Monday, April 30, 2012

All about aircraft magnetos

There are times when it feels like I just can't get over the final part of building the RV-7A. The last few hours are taking forever -- months, really -- and the most recent disappointment was discovering the other day that I couldn't get the RPM over 1900. I adjusted the throttle cable and was able to get it to 2000, but lost the ability to get it to idle below 1,000. Geez. So it appears the total throw of the throttle assembly at the fuel servo is greater than the total travel of the throttle cable. What do I do about that? I have no idea.

This revelation revealed something more serious over the weekend. The engine is seriously mistimed. We originally timed it just before first start last September. But because I had removed the ring gear and reinstalled it, the timing marks on the gear were not aligned properly so we had to determined top dead center (TDC) and made Sharpie marks on the ring gear. This was a mistake.

In recent test runs, I noticed the propeller would stop as it started to wind and then start again. It was on the cusp of a kickback,apparently.

My first step was to remove the propeller this weekend and orient the timing marks properly. That helped us determine, thanks to Mike Hilger, who actually knows what he's doing, that the impulse coupling is firing wayyyy early.

I'll need to retime things but I don't have the equipment. I ordered the Rite System deluxe kit from Aircraft Spruce yesterday and it should be here in a few days. That will help me determine -- perfectly, I hope -- top dead center of the #1 cylinder piston, which should allow me to adjust the mag to fire at the proper time.

And therein lies the problem. First, I don't know much about this process but, second, I do know enough that the oil cooler hoses have to come off, the oil filter has to come off, and some fuel hoses may have to be removed to allow me access to the mag. Also some secured probe and electrical wires may be in the way.

It feels like starting over. When I take the oil filter off, it burps oil out of the crankcase, presumably coming from the oil cooler, which sits slightly higher. To avoid the mess, I guess I'll just drain all the oil out of the crankcase. Nothing but time and money. Time and money.

In the meantime, I have to try to learn what I'm doing with the mag. Fortunately, the EAA -- you remember the EAA, right? It's the outfit that all the whiners on Van's Air Force regularly say has nothing for homebuilders anymore -- has provided this webinar on magnetos.

So, it appears reports of this plane being nearly ready for inspection are somewhat incorrect.

This week, I'll pass 3,000 hours on the project.


  1. Ugh! Hope you can wrap this up quickly.

  2. The only thing I can think of that would easily resolve the issue with the throttle is a different sized cam on one end of the cable or the other (most likely on the fuel servo end). Sadly that's why they call me "captain obvious" at work.

  3. Mike said to unthread the rod end bearing a bit and then readjust the two nuts that hold the cable in the bracket. I did that without satisfactory result. I didn't see originally how that would really change anything and, it didn't.

  4. 99% done…10% to go. We feel your pain.


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