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Engine power issue on takeoff

backcountry

Well Known Member
A couple of times during the takeoff run I?ve experienced the engine shake and loose some power for a few seconds and then return to normal operation.
This so far has only happened during the first takeoff of the day.

The outside temperature is presently lower than normal, however don?t takeoff before reaching 120 degree engine temperature.

Engine has approximately 135 hours.
 
Are you running Mogas or 100LL ?

I believe oil temp needs to reach 123 F before doing your run up. Of course, run up and ignitions switch rpm drop checks are to be done long before take off.

Let things warm up a little longer before you take off, and get back to us.

Read your POH for more familiarity with your RV-12 and Rotax 912 engine.
 
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Backcountry-

Very early on during my initial flight testing (something like the third flight or so with fall temps like we are having now) I had something similar happen during takeoff. I was climbing out and had a sudden loss of power and a rough running engine accompanied by a slight whiff of fuel. I pulled the throttle back a eased it back in little and things seemed to smooth out up to about 3900 rpm ... so I made it around the pattern and landed. My initial thought was vapor lock from the winter auto fuel ... but I was not on the ground that long, plus it was a cool day.

Upon landing, I downloaded the Dynon SkyView's recorded data to a flash drive and looked at the data in a graph generated by Savy. It was easy to see what happened ... during the initial onset of the "event" the left EGT got cooler than the right, RPM dropped from 5250 to 4000 ish, the fuel pressure dropped a little and fuel flow increased a little. By all appearances the left carb went rich.

Removed the left carb and there was nothing in the bowl, floats were just fine ... so I removed the float arm and needle then turned on the electric fuel pump so I could flush a little fuel through the system and found what appeared to be a tiny grain of sand appear in the fuel containment container. I'm guessing that tiny piece of grit kept the needle valve open and allowed excess fuel to enter the bowl making that carb go rich ... I have not had a reoccurrence since.

Using Savy really helped me get a clue as to which side of the engine needed the attention ... suggest you may want to analyze your flight data ... it may help get you pointed in the right direction.

Happy flying,
 
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I already replaced the carb floats before this issue occurred. The engine now runs extremely smooth with no gasoline fumes smell. I?ve always used 93 octane non-leaded. The issue I?m now experiencing is that sometimes only during the first take off of the day I get a brief Engine roughness and a drop in rpm?s for a few seconds.
The only time I?ve seen it is the first take off of the day and even this doesn?t always happen. So far it has only occurred twice.
During these cooler days I may try to let the engine warm up to maybe 135? before takeoff and see if that helps.
 
I already replaced the carb floats before this issue occurred. The engine now runs extremely smooth with no gasoline fumes smell. I?ve always used 93 octane non-leaded. The issue I?m now experiencing is that sometimes only during the first take off of the day I get a brief Engine roughness and a drop in rpm?s for a few seconds.
The only time I?ve seen it is the first take off of the day and even this doesn?t always happen. So far it has only occurred twice.
During these cooler days I may try to let the engine warm up to maybe 135? before takeoff and see if that helps.

Let the OIL TEMP indicated warm up to over 123F...

No one knows what you mean by letting the engine warm up to maybe 135.... based on which gauge?
 
I already replaced the carb floats before this issue occurred. The engine now runs extremely smooth with no gasoline fumes smell. I’ve always used 93 octane non-leaded. The issue I’m now experiencing is that sometimes only during the first take off of the day I get a brief Engine roughness and a drop in rpm’s for a few seconds.
The only time I’ve seen it is the first take off of the day and even this doesn’t always happen. So far it has only occurred twice.
During these cooler days I may try to let the engine warm up to maybe 135° before takeoff and see if that helps.

There are other carb problems that can cause issues that aren’t related to sinking floats

There have been many rough engine problems caused by contamination within the carb. These are often intermittent because the partial causing the problem can move around.

The temp requirement for ops above 2500 RPM has nothing to do with engine performance / operation.
It is to prevent cavitation of the oil pump.
Since the pump sucks oil from the tank, when the oil is cold and more viscous there is resistance working against the pump sucking oil from the oil tank.
 
There have been many rough engine problems caused by contamination within the carb. These are often intermittent because the partial causing the problem can move around.

Not sure what "partial" means in this sentence, Scott. Maybe a typo? Did you mean Particle? Like in a particle that would cause some sort of obstruction of fuel flow?
 
The temp requirement for ops above 2500 RPM has nothing to do with engine performance / operation.
It is to prevent cavitation of the oil pump.
Since the pump sucks oil from the tank, when the oil is cold and more viscous there is resistance working against the pump sucking oil from the oil tank.

Good information - I didn't know reason for min temp before flight.
 
That is really interesting (Scott?s data point)

One thing that I have seen that I believe is the cause of a lot of people?s ?power loss on climb out? issues is the carbs going out of sync. These carbs are different to the other designs that we are used to with the piston moving with airflow. I think a lot of people remove the float trays and clean them out and the issues still remain. If you are having issues, take the carbs apart clean all the brass items check the o-rings etc. but also thoroughly clean and then lubricate you pistons and ensure that they move up and down in their travel with no resistance.

Having experienced power loss on climb a couple of times behind a rotax, it appears that it?s a significant loss but actually it?s out of balance and in both instances when I pulled power to 4000 to 4500 rpm the engine smoothed out again.
 
One of the Tecnam P2006T twins suffered from an intermittent rough running Rotax (one of the two...) that occurred only in the climb.

We found a very small piece of orange 'stuff' in one carb float chamber that I think was probably out of/from the fuel supply hose.

Cleaned the float chamber out and the fault never reoccurred.
 
Scott, I?ve never heard of cavitation in a positive displacement pump. I?d like to dig into that just for my own personal education. Is there a Rotax document that talks about that?

Rich
 
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