I think I disagree that it's not ok. Linda said she has a barely perceptible drop (which is what I get with my C-90 in a 415C). But then you say 25 to 75 is ok and your preference. In my mind a 25 rpm drop is "barely perceptible" and about a half needle width.
Dan Caliendo Ercoupe Mach 0.14 3658H On Nov 18, 2009, at 4:00 AM, William R. Bayne wrote: > > Hi Linda, > > No, it's not OK! > > I would expect a competent mechanic to be aware that the condition you > describe is not right, (apparently) never was, and actively pursue such a > problem until it is properly resolved > > If compression is good, timing and plugs are "right", and absent induction > air leaks, more of the fuel-air mixture should be ignited and burned with > dual ignition sources during the mechanically constant power stroke of each > cylinder. When one or the other mag is not firing, combustion is less > complete and power is less from the same fuel-air mixture, resulting in a > commensurate drop in RPM. > > The mag check should be done at 1700 RPM. Between dual and single-magneto > operation there should be no MORE than 150 RPM difference, but I would prefer > to see between 25-75 RPM. The difference in RPM when operating on either > single mag. should not exceed 75 RPM, but I do not recall much variation > operating on either mag. > > An imperceptible mag drop is strongly suggestive of unresolved ignition > circuit issues. It isn't an "open circuit" or you couldn't kill the engine > with the ignition switch, but it could be intermittently "open" (you guess > when and why). This would be a very dangerous situation if present until > resolved. > > An improperly timed or malfunctioning magneto could be the culprit. > > Substandard plugs could reduce dual-mag RPM to that of single-mag operation. > > Remember, I'm not a mechanic! > > Regards, > > William R. Bayne > .____|-(o)-|____. > (Copyright 2009) > > -- > > On Nov 18, 2009, at 00:34, Linda Abrams wrote: > >> A big Thank You to everyone who sent ideas on possible sources of the >> intermittent run-up problems. >> >> If there were any drops on the mag test at all, it was almost imperceptible >> -- maybe half a >> needle's-worth. (Is that ok? ) >> >> Linda >> N3437H (Sky Sprite)
