Just remember to readjust the mixture when you go back to normal idle speeds. Since you are well outside the operating range for that carb, idle mixture becomes very critical. The trick to winning a low idle speed contest is to adjust the idle mixture for that super low idle speed. On a well tuned Onan, you should be able to run the idle speed down low enough to be able to count on each finger each time a cylinder fires. My intuitive sense of the engine speed was wrong. I dont know if this method of connection would work with a battery ignition system.Īnyway, my original concern about RPM, rather than timing, was resolved using the laser tach that you suggested, and I was able to confirm that the rpm reading was accurate. Its just an interesting observation, Im not suggesting it as a normal way to use it. As a reminder, this was the "split" magneto set up, with a separate stator coil and ignition coil, as described in the master shop manual for onans (SandDawg pointed me to the site that has that). I only used this hook up with the rpm feature of the timing light, not for timing, though I suppose it would also work for that. The current pulse through that wire is apparently sufficient to trigger the timing light. Surprising (to me anyway) it also functions well using that arrangement. Of interest, to me at least, I also tried putting the inductive pick up of the timing light on the wire coming from the iginition coil to the points. Anyway, I dont use that feature - I set the desired advance to 0, and line the flywheel mark up with the 19 degree mark on the timing gear cover (for a CCK with no centrifugal advance). So it too would be affected by the waste spark, since it uses that to calculate RPM. But that feature on the timing light must also take into account the engine RPM. My timing light is a digital advance model, meaning if I dial in the desired spark advance, then I can line the flywheel mark up with the TDC mark on the engine and have the proper advance. The quality was good too.īy the way, I was thinking more about the effect of the waste spark on the timing function. The communication was good, shipping was cheap-cheap and did not take as long as one might think. I have ordered one ebay item - some wire brushes for a dremel tool - from a Chinese seller. What I meant was that many of the ebay listings for the tachs are actually shipped directly from China. I wasnt knocking China made products so much. I'll get one from amazon - quick shipping. Any suggestions on what to use? I see a bunch on ebay, some with a laser sensor, some with a contact sensor, and some with both. I would like to check the rpm with something that measures it at the shaft. When the magnets pass the stator, it provides a pulse of current, and the points break just at the right time letting the current flow into the coil (the points actually short the coil out until they open, so its not at all like a battery ignition, which has current flowing through the coil until the points open. It is not the version where the stator and ignition are all in one. You can still buy NOS stator kits and 4 volt coils for this. A I mentioned in a previous post, it has a single stator coil, just one, not a ring of them, with one pair of magnets in the flywheel, and a separate, 4 volt ignition coil. It is in a gravely clean-cut 70, and its rated at 12.9hp. I am not sure that a conventional tach that connects to the points will work on this magneto design, however. The problem is, I dont think it actually is reading twice the rpm - but rather the correct RPM, just based on the way the engine sounds, so I am going to find another way to check it, as one person suggested. But I expected it to read twice the real rpm when on the 4 cycle setting because of the wasted spark design and the fact that it uses an inductive pick up on a spark plug. I knew that the wasted spark design would not affect the timing function. And, it has a 2 and 4 cycle setting for the reasons mentioned in the discussions. Yes, timing lights measure both timing and RPM - mine does.
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