Carb imbalance: It can sneak up on you

Self confession time. It's good for the soul, but it can be embarrassing.

I recently turned around and returned to the airport with a rough engine.

Having ruled out an ignition problem, at least with regards to cheap, easy repairs, my thoughts turned to the carburetors. There are two of them on the Rotax 912, and each is responsible for the two cylinders on its respective side of the engine, So more specifically, I began to suspect a problem with the right hand carb. Lending some level of credence to this theory is the fact that there is a current Service Bulletin in effect on the 912 engine, and it has to do with defective carb floats absorbing fuel and becoming too heavy to effectively manage the fuel volume being fed into the engine.

The serial number of my engine is too low to have been caught by the SB, but not so low as to rule it out entirely, especially given the symptoms I was seeing. The nice thing about the SB text is that it includes directions for testing the floats for an overweight condition. You simply drop the float bowl and take the floats to a scale for weighing. If the sum weight of the two is less than 7 grams, you're good to go.

Now, when I say "simply drop the float bowl," I am ignoring the fact that the drip tray underneath the carb has to be removed. I'm not sure what that entails in airplanes other than the RV-12, but in the 12 it means removing the entire carburetor. On the right side of the engine, that is a little tricky in that it is a fairly cramped area. I managed to get it done, and the floats weighed in at a healthy 5 grams.

This was good news because Rotax has yet to release a replacement float, and when they do they will be wildly in demand, It was bad news, though, because it had exhausted my troubleshooting expertise.

That's when I called in a mechanic.

He listened to the engine for just a few seconds before determining that it was, in fact, a problem with the carbs and that the most obvious thing to do was to attempt to balance them. I was still in denial. I had always assumed that as long as I didn't diddle around with them, they would stay in tune. The mechanic squashed that idea like a flatworm under a steam roller: if that was the case, why would you need a periodic inspection?

Well, yeah, when you put it that way...

http://www.schmetterlingaviation.com/2014/11/vertigo.html

A few years back there was an article in an RVator that showed pulling the choke at altitude could help clear up any partial roughness due to something jamming the needle jet (page 9):

https://www.vansaircraft.com/public/RVator/2010/2-2010-RVator.pdf

While your carbs may have needed syncing (note that the video shows you should safety wire the linkages after syncing - I haven't done this yet), I wonder given the moisture content and ambient temps that perhaps you did have some minor carb icing? However unlikely?

Was the low/fluctuating REGT cleared up?

I have had great success doing the carb sync on my RV-12 without the need for an outside "assistant" or exposing one's self to the arctic blast that is the propeller slipstream. You can extend the sync vacuum hoses (I use "equal lengths" from the carbs to the gauges - although I don't think that matters much) from the two manifold connection points back into the cockpit via any readily accessible open hole (e.g., vent, window, door ajar, etc.). Attach lengths of monofilament fishing line to the two throttle arms and run the lines straight back into the cockpit. Now you can sit comfortably and safely in the cockpit monitoring the sync gauges while you gently tug on each carb's monofilament line to determine which carb needs tweaking. Then shut-down the engine, tweak the desired carb, restart the engine, and verify your settings. It might take an iteration or two to nail it. Works great for me and no one has to stand near the whirling propeller!

This procedure allows me to self-sync my carbs while avoiding the frigid prop blast - after all, yesterday it plumetted down to a wintery 69 degrees F here in California!

This type of thread subject substantiates the reason that checking carb sync with gauges at the 25 hr inspection ( cables will stretch when brand new, etc), and at each 100 hr inspection is specified in the maint. documentation.
After having done it a couple times, it only takes a few minutes to hook up the gauges. I also route the hoses into the cockpit to make it easy to do solo (in cold weather you can route them in through the fresh air vent a and then tape over the vent to reduce the draft).

I just reinstalled my floats, bowl gaskets and bowls with the replacement float kit today...I was able to do this on the left without removing the air filters, tight fit, but just loosen the main clamp, and the top retention spring and carefully pull the whole carb assembly out of the intake gasket....the right you must remove the air filter first (at least I couldn't make it happen - too tight). Of course being careful to not get too crazy since the cables and hoses are attached still, but that gives you enough room to stick a rag underneath to catch any spilling fuel and to release or reattach the bowl catch wire.

I proceeded to recheck the carb sync and the carbs were off approx 2" Hg....so re-synced carbs....

the weather didn't bother me...

well it was a little warm when I finished everything up...74 degrees.

Carb sync will always change over time. Cables stretch, pulleys wear, parts in the carbs wear, pilots pull to hard on the throttle, ect... This should be done at the 25 hr. Warranty inspection and at every annual condition or 100 hr.

I'm curious as to why folks do a pneumatic synch if their engines are idling smoothly.