Hi Dan, You're certainly welcome to disagree, but it's more productive to debate objective points (as opposed to subjective ones).
In the cockpit during runup I have always found the sensation of an instant 25+ RPM drop going from dual-mag to single-mag operation so distinctly "perceptible" that visual reference to the tach reading is purely secondary confirmation of a successful mag. check. At the same time I keep in mind that some people involved in fields necessarily mechanical are simply not very mechanically "inclined". Back when I owned a quick printing shop, the paper feed mechanism had to be adjusted for different paper thicknesses on different jobs. I could hear a press operating two rooms and forty feet away out of my sight misfeed, indicating improper adjustment. On a single-sided print job, if uncorrected, that would result in blank (unprinted) sheets at intervals throughout the completed job. That meant that (1) if special paper was being used, less printed sheets were delivered than agreed, (2) if not, the number of unprinted sheets would go out the door as an uncompensated expense reducing profit, and (3) raise questions in the mind of the customer as to whether we knew what we were doing. If printed on two sides, every sheet with one blank side had to be (a) tossed or (b) separated out by hand and reprinted (two extra press runs). Presspersons unable or unwilling to operate a press profitably for me were either not hired or terminated, as appropriate. Visually, and with reference to the tach shown on page 51 of the Ercoupe Service Manual, the needle tapers to a point more or less at the outer edge of the 100-RPM graduation marks. While one could certainly assert that the width of the needle for most of its length would cover a range of 50-75 RPM were it that thick at the end; that would be a "red herring" statement unrelated to the sensitivity with which such a tach is normally "read". Such an "impression" is simply too subjective for meaningful or uniform anaylsis or discussion. Some aftermarket electronic tachs offer more distinct readings, and digital ones are the way to go for trouble-shooting to remove any reasonable doubt. Mechanical tachometers such as the AC unit (perhaps most common in coupes today) are notoriously inaccurate, but serve the intended purpose when properly selected and verified periodically so that any error at runup or cruise RPM is known and considered "within limits". On rereading my initial response to Linda I see nothing wrong or misleading. Regards, WRB -- On Nov 18, 2009, at 12:36, Caliendo Dan wrote: > 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)
