July 18, 2010

Small steps forward

Today my plan was to swap the upper Mikuni fuel pump for the other one I have, to see if that solved the fuel pressure problem and then to see if I could track down why my magneto earthing is not working. I realised when I got the Mikuni out of the garage that this was the one that was previously fitted to MYME, so I reasoned that there was a good chance that fitting it would solve the problem.

In fact, when I drove onto the field, I found that John and Bob were already there and that they’d dug out a Warp Drive prop pitch tool to see if it would be any good for adjusting my Arplast. This was too good a chance to miss so I decided to check the prop first.

The Warp Drive tool ideally needs two parallel faces to work accurately (front and back faces of the prop) and where you need to fit it on the Arplast is far from that shape (more of an aerofoil shape actually). So there was a bit of guesstimation involved in getting the tool in exactly the same position on each blade to check the pitch angles of each. Even so, it indicated that one blade was 1 degree out (over pitched) compared to the other two, so I changed it to make them all the same.

Then I swapped over the Mikunis and checked the connection of the earth lead that goes down to the starter switch and couldn’t find anything wrong with it, really. So then it was time to start the engine again to see how it ran.

DRAT AND DOUBLE DRAT! I’d forgotten to bring my ignition key, which was still at home in my conservatory 🙁 Only one thing for it – drive home and get it. So that’s what I did. It took an hour and a quarter what with all the Sunday drivers and when I got back, John had gone and only Bob was still there. We had a brief chat and then he also left, leaving me to see what effect the changes I had made would have.

The prop change seems to have worked a treat. Now the engine seems much smoother and I’m amazed how much difference one degree on one blade has made. I did one aborted take off run and I’m pretty certain revs at full throttle were also much closer to where they should be. So that’s that job sorted for the moment, but Kev on the BMAA forum is sending me his Arplast tool to borrow tomorrow, so I’ll check things out yet again properly next week-end.

The Mikuni change, though, made no difference and I now think I know why. MYME (which this system came off) used to have a hand primer bulb in the fuel line and was obviously being operated like that. I removed the bulb because there seemed little reason for it, what with having the electric pump and also because there have been reports of primer bulbs blocking and causing fuel starvation. But the thing is, the primer bulb has a non-return valve in it. I understand that the Mikunis do not contain proper non-return valves and it’s doubtful I think, that a worn fuel pump would effectively stop fuel from flowing in reverse when switched off and under pressure. So that leaves the primer non-return valve as the only effective barrier to prevent this, and I’ve removed it. Makes sense, doesn’t it? Anyway, it’s going back in again next week-end to see if that solves the problem – better that than to lay out £50 each for two Mikuni repair kits only to find that when fitted I still have the fuel pressure problem 😕

Now what about the mag earthing? Well, I’m still uncertain about this. Earlier I’d moved the starter switch earth connection through which the mags are earthed to a different position but the engine was still not switching off. Then it occurred to me, and I confirmed it by digging out my original diagrams and cable colour codes from when I did my MYRO’s wiring all those months ago, that the RPM gauge shares the same earth connection on the back of the starter switch, and that of course is working. This says that the earthing must be good in that case, leaving only the final possibility that the problem must therefore be inside the starter switch. This did not come as a great surprise actually, a I know that when I wired up the switch and indeed the whole of the panel, I took very great care and checked that everything worked as it should. The only mystery now is why has the switch failed after doing that? Anyway, I now need to get hold of a replacement twin-mag rotary starter switch. They can be a bit pricey, though, but there’s nothing else for it.

I’m working my way slowly through the problems but it’s a rather tedious process that’s taking me longer than I might have thought. However, each little step is taking me closer to finally getting MYRO into the air. I just hope that when I finally do, we still have some good weather 🙂