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  • Sounds like a questionable spark plug, that is not firing under compression. a spark tester seems to be in order.
    Why not get a thick piece of lexan or similar clear material, get one of your heads, plug the valves, install lexan window in lieu of cylinder.
    In lieu on one plugged valve install air fitting for compressed air.
    Get ignition going from your starter drill you can see the spark in the window, then apply shop air and determine if spark fails under compressed air

    Compression test: can you use soapy water & run engine with drill starter and get soap bubble around head gasket??

    Comment


    • Originally posted by Willy View Post

      ........
      Without a quantitative value as to compression we are just tossing darts at the target blindfolded and hoping to maybe hit the wall.
      ...................
      Yes, THIS......

      Also known as "bowling under a sheet".

      Without being "critical", which I have no desire nor intent to be, the best way to diagnose these things is to change ONE item at a time. When you change two, or many, your chance at getting any real knowledge is "drastically reduced" (eliminated, really).

      And, of course, when something is known to be questionable, that's the thing to check first. If you cannot check it, then make something that allows you to check it.
      4357 2773 5150 9120 9135 8645 1007 1190 2133 9120 5942

      Keep eye on ball.
      Hashim Khan

      Everything not impossible is compulsory

      "There's no pleasing these serpents"......Lewis Carroll

      Comment


      • We’ve been though all of this with the last engine.
        Time to pull the sail down, come back to the shore and take the time to make some diagnostic tools.

        They would serve you well on the next engine, and the next…..

        Just sayin’

        Sid

        Comment


        • Originally posted by Willy View Post

          Without a quantitative value as to compression we are just tossing darts ..........
          Quantitive what? 5:1, 6:1, 8:1, 11:1 compression? It doesn't matter.
          Brian say it bounce the flywheel back forth at compression, good enough.
          Remember that the old hit n miss engines ran just fine at about 5:1 compression. that aint much but all you need
          Ford model T is reported around 4:1

          Comment


          • Originally posted by Ringo View Post
            Quantitive what? 5:1, 6:1, 8:1, 11:1 compression? It doesn't matter.
            Brian say it bounce the flywheel back forth at compression, good enough.
            Remember that the old hit n miss engines ran just fine at about 5:1 compression. that aint much but all you need
            Ford model T is reported around 4:1
            Yes, very true, however at this point we don't know what the compression pressure is, we're not concerned what the ratio is just the pressure. Rocking the engine back and forth and saying it is okay is purely subjective.
            A method of determining the actual compression pressure is. It is the most basic method used as a diagnostic tool in order to determine the ability of the engine to actually fire on it's own reliably. No sense chasing one's tail if the engine is only capable of producing 45psi of compression.

            I have rebuilt a lot of commercial chainsaws over the last 20 years or so and find that on these engines you can pull till the cows come home at 50psi, 70psi and if the mother of internal combustion is shining on you it may start but it will be an unreliable beast at best.
            90-100psi and it will at least start reliably although it will be weak compared to the 130-160 that it should be at.

            I'm not saying these figures are directly applicable to Brian's engine, however it is the very first thing I check in order to determine which direction to take next.
            Low compression and I won't waste my time fiddling with the other aspects of the engine because no amount of work elsewhere is going to make it a runner.
            Home, down in the valley behind the Red Angus
            Bad Decisions Make Good Stories​

            Location: British Columbia

            Comment


            • Originally posted by Ringo View Post
              Quantitive what? 5:1, 6:1, 8:1, 11:1 compression? It doesn't matter.
              Brian say it bounce the flywheel back forth at compression, good enough.
              Remember that the old hit n miss engines ran just fine at about 5:1 compression. that aint much but all you need
              Ford model T is reported around 4:1
              Do you realize there is a difference between static compression ratio and cranking compression?

              Here’s what Brian actually said.

              Originally posted by brian Rupnow View Post
              Compression isn't real great, but then with these engines I build, the compression generally isn't great until the engine starts and runs for 10 or 15 minutes to seat the valves 100%--then they have crazy compression.
              “isn’t real great” and “crazy compression” means absolutely nothing. There is a tool you can use that will give you some numbers that are useful in diagnostics.
              Last edited by oxford; 12-28-2021, 02:38 PM.

              Comment


              • Maybe problem is with the fuel tank cap. That 1 mm hole might have become plugged. 😉
                Allan Ostling

                Phoenix, Arizona

                Comment


                • Some observations and suggestions (in addition to most of the above):
                  1. Seeing arcs inside the boot indicates a poor connection or poor insulation. If it sparks, it probably will not be jumping the gap in the combustion chamber.
                  2. Try machining away some of the plug housing to better expose the gap
                  3. Make the electrodes as sharp and pointy as possible
                  4. Retard timing past TDC
                  5. Use a Model T type spark coil that provides a continuous series of sparks. This will require a different cam for the ignition points
                  6. Use a solid state ignition system with Hall Effect magnetic flywheel sensor
                  7. Take quantitative measurements of the compression on your best engines and problematic ones like this
                  http://pauleschoen.com/pix/PM08_P76_P54.png
                  Paul , P S Technology, Inc. and MrTibbs
                  USA Maryland 21030

                  Comment


                  • Is the spark red or blue ? If red it has a bad condenser or bad wire. It needs to be blue to run. also check the coil is wired right + to + and - to-

                    Comment


                    • Spark is good. I've used this same ignition set up on all of my engines, and nothing has changed. Right now I'm totally buffaloed on this one. Something is basically wrong here, and I don't know what it is. I have four different spark plugs, which all fire fine laying out on the deck, but won't fire the engine when in place. Even an engine with crappy compression will fire when you're feeding it gas and turning it with a drill. The fact that my engine isn't firing at all is really unusual.
                      Brian Rupnow
                      Design engineer
                      Barrie, Ontario, Canada

                      Comment


                      • Back to the three basics. Fuel mixture, compression and spark. One of those is not correct, or it would have to at least pop.

                        Doggedly going through them is the only answer.

                        The one you are sure has to be correct....so sure that you hardly feel the need to check it, that one is often the one at fault. Seems to be a rule somehow.
                        4357 2773 5150 9120 9135 8645 1007 1190 2133 9120 5942

                        Keep eye on ball.
                        Hashim Khan

                        Everything not impossible is compulsory

                        "There's no pleasing these serpents"......Lewis Carroll

                        Comment


                        • Making a compression tester has been suggested many many times in the past. One more try.... how about a redneck compression tester that you could make in a few minutes? Make a adapter fitting for the spark plug hole (you already have this for standard plug threads but not this one). Take a piece of fuel line hose, steal a check valve from the fuel line of one of your other engines, install it close to the adapter with a piece of fuel line, get a kids balloon and put it on the other end of the line. Spin the engine with your drill and see if it blows up the balloon !

                          Engines are basically air pumps, the redneck compression tester will give you a idea of how well it pumps air (aka compression). You could even do the test quick on a prior running engine to get a feel of what to expect on how fast it blows up the balloon.

                          Other than finding a balloon, it shouldn't take you more than 20 minutes to rig this up. Unhook the actual fuel line during the test and pull the plug wire ! Don't want to fill the balloon with fuel mixture and then have it ignite !!!!

                          Last edited by Sparky_NY; 12-28-2021, 08:01 PM.

                          Comment


                          • It might be possible to rig up a tire valve stem into the spark plug hole and use a tire gauge to read peak pressure. Or get an automotive compression tester and make an adapter for the smaller plugs. But I still suspect ignition.
                            http://pauleschoen.com/pix/PM08_P76_P54.png
                            Paul , P S Technology, Inc. and MrTibbs
                            USA Maryland 21030

                            Comment


                            • Originally posted by PStechPaul View Post
                              It might be possible to rig up a tire valve stem into the spark plug hole and use a tire gauge to read peak pressure. Or get an automotive compression tester and make an adapter for the smaller plugs. But I still suspect ignition.
                              With the tiny displacement of Brian's engines a automotive tester would take forever to pressurize the hose and gauge before you would get a stable reading, way too much volume.

                              You could well be right about it being a ignition issue. Methodical process of elimination is how this sort of thing gets troubleshot, using proven methods. Randomly replacing parts is the worst way. Very similar to electronics, imagine troubleshooting/debugging something without a multimeter or any test equipment!! Experience allows one to take a very good guess at the problem but the next step is to confirm it by tests.

                              My guess is compression, there were obvious issues with the new cylinder which was repaired with a metal ring but no pics of that repair nor the new mating head. Its only a guess without some tests or at least pics.
                              Last edited by Sparky_NY; 12-29-2021, 06:22 AM.

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                              • How about spraying the whole area down with soapy water and looking for bubbles?

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