balance tube

Re: balance tube

by Roger Lee » 12 years ago

Roger Lee

131 5.38k 5.44k

The choke should do nothing at higher rpms. Try flying at cruise and apply the choke. Nothing will happen. It will affect rpm down under 3500.

---

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-349-7056 Cell

Re: balance tube

by Alan Arnold » 12 years ago

Alan Arnold

20 191 170

Bill,

Thanks for the kind words. I had forgotten about you having the same problem. I have no MP gauge or EGT gauges. I forgot to mention on my post the other day that in addition to running very smoothly during all, except WOT, rpms my "mag check" is different. Instead of having one mag drop 50 rpm and one 100, now both are 40 rpm drops.

Today I went to the airport and did a ground run with the engine "missing", but still attached to the airplane. ;) I ran it at WOT for 30 seconds after warm up, shut it down immediatly and pulled a top and bottom plug. The attached pictures are of the same plug after the run using different lighting, flash settings, and backgrounds. It's kind of hard to tell from these pictures, but the plug is a light tan with no pitting or excessive lean indications, i.e. no little melty sphereuals (sp?) attached. The other plug was identical. Since it didn't appear the "miss" was caused by lean or rich condition, I am stumped. I put three -4 washers on the throttle shaft to prevent entering the miss setting on take-off and will just wait for those on the board that are smarter than me to figure it out. This me hoping. :dry:

Alan

20131216_133529_resized.jpg (You do not have access to download this file.)

20131216_133556_resized.jpg (You do not have access to download this file.)

20131216_133624_resized.jpg (You do not have access to download this file.)

20131216_133809_resized.jpg (You do not have access to download this file.)

Re: balance tube

by bill martin » 12 years ago

bill martin

10 757 0

Alan,

That last 5% of throttle has me stumped too. Several things I have concluded after a few months of flying with the balance tube.

1. smoothness at idle and lower RPM cruising is much improved
2. fuel consumption has improved
3. spark plugs look good and EGT temps are good
4. no loss of horsepower
Before changing the tube, my prop was pitched to give me 5500rpm at WOT. After changing the tube, I still get 5500rpm with no change in pitch. This indicates to me that I am still developing the same horsepower as before. Only difference is that now I get that HP at 95% throttle.

Since the smoothness is so much better on the low end and the developed HP and temps are good, I'm going to continue to fly it. Am thinking of contacting FLYGAS in italy and see what they think. They may not want to share info (I can understand why) but I'll give it a shot. I'm not really sure it is even missing. It just seems to slow down and sounds different in that last 5%.

You can make you a manifold gauge setup to sync carbs from locally obtained materials. All you need are a couple of cheap manifold gauges from any auto supply store, a couple of small needle valves, and some rubber tubing. be glad to send you a diagram.

Bill.

Re: balance tube

by Mike Goodrich » 12 years ago

Mike Goodrich

8 123 0

This has been something I've been tempted to try for a long time. Like you, I always felt that the balance tube was too small and seeing the Italian mod confirmed my belief. However the problems you guys seem to be having have me asking the question.
"Surely if it was that simple Rotax would have done it by now"
I'm generally very suspicious of Rotax motives, I'm sure they're 99% financial and 1% technical but even Rotax must be aware of the "weakness" in their design and the vibration caused by unbalanced carbs and could see the financial/marketing gain in proposing an nice upgrade to reduce vibration that they could then sell at an attractive (read expensive) price to us operators. If they haven't is it because they never thought about it (difficult to imagine), or were they beaten to the market by the Italians (again difficult to imagine) or simply because they've done the testing and found that for some reason (maybe the problems you are experiencing) the current design is the best compromise for a bad initial design (it should have been a single carb from the start).
That's just my opinion.
Mike G

Re: balance tube

by bill martin » 12 years ago

bill martin

10 757 0

Mike,

I agree 100% with the single carb. No need whatsoever for two carbs.

Rotax filled a real need in LSA for a compact, lightweight four cycle engine that would develop 100HP.
Problem is that the only way you can get that much HP out of a small engine is with elevated RPMs. With an engine that only works well at relatively high RPMs, a balance tube becomes almost unnecessary except for idling. In my case, I must cruise in the 4000-4500RPM range. My only other option would be an in-flight adjustable prop. Too expensive. The Rotax 582 two cycle would better suit my purposes but with an overhaul every 300hrs., that means seven overhauls in 2000hrs. which is actually more expensive than buying the 912. Plus, with the two cycles, its not a matter of if the engine will fail, but when it will fail.

Then you throw a gearbox into the mix and you get more maintenance and added problem at low RPMs. Unfortunately, the gearbox is necessary. Large slow turning props are much more efficient than small high speed props. My Ercoupe can cruise at 2100RPM, which is just slightly above idle for a 912S. My 0-200 is no good for smaller aircraft though, just too heavy. With that in mind, I think what Rotax has come up with is a pretty good compromise, but not a solution, for a compact, high HP engine.

The balance tube mod. is working well for me. The 95% throttle solution was just a matter of not allowing the throttle plate to open any further. All other parameters are the same and fuel consumption has improved. I'm just curious about the throttle opening. Any ideas would be greatly appreciated.

Bill.