So remember when I was trying to debug a nasty popping, backfiring, horrible magneto? Yeah, like it was yesterday. Matter of fact, it was yesterday. And today. Every engine test since first start has been a mess and the reasons are plentiful, going back to when David and I first tried to get it running a while ago. This has been going on for 3 weekends now.
First, I modified Bob Nuckolls’s electrical system diagram to include a pushbutton starter switch in addition to the spring-loaded (ON/OFF/(MOMENTARY ON) switch. Turns out you can’t have both. What wound up happening here is that when the spring-loaded switch was set back to center ON, it was grounding out the one magneto capable of running the engine. I’ll get to that in a second. In addition to that, we were never sure which wire went to which magneto, because those two bits of shielded cable never got labeled. Lesson #1. We did eventually test that and got the wires running to the correct mags. We think.
If I wanted to keep my stick pushbutton start, the On/OFF/On switches would have to go and be replaced with dual-pole ON/ON switches, which would allow me to disengage the start circuit as well as ground the mags, making them cold and safe. I replaced those switches, and maybe here, maybe the step before, the wires got switched again and left went to right and right went to left. The good news is that with these switches installed, the engine fired up. Ran a little rough, but it was a damn sight better than last week. OK, shut down right mag for RPM drop. No drop. Shut down left mag for RPM drop. Bang, boom, sput, cough.
As you might expect, shutting down the “left” mag caused the engine to pop and misfire, so it was assumed that something was wrong with the right magneto. First thing we did was switch it out for a good one, the one my hangar mate had in a plastic bag for eventual installation on his RV9. Timed it to the engine, buzzboxed it, fired the engine up. Same thing. At this point it was a head-scratcher, because here we’ve just put a brand new Slick 4300 series magneto on the right side of the engine and we’re still getting popping, misfiring, the same thing.
Oh, one important detail I’ve left out of this story so far: I don’t know how many times we pulled the magnetos and checked and rechecked the engine timing. During this process somwhere, I the timing pin (an allen wrench or a pop rivet) in the distributor block when we pulled the engine through to set up the engine timing. Maybe I even did it at home before moving to the airport. At one point, I pulled a very bent allen wrench out of the left mag and a bent pop rivet out of the right one.
I assumed that because the “left” magneto was working fine, it was the right one that was messed up, but that couldn’t be because I’ve just replaced it with a brand stinking new one. Matter of fact, when I opened up the mag I’d pulled out of the airplane, everything looked fine. The distributor gears were OK, the rotor was fine, and the distributor block wasn’t cracked or anything. I left Ron’s spare mag on there because I had more debugging to do and I didn’t want to change back to the original mag, which may have been compromised in some way, even though none was evident.
Yesterday, I thought, hm, maybe the wire going to the magneto is shorted somewhere. So I disconnected the terminals from the mag (and took the harness cap off to be safe) and tested them for continuity. With the switch set to “ON” there should have been no beep from my multimeter. I got a beep. AHA! There’s a short in the shielded P-Lead wire! So what do i do then? Cut the ends off the wire to the “right” mag at the switch, thinking maybe the connection shorts intermittently because I cut the insulation of the wire and the shield’s touching it. Still beeped. OK, that wasn’t it.. Maybe it’s at the mag end. Cut that off. Strip back some cable so I can test the ends. Still beeped. At that point the day was pretty much over.
Last night I was falling asleep reading Book 7 of Stephen King’s Dark Tower cycle and it hit me. What if the wires were on the wrong switches? Left is right, right is left. That would mean three things: One, the “short” in the P-lead cable isn’t a short at all. The other switch was cold, so the multimeter would have beeped no matter what, because disconnecting from the switch and still connected to the other mag, there’s enough continuity for a beep. At the mag end, testing the wires would produce a beep because they’re going to a switch in the cold/safe position. Two, this would also mean that I’ve been debugging the wrong magneto for the last two weekends in a row. All my attention was focused on the wrong part of the system and it didn’t occur to me to check something as simple as wiring because I’d already gone down that rabbit hole and figured I’d verified it multiple times. Three, I’m an idiot. Once for leaving the timing pin in the left magneto, twice for not checking the wiring, and three times for assuming things were correct in places where they weren’t.
I hadn’t planned to go to the airport today, but I did. I figured it would only take me a couple of hours to sort this out, so I loaded up the bike and headed to OXR. First thing I did was pull the right magneto and check the internal parts for damage. This was onerous. I routed my hoses and cables so they trap the magneto in a cage with no hole big enough to let it exit. I had to take the oil pressure sensor hose off to extract the magneto. It always makes me nervous doing that. All I need is for a lock washer to fall down inside the engine accessory case and I’m totally screwed. I did manage to extract it, and got it up on the bench, where I was able to remove the back cover and the distributor block. Guess what I found:
That right there is the rotor gear. This spins on the shaft that opens and closes the points, making the spark. Notice that there are two teeth missing, and several of the others are damaged. The bottom line here is that no matter how carefully the engine timing is done, there is no way in hell this gear will drive the distributor gear in any way that will provide a spark at the right time. To prove this out, I reassembled and installed the old right mag on the left side of the engine. I have two impulse-coupled mags, I can do this. I remembered to take the timing pin out, say thankya and may it do ya fine (Dark Tower, remember?). Then, miracle of miracles happened. The buzzbox timing process, which I am now intimately familiar with, went just like all the documentation, descriptions, and YouTube videos said it would. Snap the impulse couplers, back off enough to get rid of gear lash, then move the prop forward to 20 degrees BTDC. twist one mag until the light just goes on or off, depending on what it’s doing when you turn the box on. Back the prop off again, then move it back to 20. The light should go on right there. Twist the other mag until the same thing happens. The trick is to get both lights to come on at the same exact time.
With two teeth missing off the rotor gear that wasn’t ever going to happen either, so wiggling the mag back and forth on the dead spot betwene those teeth isn’t going to do squat.
But joy of joys, sing hosannah to the heavens, with two properly timed and synchronized magnetos, that engine runs smooth and strong, like a big cat purring. Ron helped me push the ship out into the sun and I fired it up. Rock solid, dead on. RPM drop on both mags, just like that beat-up Cherokee I learned on.
Now I get to go back in and clean up the mess I made testing everything. Re-wrap my wires, safety-wire the governor, everything back int its place.