I have a manifold pressure guage for a twin sitting in front of me now. It’s a weighty thing and would not be something you would want to add to the empty weight of a ULM.
Obviously the electronic equivalent would be less weighty, but it was regardless just an idea. I think if you were flying a 912 it would be nice to have real time info on carb synch. Checking it otherwise is a time consuming procedure. It’s not like a twin carb motorcycle where you can check it after a ride occasionally (before shutdown) with synch gauges hanging on the garage wall for that purpose.
You can synch the carbs with a balance tube in place – there is no flow down that tube when the throttles are in synch. The reason you very often do it by pulling the balance tube is because that’s the easiest way to access nipples to connect the gauges. That does not preclude having permanent MP connections made elsewhere and using them to synch the carbs while the balance tube is in place.
Twenty years ago I thought I might like a Tecnam and with maybe 20 more years to fly I occasionally think about what I’d do if FAA medical qualification became problematic and I wanted to stop getting medicals and fly an LSA eligible plane. Rotax engines weigh into that thinking but for me any aircraft powered by them would be costly to own in comparison to keeping my low hassle O-320 (that makes 50% more power) running beyond its current 52 years since assembly. If I do need to overhaul it sometime after I retire, it’ll be about $15K and I’ll do it myself. Simple works for me.
We did the gearbox at 600 hours and it was inepect, repair or overhaul as necessary. As dictated by the chap who did the work.
In fact the only reason we pulled the engine was we were told the gearbox wouldn’t overhaul again and I couldn’t see any point in buying a new gearbox to put on such a high time engine.
On stripping the only part of the engine to show wear was the camshaft.
I can only comment on the 912UL engine and I’ve been very impressed with it.
My first one was replaced at 5500 hours. In that time we never had a problem with it.
All we did was service the aircraft as per the manual. All of which was done by myself apart from the gearbox overhaul.
It did have the starter replaced and the carbs were overhauled which showed dirt to be the cause of rough running at full power.
I then replaced with a new engine at 1/3 of the price of a new O-200 and after 600 hours is running just as reliably as its predecessor.
You cannot really synch in the air because there is a crossover tube between the manifolds. When syncing (on the ground) that tube is removed, and you use the nipples to connect to the meter. The crossover tube is much smaller than the manifolds, but large enough to make a difference.
I have a manifold pressure guage for a twin sitting in front of me now. It’s a weighty thing and would not be something you would want to add to the empty weight of a ULM.
If it would help to sync the carbs on the ground without mounting it in the aircraft then it might be useful. I’ll give it to the guys here so that they can experiment if they wish.
It occurs to me you could use a manifold pressure gauge from a twin engined aircraft to monitor Rotax carb synch. Then if the cable adjusters were at the cockpit end you could synch them without removing the cowling, at least you could if the idle synch was still OK. You could do it in flight 
Bing CV carbs are very easy to overhaul and based on decades of use on BMW motorcycles (their original application) I think the main thing that needs to be watched is the slide diaphragms, which I would personally replace yearly in aircraft service. It’s easy enough to do without removing the carbs from the engine and if a carb diaphragm cracks in flight the slide closes and you’re instantly running on only two cylinders. To change them you just remove the top of the carb with two Phillips head screws, lift out the slide, replace the diaphragms, reassemble and resynch. The latter is required because for some reason Bing used the top of the carb to anchor the outer cables, not the body, as shown by this photo.
The main long term ownership issue with any of the Rotaxes in my view is the pressed up needle bearing crankshaft and con rod assembly. It’s reliable but when overhaul time comes the whole assembly is a throw away unless you are very well tooled up. You can apparently send it out to be overhauled for $6126. One reason for this design is that because the rod journals are ground to size before assembly the crankshaft can be slightly shorter (this photo shows it if you look closely) but it’s not something many other four stroke engine manufacturers for any application have done for years: Porsche similarly used it for flat four racing engines (only) in the 1950s and some motorcycle engines (Japanese and Ducati) were built this way until the early 1980s. The Rotax was designed in the mid-late 80s by which time this was a bit archaic but they had a background in two strokes which are all done this way so presumably they went with what they knew. It is anyway a pain to overhaul.
aart wrote:
It’s all rubber parts, not just the hoses, so also the rubber parts in the carbs. Again, haven’t been through it yet. I suppose the materials alone could be between € 500 and € 1000, and it would likely take a full day of work, if not more. Again, on the forum you will see quite some debate whether to do this of not. I certainly will.
It depends on the aircraft. I just did that on the Alphatrainer, and it was a PITA. Maintenance must have been way down on the list of priorities when they designed that plane. But, Pipistrel has excellent service. They have “5 year kits” for every airplane. I think I paid €700 for the kit, and it contains everything needed (all the hoses, wires, clamps etc and then some. It came in a big box), everything is top quality from European manufacturers as far as I could see.
However, you should read carefully what the Rotax manual actually say about the “5 year hose replacement”. It’s really an inspection rather than a replacement. How are going to inspect the inside of a rubber hose or rubber gasket? You have to remove it from the engine in any case. Installing old rubber when new rubber is cheap and readily available is just silly. Rubber will deteriorate no matter what, and age is the main factor regardless of usage. The carburetors need service more often than each 5 year IME. You can do it yourself, but around here we just send them to Edge Perfomance.
The Rotax manuals are excellent also, and always available online. They also have videos. There are lots of opinions on forums, and lots of people who think they know better than Rotax themselves. My experience is that unless you are a true engine expert with years of experience with Rotaxes, then follow the manual.
Synchronizing the carburetors, you simply have to learn. You will do it a lot, and it’s a simple thing to do. You will also need a carb synchronizer. Not needed with the iS engines of course.
Thank you all for your comments and experiences.
It’s been very helpful, and I’m sure it will be very helpful to others looking at it too.
Ha I don’t get involved with the actual purchase I only really, put fuel in it check oil water and other connects including all the airframe bolts and things and then fly it. Fortunately we have a lot of good mechanics around both within the club and on the airfield including Rotax trained personel and Part 66 all waiting for me to come back and say “I don’t like that feeling or that noise” so that they can delve in and do typical engineering stuff.
Couple of weeks ago my friend who flies the glider tow (Ikarus C42?) happened to make a comment about something needing looking at He went for coffee and.when he came back the aircraft had been totally taken apart with a bunch of people around it discussing what to do next.
And I do mean taken apart the whole fuelage been taken off so that all cables were exposed whilst someone else was checking engine mountings. My friend said to me that he thought all that was needed was a bit of duck tape.🤪
