Hi Linda, I'd first suspect a malfunctioning plug or plug wire, but I'm no mechanic.
If a plug is dropped on a hard surface while being removed or reinstalled, it isn't generally considered good practice to re-use it. Unfortunately all mechanics aren't necessarily forthcoming when this happens (and there are no witnesses). ;<) A bad plug will show up as a greater than normal mag drop on one mag., and may look and test just fine. Plug wires age and can fail in the manner described without warning. They are also sometimes abused by the less talented or more hurried mechanics in the process of removing and replacing plugs. In some cases "standoff" ties, etc. are not reinstalled as before, loose wires can position themselves such that any weakness in the insulation can establish an unintended and intermittent ground somewhere, which will show up as a greater than normal mag drop on whichever single mag is affected. When the weather is more humid than usual, dust and dirt that is non- conductive can become conductive; and back again. This could affect mag drop on both mags. Some components of older magnetos can be adversely affected by excessive moisture. Harness insulation that breaks down only when moist can be devilish to confirm, locate and correct. Since coupes with the Stromberg have no idle cut-off, they are "killed" with the mag. switch. When the engine dies, you have, in effect, verified that no dangerous "open circuit" exists and each mag is properly grounded and not "live". A "live" mag is extremely dangerous should the prop be moved so long as a combustible fuel/air mixture remains in the cylinders. True "dieseling" is usually caused by a bit of carbon, etc., in one cylinder that remains hot enough to ignite the fuel/air mixture after the mags are turned off. But what if the shudder when the engine quits firing causes a momentary "ungrounding" which makes the mag circuit "live" just long enough to fire one cylinder, at which time it grounds again? Several short cycles of this could appear to be "dieseling". If the harness is installed such that each wire is properly aligned and supported, this is not possible...but I can also imagine a forgotten tool, metal pen or pair of glasses bouncing around above the baffling to the same effect! Such a problem should affect only one mag. To someone that is not doing the actual work (or watching as it is done), such problems are usually unexpected, invisible, and seem inexplicable. Regards, WRB -- On Nov 8, 2009, at 23:33, Linda Abrams wrote: > 'Coupe Group, > > This one needs the collected wisdom of the Tech List. (The nice > local mechanic helping me sounds stumped; fortunately, he has shown > that he welcomes suggestions from the Tech list.) More than 2 weeks > ago, I tried to depart for the Copperstate fly-in, but the run-up was > uncharacteristically bad. Usually, there's barely any mag drop when > I do run-up. This time, one mag dropped 200 RPM & bounced back up; > the other dropped 200 RPM...and started slowly dropping more. (My > checklist says "max. 75 RPM drop") I repeated the mag check 3 times > with the exact same results, then phoned the local mechanic who is > convenient to my field, and taxied back to my tie down. He came > right over, cleaned the 4 spark plugs he could easily access on-the- > spot, then tried the run-up himself. He said it started rather > slowly and one mag was still dropping too much & running very rough. > So I left it with him. > A couple of days later, he said he cleaned ALL the spark plugs > thoroughly, but the run-up was still bad. A couple more days after > that, he went out to the plane and -- WITHOUT him doing anything else > -- the run-up was then perfect. But it diesel'd slightly on shut down. > I was concerned about possible mag trouble, but called the more- > experienced AI who did the annual in July (he's at a different > field), and he said that it's not likely mag trouble, because mags > don't die intermittently: if they go bad they stay bad. > So I went out myself to try a flight test...but now the engine > wouldn't start at all! It cranked the prop around very sluggishly > once or twice on the first try, making a noise like a car does when > the battery is weak; on the second try it wouldn't even do that > much. Left it with the local mechanic again. > A couple days after that, he went out to try it, and it started up > fine for him, 3 times in a row! Though it still diesel'd maybe 3 > seconds on shut down. > On his next opportunity to try it, a few days later, it did start > very slowly, so he took out the battery and gave it a long slow > charge overnight. I guess the battery may have become weak because > of all the times we tried starting &/or run-ups, without flying it or > running the engine very long. > But that still doesn't solve why the run-ups are going suddenly very > bad, and then back to fine, apparently on their own. Nor does it > explain the brief dieseling on shut down. > I've asked him to check that the P-leads & connections look ok. > Another 'Couper suggested I make sure he also checks the spark plug > wires, and the mag timing. Is there anything else that could be > causing run-ups to abruptly go from good to bad and back again, and > also might be causing slight dieseling? Neither problem developed > slowly. The mag drop problem seems to be binary: either it's > perfectly fine (barely drops on either mag), or terrible (200+ drop), > and has been intermittently terrible for over 2 weeks. > I'm getting flying-deprived-withdrawal and getting really antsy to > get this solved! Does anyone know what we're overlooking? Thanks! > > Linda > N3437H (Sky Sprite) > 1946 415C, with a C-85 engine & Stromberg carb
