Old 912UL with Silver Box Ignition
I have an older 912 that I bought for my KitFoxIV when I did the rebuild on her 5or 6 yrs ago. My Fox had a 582 Greyheadand I was able to find a 912UL for about $3000. It seemed like a nobrainer to me and I was able to sell my 582 for just slightly short of$3000. The motor had 1000hrs on it and was a 600hr TBO motor. StillI fly in the flatlands of Albertaand never worried about having a high time engine. Desert FoxIV told mehe had an older 912 with the Silver Box ignition on a Model 3 and last he heardit had 3000hrs and still running well.
I did a complete restoration on my model IVincluding things like a wide body mod, Arizonabaggage compartment, and the 912 swap. It took me three years but I wassuper happy with the end result.
KitFox Ram.jpg
When I first ran the 912 it ran rough on one mag. The issue was easily solved by moving the plugs on the top of number 2 and number 4 cylinder. I could not find any info anywhere on the Silver Box ignition so it was a hit and miss and try this approach to troubleshooting. I put 450 hrs on my new to me 912 including a trip across the Prairies and into the Canadian Shield to the town I grew up in. I was a pretty happy Camper. Yes it took me some effort and a few dollars but I was flying the plane I wanted to fly and it brought me to my old home and back.
This past fall my plane lost power and started to run rough. I did some trouble shooting and after compression and leak down tests, I determined that it was time to pull the jugs to inspect. An inspection of the rings with feeler gauges confirmed the end gap was three times the gap of factory spec. I talked to my buddy Hal at Zipper big bore and he offered me a complete set of jugs, pistons, and rings that were takeoffs and minimal wear. Indeed up to factory specs. Great just what I was after. But then he threw a wrench in the works and offered some ULS pistons, Jugs and rings for not much more. It might give me 90 to 95hp. I thought about it for 2 seconds and said yes let's get a little more power. Hal machines the pistons so that they remain a low compression piston as is the UL and then machines the Jugs to fit the UL heads. Hal sent the Pistons, and parts to me and I sent him the check. My parts arrived to me the same day Hal got my check in the mail.
The weather had been good as we moved into January but when I began work the temps dropped to -30C or -22F. To cold to be working in an unheated hanger but pilots are a hardy bunch especially when it comes to getting our wings back in the air.
With the hard work done and some breaks in the weather it was time to do some testing. The results were mixed. There seemed to be incredible power but the motor was not running as smoothly as the old combination. It could be the power increase was the cold air or maybe the old motor was that tired.
Still it would backfire at low speed and run poorly at times. Some research led me to go for a complete carb teardown install all new O rings and diaphragms, and change the Jets to the standard Jets for a ULS rather than a UL jets. Well that took care of a lot of issues. The motor no longer backfired when I was taxiing to the hangar, it started easier, just overall ran better but I still had an issue. By the way I could see 5800 on take off now where as previously I would see 53 to 5400 at most.
The only issue left was it seemed to miss at cruise. Just flying along it would give a miss; run rough, just not a motor I was sure of. I ran it like that for about 10 to 15 hrs thinking it maybe needed to settle in but it was not running as smooth as I wanted. I stayed mostly in the flatlands in case something went very wrong.
My mag checks at 2000rpm showed nice drops on both sides of maybe 75 to 100 rpm. So I was sort of stumped. Then about a week ago I did a mag drop at 4000 rpm and found a significant difference between mag 1 and mag 2. I decided I would investigate further.
In the last 3 or 4 years I have been able to come across the wiring diagram for the old Silver Box Ignition and I am including in this post to help others that may have this ignition and want to keep it running. The only assistance I was offered when looking for information on this box was "that box is no longer supported by rotax change to the new system"
I took my papers to the hanger and got ready to trouble shoot one step at a time. What I found was one ignition CDI box was feeding all the top and the other CDI box was feeding all the bottom. The book says one box should feed two top and two bottom. The other box should feed the other two top and other two bottom. That would certainly cause a larger drop and the bottom plugs could tend to foul more I suppose. But I was not done yet I decided to check every line back to the coils and make sure every wire was going to the plug as suggested by the book. AHA another Eureka moment. I had the top plug of number 3 and number 4 exchanged. I corrected that and went for a couple of quick circuits. It seemed better. The guys on the ground both commented on how much better it sounded. The comment was "It used to sound like a Volkswagen, now it sounds like a Rotax."
I went for another flight this morning, more like a half an hour between the rain showers and yes it is smoother, no more misfires at cruise.
Interesting 450hrs on the old setup and it seemed to run ok. It really wasn't. That was just the old two stroke guy in me accepting less than this motor can deliver. I do look forward to going for a good couple hour run someplace and enjoy the airplane the way it was meant to be enjoyed. I do have one issue that is unresolved. The mag check at idle is great but mag checks at the 4000rpm range do give a big drop. I will continue to work on that. If anyone has a suggestion I'm listening.
Sorry for the long post but I hope this can help someone else that might be experiencing similar issues.
If anyone needs this in PDF format I can send the entire manual if requested.
ignition.jpg
ignition2.jpg
ignition3.jpg
Reply With Quote
Originally Posted by Dave S
