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

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  • I thought I had mentioned this in my previous post, but I see that I didn't. What I did this morning does not take into account any valve lash. I set my valve lash for 0.010" after the cams are located and locked in place. In reality, the valve lash means that the cam has to turn a very little bit more than perfect design parameters before the valve starts to lift.
    Brian Rupnow
    Design engineer
    Barrie, Ontario, Canada

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    • The Thumper engine looks like a first rate build! Now that you have a tig welder, You can do as all the top machinists do - Measure twice, cut once, weld, repeat.
      Kansas City area

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      • YES!!!--The engine runs. It had fairly weak compression to begin with, gave a lot of pops and farts, but didn't want to run on it's own. I advanced the timing about 10 degrees, but no joy. Everything was set to "optimum" but it didn't want to keep running on it's own. I have a new (used)1/4 horsepower electric motor that I bought a couple of months ago. I set it up and ran a v-belt from one of the motor pulleys to the offside flywheel, turned on the switch and let the electric motor run the engine. The engine was firing right along with the motor running it. The cylinder got warm and the exhaust was warm, so I just let it run. I knew that with combustion happening, the valves would begin to seal better and the rings would seat. After about 15 minutes I stopped everything, took the belt off, and started the engine the normal way, with my electric drill. I am very pleased, the engine is a runner. Now it's just a matter of taming it a bit to get a good idle and throttle response. Now that I know the engine runs okay, I will sell the plans for $25 Canadian---contact me by email. I haven't been able to get Youtube to work for me for the last month or so, but tomorrow I will get a video of it with my camera and see if my wife can open a new account on Youtube. If she can't I'm going to have to figure out another way of getting a video to post.---Brian
        Brian Rupnow
        Design engineer
        Barrie, Ontario, Canada

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        • . If I have one complaint about Traxxas 4033 carburetors, it's this. If you don't have some over-riding control on the throttle, then as soon as your engine starts it will open the throttle wide open. The only way I have found to get around that is to make up my own throttle lever with a good heavy compression spring at the pivot point and a lever connecting it to the Traxxas throttle lever. That way the engine stays at whatever throttle setting you choose. It is very exciting to have your brand new engine start and immediately rev up to the point where you are afraid it may explode before you get the ignition switch shut off.
          Brian Rupnow
          Design engineer
          Barrie, Ontario, Canada

          Comment


          • That IS a big engine.... sounds as if the HP might need to be measured... maybe an engine performance stand is somewhere on the list?

            We're hoping the YouTube thing gets straightened out. We all want to see and hear!
            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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            • Quick question--What degree of ignition advance do you guys use on single cylinder four cycle engines? I have always set my ignitions up for 10 degrees advance. I am into fine tuning on this engine, and I'm curious. I tried to google it and got 4 billion answers and non of them really answered what I was trying to ask.
              Brian Rupnow
              Design engineer
              Barrie, Ontario, Canada

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              • Okay---I've conducted a poll. Art says 14 degrees. I thought it was 10 degrees. Three small engine shops I called answered 28 to 30 degrees, one guy was very certain that on single cylinder engines with no mechanical advance it was 23 degrees, and a third shop owner said he always set for 25 degrees before top dead center. Unless somebody can offer me up a better solution, i'm going to set my ignition timing up to fire 15 degrees before top dead center.
                Brian Rupnow
                Design engineer
                Barrie, Ontario, Canada

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                • Have you looked at Vimeo for posting your videos? I use YouTube and Vimeo. Vimeo is better quality and has better player window options than YouTube. I have paid subscriptions to both and Vimeo is by far better in my estimation.

                  Here is a link to a video I just grabbed off Facebook. A grill I would like to build. https://vimeo.com/414025318 You can get the details of an account there.
                  Last edited by Black Forest; 05-01-2020, 02:40 PM.
                  Location: The Black Forest in Germany

                  How to become a millionaire: Start out with 10 million and take up machining as a hobby!

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                  • Thank you BlackForest.--I will check out Vimeo.---Brian
                    Brian Rupnow
                    Design engineer
                    Barrie, Ontario, Canada

                    Comment


                    • It's not often that I learn something new which impresses me, but it does happen. I have always set my engines ignition timing by ear and "about 10 degrees advance". This morning I was calling around to small engine repair shops to get a concensus on this timing. I never did get a clearly cut concensus on this, but ended up taking 15 degrees before top dead center as the best time for spark. However, I heard the following method of setting ignition advance, and it just blew me away. The first step is to use either cad or trigonometry to see how far up the piston moves between top dead center and 15 degrees before top dead center. In my case, with a crank throw of 0.687", the piston moves up 0.027". Pull the cylinder head off, and with the piston at top dead center, measure from the top of the cylinder down to the top of the piston. In my case that was 0.106". add the 0.027" to that, and you get 0.133". Make a disc that thickness, slightly smaller than the piston diameter. Set it on top of the piston, then bolt a bar across the top of the cylinder and then turn the crankshaft in the correct direction until the piston presses the disc against the underside of the bar. that is exactly where you want the spark to occur.


                      Brian Rupnow
                      Design engineer
                      Barrie, Ontario, Canada

                      Comment


                      • Remove the flywheel on the side where the ignition points are, set the ignition points for a 0.020" gap, then loosen off the grub screws in the ignition cam. Hook up the battery and sparkplug and while still holding the piston tight against the underside of the bar, rotate the cam in the right direction until you get a good fat spark. Lock the cam grubscrews there, and the engine will be timed to exactly 15 degrees before top dead center. Remove the retaining bar and the disc and bolt the cylinder head and flywheel back in place.

                        Brian Rupnow
                        Design engineer
                        Barrie, Ontario, Canada

                        Comment


                        • Originally posted by brian Rupnow View Post
                          Okay---I've conducted a poll. Art says 14 degrees. I thought it was 10 degrees. Three small engine shops I called answered 28 to 30 degrees, one guy was very certain that on single cylinder engines with no mechanical advance it was 23 degrees, and a third shop owner said he always set for 25 degrees before top dead center. Unless somebody can offer me up a better solution, i'm going to set my ignition timing up to fire 15 degrees before top dead center.
                          The figure that I've always used and heard for small Briggs and Stratton engines is 28-30 ° BTDC. Of course that figure will vary depending on design and operating conditions, but that does seem to be a close approximation, however this figure is not carved in stone. Ease of starting,fuel consumption, and power delivery are all influenced by timing and all of these operational aspects will be a compromise solution on any engine with one preset spark timing for all speeds and conditions.

                          Setting timing on a Briggs engine.

                          http://www.tsracing.com/Manuals/BriggsTiming.aspx


                          Install a top dead center finder tool to the top of the cylinder along with a degree wheel and pointer. Rotate the motor one direction until it stops, read the number on the degree wheel. Rotate the other direction until it stops, read the number. Keep doing this until the number comes up the same, turning both directions until it hits the stop. For more detail on this procedure see - TS Degree Wheel set up.
                          After finding top dead center (TDC) install a bracket over the piston. Rotate the motor back and forth and set your indicator to 0 at TDC. At this time you'll notice that the magnet is almost past the ignition. Back the flywheel up until the right edge of the magnet (not the aluminum) is directly under the point where the right edge of the right leg of the coil meets the flywheel. Look at your degree wheel. It should read 28 - 30 degrees before TDC while the dial indicator should read .195 - .210". Adjust by changing key numbers starting with a #4 key. You may have to file the key to get your desired timing. Retorque the flywheel to 75 foot pounds.

                          Edited to add:
                          I should add that with points a test light or ohm meter should be used to precisely indicate the exact point that the points open and the subsequent point that the spark occurs.




                          Last edited by Willy; 05-01-2020, 03:46 PM.
                          Home, down in the valley behind the Red Angus
                          Bad Decisions Make Good Stories​

                          Location: British Columbia

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                          • I think small engines have rather high spark advance because they typically run at 3000-4000 RPM or so. At very low speed, it seems obvious that the timing will approach 0 degrees and perhaps even need to be retarded past TDC. This is why IMHO it may be necessary to implement some means of adjustable timing advance based on RPM. Personally, I would design an electronic ignition, but for those who are more mechanically inclined, a centrifugal advance mechanism might be easier. The timing in degrees is really a way to achieve the maximum force from the power stroke, and accounts for the delay from the spark to the explosion of the air/fuel mixture. But it also probably has to do with the ideal amount of compression at which to ignite the mixture. Consider that 3600 RPM is 60 revolutions per second, or 16.67 mSec/rev. 36 degrees at that speed is 1.67 mSec.

                            Here is a paper which determines optimum spark advance for an engine operating at 2000 RPM, and it apparently varies from about 15o to 35o BTC, although the figures are hard to read, and it seems to vary in a non-linear manner according to load. That may be why vacuum advance systems may have benefit.

                            Some additional material relating to optimum ignition timing:


                            https://www.sciencedirect.com/topics...g/engine-speed

                            In the wiki entry for "Flame Speed", I found this:
                            A 12:1 compression ratio gasoline engine at 1500 rpm would have a flame speed of about 16.5 m/s
                            So if the combustion chamber is about 50 mm, it would take about 3 mSec to traverse that distance, which would be about 36o ignition advance. Check my math.
                            http://pauleschoen.com/pix/PM08_P76_P54.png
                            Paul , P S Technology, Inc. and MrTibbs
                            USA Maryland 21030

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                            • Comparing Brians engines to commercial 4 stroke engines for iginition timing may not be valid. Brians engines are low rpm, low compression and diffferent fuel ALL of which effect optimal timing. My guess would be Brians engine would require less advance than its commercial brothers. Brians guess of 15 Before TDC is probably a real good guess for such a mild engine as a starting point. Little experimenting later for fine tuning.

                              Looking forward to a video ! Without pics it didn't happen as they say LOL

                              Also looking way ahead yet, the end goal of running that edger is getting very close now.
                              Last edited by Sparky_NY; 05-02-2020, 07:39 AM.

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                              • Hey Brian, misery deserves company !

                                I ordered a tig welder a couple days ago and got the shipping tracking last night ! I have been tempted to get one of these for a couple years now but successfully resisted. This thread no doubt helped put me over the edge, thanks a lot (I think LOL)

                                https://ahpwelds.com/product/ahp-alp...orders-end-may

                                Supposed to be back ordered for about a month so I was shocked to get the shipped notice and tracking info a couple days later !! I had a tig years before but it was a old transformer one, nothing like this one with all the bells and whistles.
                                Last edited by Sparky_NY; 05-02-2020, 07:49 AM.

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