unloaded would that be about right, and idle RPM falls to where ever the lever drops it to?
Second is a 5hp tiller. Briggs Engine 135202, Type No. 0119-01 The only spec I can find on RPM reads, "To adjust IDLE RPM, rotate throttle linkage counter-clockwise and hold against stop while adjusting idle speed adjusting screw to obtain 1750 RPM. Release throttle linkage." Then it goes on to say that the high speed screw is factory adjusted, but it doesn't give a guideline to see if the engine is still in spec. Any suggestions?
Going to vary a bit depending upon the condition of the engine & the equipment it is powering .
Ideally it would run just past the peak torque RPM
This is because when the engine is under load & slows down a little the available power ( torque) increases to overcome the extra load .
So you will need to do some testing
The running speed needs to be high enough that when a heavy load is applied the revs do not drop below peak torque and that will depend upon the inertia which is basically the weight of what ever is being spun.
So I really could not give you a magic number .
The factory specs are where you start, then try a little faster & a little slower to see where your machine works best for the purpose you are using it for.
Eg
A rough cut mower will often spin slower than a fine cut mower .
#6
sgkent
so where do I get factory specs for a tiller and an edger? The manuals and Google do not reveal specific numbers, only numbers from general discussions.
The edger will be the same as a lawnmower 3000 to 3600
Tiller probably a bit slower 2700 to 3000
They would have been in most owners manuals , any idea where yours is ?
The bigger problem is over reving as the con rods are not strong enough to handle 5000 rpm and the splash lubrication system breaks down at higher speeds .
#8
sgkent
The manuals are sitting next to me. I went thru each one and they give things like the typical parts blowup, parts numbers, how to operate, how to set the mixture etc., but the highspeed RPM is not there. These are all 25 years - 28 years old, have had numerous carb rebuilds over the years, and I should check the RPM to be sure it is still close to spec.
#9
sgkent
Not a mower. It is for a 26" front tine rototiller, and a three blade lawn edger. I know what the RPM should be on the mower.
Bert said your edger would be the SAME as a mower. The numbers he posted are the ones I would use, as he also said, NO MAGIC NUMBERS. Very uncommon to find RPM specs for the older engines.
#12
sgkent
Thanks. So a 21" mower blade is 21" x 3.1416 = 65.97" \ 12" = 5.50 feet per revolution. I have read that the suggested current tip blade speed is 215 mph which is 18,920 feet per minute. 18,920 feet per minute \ 5.50 feet per revolution = 3440 revolutions per minute unloaded at high speed. That is really close to the 3600 max RPM so it would be hard on engine life, However Toro customer service specified for my 21" mower (Honda / Toro) to set the engine RPM to 3000. I guess I will set everything to 3000 unloaded and the tiller to 2700 RPM fast / 1750 slow. Thanks everyone. That pretty much agrees with what everyone has said also.
