New guy from Norway here. I have just finished restoring Air-Bike s/n #33, built in 1996. It has previously been owned by, among other, Knut Aunan, a fellow Norwegian and a member of this forum. I bought the Air-Bike two and a half years ago, and have spent a couple of hundred hours working on it. It has just been inspected and found airworthy. It will need to be test flown by an instructor before I am allowed to fly it, so it is pretty exiting times here. Unfortunately I have discovered a problem with the Rotax 447. Below 2200 rpm it shakes so vigorously I believe the whole airplane will fall apart. I have found som play in the engine mounts, which I believe are the originals from 23 years ago, so I have ordered new ones. I hope this will cure the problem. If not, some serious thinking has to be done.
This forum has been a lot of help during my restoration, and a big inspiration. I’m impressed that so many people care about these small airplanes.
welcome to the forum Petter. most rotax 2-stroke engines have heavy vibration below 2000 rpm thats why i set mine at 2100. if your tach is reading 2200 make sure it is reading correctly especially if the engine smooths out at higher rpms.
If you are new to two stroke engines you might not know that it's perfectly normal for them to not fire on every revolution when at small throttle settings. The engine shaking the whole airframe - that does not sound normal. None of mine ever did (but never had an airbike).
Do you know the engine's history? Does it spin up to something like 6000 RPM when tied down to the ground? There is a lot of info on the Bing 54 carb on the internet. Is the carburetor air screw set correctly? If not, the engine could run roughly at near idle speed. Intake silencer removed? It's optional but different jets are needed with and without. Has anything on the carb been changed away from standard for any reason?
Thank you for replies. My knowledge about two strokes are limited, but I have had experience with a single cylinder moped motor in my youth, and my single cylinder 200cc paramotor in newer days. Otherwise I try to read what I can about ignition, carburetor and propeller setup on these engines. My engine (250 hrs), carburetor and mikuni pump have been disassembled and assembled with new parts and gaskets (excluding piston rings). The carburetor uses an K/N filter and is set up according to this, as is the 1,5 turns out on the air regulating screw. It also has a new needle and needle jet. The float level should also be ok. But, as I am writing this and consulting the parts catalog, I see now that I miss the o-ring associated with the air screw. So I probably have a small air leak there. I would probably have missed this if I didn't have your inputs Found a picture I took during reassembly, and the screw had the o-ring installed. Otherwise the engine reaches 6150 rpm on the ground, which should be OK according to the manual (6000 rpm-6200 rpm static on ground). I have both a Tiny Tach and an "standard" rpm indicator. And from what you tell me, it idles a bit rough below 2000 rpm. I will keep that in mind.
I have no contact with Knut. But the owner after him said that he had moved to Thailand due to health issues.
A/B Plans #112 has 1008 hours as of 02-09-2015 Ace
Posts: 1,694
Time Online: 36 days 13 hours 14 minutes
Hi Petter, welcome to the Buzzards Board, there is a wealth of information here. Generally it is not a good idea to run the Rotax 2-stroke in the low 2000's, vibration at that RPM is a characteristic. I never run my 503 below the 2400 RPM range to avoid the potential damage to the AirBike airframe or engine. There is no need to run the engine in the low 2000,s. However, there is a prop balancer available from leadingedgeairfoils.com that likely will be a good solution to your concern. I have one on my Rotax. After reviewing and attending to the suggestions made by other, and if all of that checks out, I most certainly would suggest purchasing the prop balancer. You may have other suppliers of this type of produce in your country that would be more convenient than what I suggested.
One last thing, but ONLY if nothing else checks out is that the 4-Dog Hub in the "B"-type gearbox may be damaged (from excessive running at low RPM or wear over time such as somewhere around 1000 hours). That part is the connector between the prop shaft and the gear train to the engine. I hate to even suggest this as a problem because it is so remote as a problem and I doubt if Kurt has many hours on that engine or ran it at low RPMs. If everything works well above 2400 rpm, call it good.
Have fun flying you outstanding choice of an aircraft. check out my website for how to have fun with an AirBike. I also have a new facebook page called The AirBike Ace. Good luck Petter and have fun.
Hi All Am I wrong, but, I think Knut had a half Vdub on his airbike. Remember the time he lost one of the exhaust pipes and it landed on a farmers barn roof? Bob
Paul: Thanks for your advice. That propeller balancer looks a bit like woodoo, but I might give it a try. And my next step on the list of things to do, is to check the gearbox. I was thinking of running the engine with the propeller off, to take the load out of the gearbox. I understand that I have to be careful with the rpm, so it don't overspeed. I also have moved my rudder pedals forward, as you described it on your webpage. Worked very well
Bob: I believe Knut had a Z-Max. It might have had a 1/2 VW installed. Hope I don't loose my exhaust on somebodys roof
Paul: You mentioned the dog gear. I disassembled the gearbox yesterday and found what you see on the pictures. It doesn't look good. I would guess that the surfaces should be smooth, and not pitted like this. So, I tried sanding the dog gear. It came out quite good, so I hope the mating surfaces will have a smoother contact area. When installing the parts again I will rotate the dog hub so that I use the two "knuckles" that are not worn. Maybe this will make the gear last some more hours. Next step is to check the preload of the springs in the gearbox.
Actually Petter the dog ears in the pict doesn’t look that bad. The preload and the inertial mass of the prop is important along with setting the idle to 2000 rpm.
A/B Plans #112 has 1008 hours as of 02-09-2015 Ace
Posts: 1,694
Time Online: 36 days 13 hours 14 minutes
Wow, Petter, you are the kind of guy we like to see on this Buzzards Board! It takes a lot of nerve to get into the innards of things like the gearbox and engine. I agree with Monte, the chipping does not look too bad, actually very close to verifying that the dog is not the source of the vibration problem. Good plan to reassemble with new dogs engaged. I, however, do think that 2000 rpm is too low. I can't recall where, but I thought that the minimum RPM was stated somewhere in a Rotax manual. When I have time, I will look through my manual and try to find something definitive. I don't let my idle go below 2400, the plane doesn't go anywhere at that RPM. By now you should have the new engine mounts and perhaps the balancer I mentioned earlier. BTW, my engine mounts are the same ones that have been on for 20 years. Does anyone know if there is a "lifetime" in years that these are good for? Mine have not deteriorated.
Thanks Paul. It is good to find out what works or not. Now I know that the Gearbox is not an issue, and I hope my engine mounts arrive by friday. I did find some play in my forward mounts. When I lifted the engine by the propeller I could feel and see the play in the mounts. I have not ordered the balancer. Yet. I have also found that I might have had my carburetor air screw adjusted too far out. The Rotax manual says 1,5 turns out, but when reading the advices from Mike Stratman, and Bryan Carpenter at Rainbow Aviation, they say 0,5 turns out. This is the next thing I will try. Another thing. I found an article from Mike S that talks about troubleshooting the Ducati ignition. My E-box/coil does not give the correct ohm on all of the circuits. My E-box also comes with one yellow colored wire, instead of a white, so I start to wonder if I have the correct E-box. I have noted that I will not make it idle as low as 2000 rpm, and see If 2400 is better. So hopefully by friday I will try out my new parts and adjust the idle screw to see if it idles better. Thanks for all advices so far. Attached is a screenshot from the Rotax 447, 503, 582 operators manual.
Petter i'm sure your carb is a bing 54. there was an AD that came out about the needle clip is prone to failure which will cause the engine to stall. the new style clip is stronger and has a small o-ring to stop any rotation of the needle in the clip. might check that and your jet sizes that rotax recommended for your altitude.
Monte: I have replaced the needle, jet, and clip with new ones, and I have installed the o-ring modification to the needle. I have also checked the jet sizes, and they are according to spec.
Petter, sounds like you are on top of things. I would bet that adjusting the idle to 2000-2200 rpm and fine tuning the air screw will make the engine run much smoother. Your plane is a beauty.
So. I have replaced the engine mounts, checked the gearbox, new igniter plug caps, adjustet the carburetor air screw to smoothest running, and set the idle rpm to 2200-2300. The engine accelerates very good, with no hesitation. Runs good at max rpm, and holds the idle speed without wanting to stop. I feel it could have idled more smoothly, but having nothing to compare with I´m not sure. So, I think I leave it where it is now.
Found this video of a Mini-Max with a Rotax 447. Youtube https://www.youtube.com/watch?v=NR63x5riSw4 at around 3:00 minutes when idle gets below 2000 rpm, it sounds just like mye engine. And when it goes above, it also sounds like my engine. So I guess this is the normal...
Thank you all for contributing, I really appreciate your help. You will be updated after the test flight is done
Petter if it was me I would tether your Airbike so you can check the temps and prop load. I always do this when I work on my 503 after having 4 forced landings. Don’t want any surprises after takeoff.
Monte. Thanks. I have done this. Tethered the AirBike and run the engine at max rpm for about 5 minutes. Also tested the Acceleration several times from idle to max rpm. It seems to work fine. Will probably do more of this as my test pilot would like to test run the engine too.
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