Dgamble
Well Known Member
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
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