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Engine Vibration

PostPosted: Tue Jul 07, 2015 7:29 pm
by User GDS
I posted this on the Rotax Owner's forum, if anybody on here has ideas I would appreciate it:

I have a vibration issue that started some time ago and has been getting worse. The vibration is not ordinarily present on ground run ups but begins after flying for about 5 minutes (after engine is fully warmed up?) It is very noticeable between 3000 and 5400 rpm and is worse if I yaw the plane back and forth. If I pull power and go below 3000rpm, the vibration stops, even if I maintain airspeed. At around 5400rpm, I can turn the vibration off and on by changing rpm. IOW, at 5390, vibration, at 5420, no vibration.

Gearbox situation: I sent the gearbox out and it was re-shimmed and they adjusted the clutch. No change in vibration issue.

Propeller situation: I have a three blade warp drive that I have used since first flight, it has been checked for consistent pitch and dynamically balanced. I recently installed a Prince propeller, had it dynamically balanced, and have exactly the same vibration problem as with the warp drive.

Engine Maintenance: I do 50 hour oil and filter changes and follow the current Rotax maintenance manual inspection checklist very carefully. At around 100 hours I found a small metal chip on the magnetic plug, nothing but the normal amount of dust since then. Carbs were balanced and checked for float buoyancy at the annual C.I., April, 2015. There are no mods to the engine, I use nothing but the recommended spark plugs, oil, filters.

It was recommended that next time I fly I do a mag check while the engine is vibrating. The motor mounts are tight and do not appear to have any wear, but I'm out of ideas and thinking of changing them.

I push the engine pretty hard but take good care of it. The vibration is getting worse, any ideas would be appreciated.

Re: Engine Vibration

PostPosted: Tue Jul 07, 2015 10:12 pm
by R Rinker
Sounds like a classic harmonic. I assume when they had the gearbox they checked the thrust bearing?

Re: Engine Vibration

PostPosted: Wed Jul 08, 2015 12:58 am
by danerazz
See, this is why you should ditch the rotax for a rotec, then you get nothing but silky smooth radial vibes!

Re: Engine Vibration

PostPosted: Wed Jul 08, 2015 11:40 am
by Augustus
Carb balance?

Re: Engine Vibration

PostPosted: Wed Jul 08, 2015 3:06 pm
by scubarider2
I know you said you did a carb balance but it is difficult to get it right. Did you "green light" at higher rpms or idle?

Re: Engine Vibration

PostPosted: Thu Jul 09, 2015 12:17 am
by Av8r3400
Carbs. Been there done that, still wear the t-shirt.

Check floats, fully clean (best is a heated ultrasonic), readjust all jet screws, sync and check that both enrichers are fully moving both directions.

Re: Engine Vibration

PostPosted: Thu Jul 09, 2015 8:00 am
by messenger
I agree, it is most likely carbs, in my case, the choke was sticking just slightly on the left side. that caused a miss in the 3500 to 5000 rpm range

Re: Engine Vibration

PostPosted: Thu Jul 09, 2015 1:34 pm
by Gil T


That's got to be frustrating when you cant pin down the source without taking everything apart. Look at this picture and I didn't notice any particular shaking or vibration. However when I noticed this I grabbed hold of the prop and was able to move the engine a significant distance. This is a good illustration of why you should RTFM. I used the shipping bolts which are way too short. I believe you need 25mm of threads into the engine case. I guess if things had got much worse I would have had to recalculate my CG before landing.

Not much flying the last few months because of family sickness and now its 110+ so that's going to shut me down for a few more. I lose my ground crew when it gets past 95.

Let us know what your fix is. See you later.

Gil T

Re: Engine Vibration

PostPosted: Thu Jul 09, 2015 3:14 pm
by User GDS
Yesterday I pulled the cowling to do some more checking and found three spots where the cowl was damaged from wear against the engine. Biggest spot was on the flange for the prop opening, it was hitting the base of the fuel pump. The engine is definitely shaking and I'm grounded until I get this figured out.

I checked the run-out on the crankshaft and on the prop flange, both are less than .002" total.

I did an in flight mag check and it behaved normally. The spark plugs look good.

I re-checked carb sync this morning, it is off slightly, 1 dot on the carb mate. When I sync the carbs I do it at 4000rpm, when I get it where I want it, I run it all the way through the range, from 2000 up to 5400. I don't go more than 1-2 dots off even with the throttle jockeying. I've had it go farther out of sync than it is now, I don't think that's it, but I don't know what else to work on. I weighed the carb floats in pairs, they were 4 grams and 5 grams, well under the 7 gram limit. I guess I'll pull the carbs and do a thorough cleaning and re-balance.

Gil, when I was building, I safety wired the bolts on the engine side of the mounts, on your suggestion. (Glad to see you back here, best wishes with the family) I thought about changing the motor mount rubber, but at 320 hours they look great, no oil contamination and are torqued properly.

Re: Engine Vibration

PostPosted: Mon Jul 13, 2015 1:17 pm
by Johnny C!
I am sure that your issues are not
as simple as mine, but yesterday
evening, about 2 minutes after
wheels up, Lima Charlie developed
a noticeable vibration. I returned
to the airport with 6 minutes on the
Hobbs. I pulled the cowl and began
looking for the usual suspects, but
found nothing out of the ordinary
there. Then I noticed I had lost
almost half of my leading edge
protector on my 2 blade Ivo.
I have a call into them to order
enough to replace mine, plus
some spares. It wasn't anything
terrible, but enough for concern.

I feel sure this was my issue, so
hopefully I can get the new ones
here this week.

Best of luck on yours.


Re: Engine Vibration

PostPosted: Fri Aug 28, 2015 7:17 pm
by User GDS
This week I finally resolved the vibration by changing out the gear set in the gearbox.

I isolated it to the gearbox by borrowing the gearbox off of my friends airplane and flying it, it was a smooth as when it was new. I sent mine to a service center and they re-shimmed it to increase tension on the springs. When I got it back, it still had the vibration. Then I took it off and sent it back to the service center, they shimmed it as tight as it would go and changed the front bearing (at my request), it still had the vibration. Based on advice from a couple of people, I decided to change out the gear set. I ordered the gears, built a tool and installed them myself, the vibration went away.

I could see nothing wrong with the gear set, but the cams that engage with the slipper clutch (the "dogs") showed some wear. I don't understand how wear on the dogs could cause vibration, but maybe they chatter or something.

The shimming, bearing and shipping costs were about $500. The gear set cost about $1000. I don't blame the service center, they could not see anything wrong with the gears and were trying to save me the $1000.

It was frustrating, but I'm glad it's fixed. I really like flying Rotax engines, but I don't like having a gearbox. My neighbor just took delivery of a brand new Carbon Cub, it's beautiful, expensive and doesn't have a gearbox. I'd like to have one, but at around $230K with tax, it's too expensive and pretty to bang around in the desert like i do with the H.

This morning I landed on top of a desert mesa with a 100 yard flat-top, it was all worth it.

Re: Engine Vibration

PostPosted: Fri Aug 28, 2015 10:28 pm
by Johnny C!
That wasn't a cheap fix, and I am glad
you have it corrected.

Be safe!


Re: Engine Vibration

PostPosted: Sat Aug 29, 2015 8:14 pm
by R Rinker
There doesn't have to be anything broke to get a harmonic vibration, just something a little different. Best example is a buzz in a violin setup. It can drive you crazy finding the cause.
I have a friend that just got his homebuilt flying after an all summer Rotax fiasco. He used a brand new engine, went through the factory engine training course, and really did it all to get it right, but still couldn't get the engine to run normally. After losing the summer to this, he finally found out there was defective parts in a carb. This is so common obviously Rotax still doesn't have a bolt it on and fly it engine like we want, although everyone tries hard to convince themselves. So often its-- get it going & mess around with it, fly a little bit, mess around some more, and on and on. That's why I'm really hoping the Gemini works. With no carbs, valves, etc. it just may be the simple design that eliminates a lot of the common problems. Instead of a gear 'box', it has a geared prop, which is different. Radials deal with the harmonics by using odd number of cylinders. The Gemini has 3 cylinders and 6 pistons, which just might be an effective design to eliminate the vibration. The smoother it runs the better it is for absolutely all other components. Also, GDS, for every person like yourself that shares your experience as you have, there are 'many' more that we never hear about.
What I wonder is: how many have a friend that can loan out a gearbox to diagnose or troubleshoot?? Where would you be if that hadn't happened? You sent the gearbox out so that base was covered. I suppose you would give up flying or replace the entire engine?

Re: Engine Vibration

PostPosted: Sat Aug 29, 2015 8:46 pm
by BucF16
Yep, The Rotax is an engine that we have been stuck with for too long; But hopefully not forever. If the Gemini is in the weight realm, then we have a new partner to play with. I've built 3 aerobatic mounts all Lycoming of course. Easy to adjust, easy to annual, but too heavy for our purposes.

I'm hoping that in the next 24 months that "technology" will offer me a choice to install a new breed of engine on my Highlander. Max (Blue Max) and I Start our Projects this month (Sept).

Cheers to all. Stand by for many ?s ;-)

Re: Engine Vibration

PostPosted: Sat Aug 29, 2015 10:28 pm
by R Rinker
Gemini weighs close to 200 lbs, but doesn't need a lot of the stuff Rotax needs, so the actual real world difference may not be much. I understand it's designed to replace a Rotax, actually is a little smaller overall with no carbs.