Carb balance causing BAD vibration?

[glenadavis]

I have a 2013 RV – 12 that I bought about a year ago. I recently brought it to a shop (admittedly not a very RV knowledgeable shop) to have some avionics work done. While it was there, they balance the carbs. Now, I get a fairly aggressive vibration between 3000 and 3700 RPM. It goes away below 3000 and above 3700. Could this be the result of them balancing the carbs incorrectly? I don’t know if they removed the prop but if they did when they reinstalled it might something have gone awry, causing that same vibration. This aggressive vibration was not present prior to this time in the shop. I’m bringing it back to them on Monday morning and was hoping to be able to give them some information from the THINK TANK.

 

[JP4]

I have a 2013 RV – 12 that I bought about a year ago. I recently brought it to a shop (admittedly not a very RV knowledgeable shop) to have some avionics work done. While it was there, they balance the carbs. Now, I get a fairly aggressive vibration between 3000 and 3700 RPM. It goes away below 3000 and above 3700. Could this be the result of them balancing the carbs incorrectly? I don’t know if they removed the prop but if they did when they reinstalled it might something have gone awry, causing that same vibration. This aggressive vibration was not present prior to this time in the shop. I’m bringing it back to them on Monday morning and was hoping to be able to give them some information from the THINK TANK.

Improper carb balancing wil absolutely cause the vibration you describe. It can be tricky to do correctly and if the shop is not really experienced with Rotax dual carbs, it's unlikely they got it right on the first try. Done poorly it can cause really bad vibrations. It's a pretty straightforward procedure that starts with setting up the carb linkage before moving on to actually syncing the carbs. The Rotax manual is pretty easy to follow, but it still helps to have a little experience doing it.

 

[SPRV12]

When I perform carburetor synchronization, I check the vacuum at 1800, 3000, and 4000 RPM; some only check at 1800 and 3000. There is always some vibration at a certain RPM range, usually 3000 to 4000. If the prop was removed, the prop pitch may have changed. I would have it checked before prop balancing.

 

[Piper J3]

Keep in mind that carb balance is done by adjusting the position of cable sheath. You can try to find an optimal cable position to get good synch at low, mid, and high range, but its usually a crap shoot. You’re primary concern is to obtain good synch at idle and off-idle. This is necessary for gearbox longevity. So, you balance for idle, and then if you like, you can check mid-range. Highspeed will always have good synch because of large mass airflow with throttle plate nearly wide open.

Mid-range is where synch differnce shows up. My theory is.... don’t sweat it. Engine gearbox can tolerate vibration at mid-range speeds. The engine doesn’t normally operate at mid-range in flight. Speed transition low – high and high – low progress thru mid-range quickly. Only time I can think of for running mid-range is to extend a glide and then it is ony for a short interval…

My RV-12 is very early SN and now has over 1000TT. Most of that time has been with vibration at mid-range. I’m still running original gearbox with no internal engine maintence….

 

[docjjrmd]

Keep in mind that carb balance is done by adjusting the position of cable sheath.

When I perform carburetor synchronization, I check the vacuum at 1800, 3000, and 4000 RPM; some only check at 1800 and 3000. There is always some vibration at a certain RPM range, usually 3000 to 4000. If the prop was removed, the prop pitch may have changed. I would have it checked before prop balancing.

I agree most of the sync process is making sure throttle cables are adjusted correctly, its a coarse adjustment. Carbs should be balanced with equal vacuum draw after coarse cable adjustment to be complete and correct. You will hunt for optimum as you set low, mid and high. Low being the more important in my mind. Also remember that throttle cables must be in their final position when adjusted. Moving or changing bend radius will affect the cable throw enough to affect carb balance. I've seen perfect balance go haywire after cowl is reinstalled because of cable position change.

 

[CTCole]

Sounds like the shop has no experience with carb synchronization, let alone Rotaxes. Make sure they have the Mainenance Manual - Line for the 912, and make sure they do both the mechanical and pneumatic steps of synchronization. They'll probably need to get/make a dual vacuum gauge setup for the latter. Also make sure they don't set the idle too low, or the clutch dogs will chatter & wear.