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Thumper---A new 1 3/8" bore i.c. engine

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  • Originally posted by brian Rupnow View Post
    I was asked yesterday what rpm the engine was running at in the video. I checked it this morning with the engine driving the clutch. It seems very comfortable at 975 to 1000 rpm. If I turn the idle screw to let it run slower, it stumbles and stalls.
    Thanks for taking the time to do this Brian, much appreciated.
    The reason I ask was to compare comfortable idle speeds with various small engines I have around the yard, that while small, are still much larger in displacement than your engine is.
    I checked two of the smallest, both 5 HP engines, one GX OHV pushrod Honda the other a flathead B&S.
    On both high idle is governed at 3,200-3,600 rpm while a comfortable low idle is set at 1,500-1,800 rpm. I don't think they would idle comfortably any lower.

    Personally I think the speed you are getting you engine to idle at is very good.
    Plus, unlike the Honda or B&S, yours is oh so much nicer. Well done sir!

    Home, down in the valley behind the Red Angus
    Bad Decisions Make Good Stories​

    Location: British Columbia

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    • Thanks Willy---I've spent much of today "dialing and tweaking" to see how slow I could get the engine to idle. It idles at about 950 to 1000 rpm with no load at all on the engine, and idles at 800 rpm quite happily with the load of a driven clutch on it. If I try for anything lower than that, it just gives up and stalls.
      https://www.youtube.com/watch?v=yktU...ature=youtu.be
      Brian Rupnow
      Design engineer
      Barrie, Ontario, Canada

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      • Originally posted by brian Rupnow View Post
        Thanks Willy---I've spent much of today "dialing and tweaking" to see how slow I could get the engine to idle. It idles at about 950 to 1000 rpm with no load at all on the engine, and idles at 800 rpm quite happily with the load of a driven clutch on it. If I try for anything lower than that, it just gives up and stalls.
        https://www.youtube.com/watch?v=yktU...ature=youtu.be
        What is the upper rpm range? Curious what sort of operating range you get.

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        • Sparky--I have never tested any of my i.c. engines to determine the upper limit of rpm's. I don't think it would be much over 3000 rpm., based on the cam design. I also have a reasonable fear of them flying apart in my face. I'm always interested in how low an rpm they can reach without stalling, and any that I have had working my various machines never exceeded 2000 rpm.
          Brian Rupnow
          Design engineer
          Barrie, Ontario, Canada

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          • Nice job Brian! You do a magnificent job, and appear to have fun. Been working engines all my life. Been in the service business for 30+ years. From my view point the ability for engines idle a slow speeds went away with the
            carbs that had a idle mixture adjustment. Those old one lungers would idle down real slow if you knew how to tune
            the carb settings.
            olf20 / Bob

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            • So---What was the outcome of this exercise? Not what I would have liked, but I will explain. "Thumper", with it's larger bore is a lot more powerful than any of the other 1" bore engines that I tried to run the sawmill edger with. I can easily start the engine with the disengaged clutch driven by three rubber o-rings. With no o-rings in place, the engine idles at around 1000 rpm. With the load of the o-rings driving the clutch (while it is disengaged), the idle rpm's drop to about 800 rpm. However, the edger is designed to have an input of 2000 rpm driving the 16:1 gear reducer. This should turn the saw-blades at 1000 rpm, and turn the infeed rolls at a lower rpm so as not to over-run the saw-blades. I adjusted the engine to run at +/- 2000 rpm, and engaged the clutch. The engine took it all in stride, and didn't stall. There was a noticeable drop in engine rpm, but it drove the edger mechanism without any noticeable bogging. However, when I went to feed a board through the edger, the board would get about half way thru the saws, and then stall the engine. I repeated this about 10 times with different settings and adjustments, but it became plain that the new engine simply didn't have enough power. I have a theory, but at this time it is only a theory. With the ignition set to fire at 15 degrees before top dead center, the engine starts and idles very smooth, and revs up very well in response to opening the throttle. I have the feeling that at 2000 rpm, the timing is too slow for that rpm. The engine doesn't have any mechanism to advance the spark timing with changes in engine rpm. I believe that if I were to modify the engine so that as the throttle opened the ignition timing would advance automatically, the engine would be much more powerful at the 2000 rpm it is being asked for. I have decided to back away from this project for a while and do something else. I will probably come back to this and redesign the engine to give it an automatic spark advance, but for now I'm burned out on it.
              Brian Rupnow
              Design engineer
              Barrie, Ontario, Canada

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              • I suppose it could not be something like a small misalignment that causes the board to actually jam at that point?

                Easily checked. Could explain a lot, as the load should not change much once the saw is fully in the wood.
                CNC machines only go through the motions.

                Ideas expressed may be mine, or from anyone else in the universe.
                Not responsible for clerical errors. Or those made by lay people either.
                Number formats and units may be chosen at random depending on what day it is.
                I reserve the right to use a number system with any integer base without prior notice.
                Generalizations are understood to be "often" true, but not true in every case.

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                • Brian, a relatively simple way to prove the problem is to rig a manual advance for the ignition. I drove a farm tractor years ago with manual advance via a lever on the steering column. Postions were marked for starting and running with continuous adjustment slow to fast.
                  Southwest Utah

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                  • You might also need to figure out a way to adjust the air/fuel mixture ratio, making it a bit richer at higher speed and load. That is usually accomplished by carburetor design, which may have separate low speed (idle) and high speed jets. The throttle plate may be designed so as to "hide" the high speed jet when mostly closed at idle, and then expose the larger high speed jet when the throttle is opened further for power. Here are some links I found relating to air/fuel mixture and performance:

                    https://x-engineer.org/automotive-en...e-performance/

                    From Quora:



                    Jason Spenny, 10+ years in the auto industry Thermal systems Engineer.
                    Answered Apr 1, 2017

                    It depends on the engine but I'll give ya a couple reasons why that happens.
                    1. Cooling. Fuel is liquid. If you squirt more fuel into the cylinder than will actually burn, the unburned fuel has to vaporize under the heat, which absorbs heat. This prevents overheating under max load.
                    2. Peak power actually occurs just rich of stoichiometric due to the dissociation of CO2 into CO and extra O2
                    3. Fuel distribution is not always homogeneous which means some areas could be locally rich leaving other areas light on fuel, so you can't develop peak power. Some calibrations dump extra fuel to avoid this condition.
                    4. In carburetor engines, you don't have a computer (typically) controlling fuel timing so the tune is set. In order to run nice at some point, the result might be rich at WOT as a compromise for mid power smoothness, or soft idle.
                    5. Spark advance. If you want to advance your spark, you can get more power but at the risk of knocking or pinging. A bit extra fuel cools the cylinder and would reduce that risk.
                    These are the best I can think of









                    http://pauleschoen.com/pix/PM08_P76_P54.png
                    Paul , P S Technology, Inc. and MrTibbs
                    USA Maryland 21030

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                    • Along with ignition timing, wouldn't hurt to invesrtigate valve timing also. A common small briggs and stratton would be a good data point, they max out at 3600 rpm so valve timing should be representative. I think Brian uses very similar valve timing for all his engines, none of which are optimized for power.

                      Paul: Brian is using a commercial RC carb, they have provisions for mixture control idle thru high speed and work very well. Worth noting that on RC planes you adjust the main needle at full throttle which is also max load due to the prop, then set the idle air bleed. Don't know if Brian attempted setting the main needle at high rpm and loaded, if not it may be leaning out with the edger.

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                      • Brian I know you are perfectly capable of adding manual advance and retard and I am just old enough to have driven a couple of old cars with the manual control, tractors too.

                        In the case of one tractor (green era Massey-Harris Pacemaker) the procedure was to put the tractor under load then advance the spark until the engine pinked then a small amount of water (steam?) was added to the induction mixture. The cars (1930 era American De Soto etc) had manual spark control, full retard for cranking (manual or electric) then advanced for normal running, when faced with a steep hill if the engine pinked the spark was retarded until you are forced to change down a gear.**

                        I guess you do not want to futz with the steam generator but at least the ignition timing control may be worth a try.

                        **BTW changing down a gear on a hill required a lot of skill to double the clutch, change the gear and handle the throttle all the while hoping you were not forced to stop and then be faced with a long slow climb in low gear!

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                        • Manual ignition advance is nothing new to me. My first three cars were all model A Fords with two levers right behind the steering wheel. One was for throttle and the other was for advancing the ignition.---And woe betide you if you forgot to retard the ignition and tried to crank start the engine.--Many wrists were broken from that.
                          Brian Rupnow
                          Design engineer
                          Barrie, Ontario, Canada

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                          • Originally posted by brian Rupnow View Post
                            Manual ignition advance is nothing new to me. My first three cars were all model A Fords with two levers right behind the steering wheel. One was for throttle and the other was for advancing the ignition.---And woe betide you if you forgot to retard the ignition and tried to crank start the engine.--Many wrists were broken from that.
                            Brian I was not suggesting the steering wheel levers would be new to you but we may have younger readers, so sorry about that.

                            Re the broken wrists I recall keeping the thumb out of the way was part of the first driving lessons!

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                            • On the old Harleys you had to reach down and turn the magneto by hand after starting. If the plug wires were faulty you would find out pretty quick.
                              25 miles north of Buffalo NY, USA

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                              • Originally posted by nickel-city-fab View Post
                                On the old Harleys you had to reach down and turn the magneto by hand after starting. If the plug wires were faulty you would find out pretty quick.
                                How old are you talking about?

                                My first bike was a Harley (it was all I could afford) - 1936 EL (61 CI). It had a battery/coil ignition and the spark retard/advance was on the left twistgrip. A couple of times I forgot to retard the spark when starting and the beast threw me over the bars. Or nearly. I learned.

                                Complicating the issue was that my friend had an Indian Chief (80 in. flathead) and we'd occasionally swap bikes. His had the throttle on the left side and the spark on the right. Easy to get crossed up...

                                -js
                                There are no stupid questions. But there are lots of stupid answers. This is the internet.

                                Location: SF Bay Area

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