Max. 2500 rpm Problem Solved

[Onex107]

This afternoon, with Bill Larson’s help, we chased the problem to it’s source. The culprit was the magnitron switch. Not as we suspected, a bad switch, but the wrong one. The Aerovee manual calls for a Double Pole Single Throw and over a year ago I made the installation with a Single Pole Single Throw. The mags provided with the engine kit worked fine on that switch for over 80 hours. Why would anyone suspect the switch. I think the problem started with the top mag moving enough for the magnet to rub the yoke over the coil. It smeared the laminations to effectively weld them together. The problem was centered in cylinders #2 and #4. In addition I had a burned plug wire on the bottom #1 plug so I bought two new mags. I replaced the bottom one, thinking that the top mag was O.K.. But replacing the bottom mag had no effect on the problem. I then replaced the top mag and saw the damage but when I did a run up, both new mags were dead. Bill and I checked the switch and wiring with an ohm meter and found nothing wrong. I went out today to remove the mags and send them back and decided to do one more run up with the P leads disconnected from the mags. to eliminate the wiring and the switch. They preformed as advertised,2900 plus on both ignitions. . We then connected the P leads to the mags and disconnected the P leads at the switch. The Mags were still dead. It then dawned on us that the P leads were crimped to each other at the singe pole push on connection and were shorting each other. Cutting one wire did the deed. A Double Pole Single Throw switch is on order.

The new mags didn’t like the setup. I don’t know why the two supplied with the engine did not object for 80 hours and if I hadn’t replaced both mags I would probably still be looking for the problem. I don’t know why it wouldn’t run on the secondary, unless the damaged top mag was still putting out in spite of the P lead. It’s difficult to tell if the P leads are good unless you do a P lead shut down test.

It’s taken over 30 days and a detailed and frustrating inspection of the whole engine to find a $20 mistake that was made over a year ago. Thank you for all your suggestions. Your support is really appreciated. This is one you can add to your list. If you suspect a bad magnetron, disconnect the P leads, the engine will run fine without them. If it doesn't, they could be the problem.

[Rynoth]

Glad you found the problem! I for one can verify that there are bad mags out there that can't be grounded/killed, I had a pair of bad ones in my aerovee kit. I encourage anyone and everyone to do mag checks as a routine before every engine shutdown, basically just quickly flick the ignitions off and on again just before shutting down to verify everything cuts off correctly. Otherwise you have hot mags at all times, which can be dangerous. Any fuel in a cylinder when turning the prop by hand could be very nasty indeed.

Basically, for those first 80 hours you had hot mags even when the engine was off.

You should probably do some secondary ignition troubleshooting/tuning as well (which should be easier to do now that you can turn off the mags!), since they obviously weren't able to run the engine at full power either. Since the mags weren't grounding, your initial tuning/timing of the secondary was probably being masked by the mags that kept firing.

[kmacht]

I gave heard it a number of times but exactly how is a hot mag dangerous on an aerovee with an aero carb. If you shut down the engine using the mixture there isn't any fuel in the system to start the engine. Unlike a normal Cessna there isn't a fuel bowl in the aerocarb to suck up into the intake.

Keith
#554

[radfordc]

Since magnetrons also require the engine to be turning over at approx. 200 rpm before they fire, the chance of accidentally starting is about zero.