What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Rotax 912 ULS Running Rough

bob1393

Active Member
I'm hoping Scott from Van's will jump in here and offer some ideas for us.

Last Thur, my wife and I, after doing a run-up in our RV12 prior to take off (run-up and all engine gauges perfect and in the green), Proceded to the runway, added power, and started take off roll. Everything fine until moment of lift-off. Engine starting running real rough, lots of vibration, loss of power. I immediately pulled throttle to idle and landed without any problem. Taxied off the runway (engine running great now) to a safe spot, then tried to do another run-up. Engine ran great until just under 4000 RPM, then ran rough again. It feels like the engine is missing or not running on all 4 cylinders. Tried several times with same result. Taxied to hangar and shut it down for the day.

The next day, my partner beat me to the hangar, he started it and it ran perfect. Even up to full power. He even flew around the pattern and did a couple of T&G with no issues. My partners and I have flown it several times this week with no problems.

I want to add that concurrent with the aborted take-off the Ducati VR failed. We replaced it. However, engine was running good when my partner started it before replacing the VR.

So the question is, can you Scott, or anyone else, provide any hints to us as to what we should check or look for? Would the VR failure cause it to run rough? When we started it the next day with a bad VR it ran great. BTW, engine has 445 hours, same as plane. Certified Oct 2015. Partner and I built it. Running flawless since certified.


Bob
 
Last edited:
Sure sounds like a bit of debris in one of the fuel bowls temporarily blocked a jet. Right after the event, did you pull off the two fuel bowls and look for debris? Since the engine is now running OK the debris may or may not still be there. You can look at your Dynon engine performance print-out to see if there are any noticeable anomalies or trends. I do not think that your temporary rough engine issue is associated with the Ducati R/R.
 
Prior post to check for debris in carb bowls makes sense.

Based on prior experience, you may want to check carb floats. They have had ongoing issues with absorbing fuel and gaining weight.

Also - carb balance.

I was in a friend's RV-12 when he had "sunken" carb floats and the behavior was pretty much identical to what you describe - so much so that takeoff was aborted.
 
I've had many overweight floats in my RV-12. In my experience, overweight floats do not create much of an issue at high-power settings -- the engine runs fine. The high fuel flow at high-power settings seems to diminish the negative effect of heavy floats. If I get a whiff of fuel odor during the engine's operation or see fuel residue in the carb trays, I suspect heavy floats. In any event, its a good idea to check the floats' weights and do a carb balance.
 
Last edited:
The voltage regulator does not supply electrical power to the ULS engine.
Other possible causes of rough engine are an engine driven fuel pump going bad or using winter gasoline when it is very warm outside (vapor lock).
 
When was the last time you had the carbs serviced? Rotax recommends every 200 hours.;)
 
I've had many overweight floats in my RV-12. In my experience, overweight floats do not create much of an issue at high-power settings -- the engine runs fine. The high fuel fuel flow at high-power settings seems to diminish the negative effect of heavy floats. If I get a whiff of fuel odor during the engine's operation or see fuel residue in the carb trays, I suspect heavy floats. In any event, its a good idea to check the floats' weights and do a carb balance.

My first set of bad floats acted OP's symptoms ..
 
Bob,
Particularly since it sounds like the problem is intermittent, in all likelihood it is a carb. / fuel delivery related problem.

What you are seeing is a common symptom of a carb. with some contamination or an out of spec. float.
Contamination can be a problem to resolve because sometimes you can even make an entire flight and not have it act up, but then it does again on the next.
A float problem is easy to identify by just removing them and checking the weight.
 
Thanks Scott and others for the reply. We have the new floats and did check them at 200 hours and they were fine. We did not check them again at 400 hours. We will check them now and the gascolator also to see if there is any debris or contamination as well as weigh the floats.

Bob
 
We will check them now and the gascolator also to see if there is any debris or contamination as well as weigh the floats.

Bob

If it ends up being debri causing your problem, it will be in one (or both) of the carbs. Finding nothing in the float bowls also doesn't mean it is not a contamination/debri problem. Disassembling the carbs and blowing out passages over a white sheet is sometimes the only way to tell that anything was there.
 
Thanks Scott and others for the reply. We have the new floats and did check them at 200 hours and they were fine. We did not check them again at 400 hours. We will check them now and the gascolator also to see if there is any debris or contamination as well as weigh the floats.

Bob

Still curious if this was an engine started from cold or was it warm(hot) from a previous flight...
 
Thanks Scott and others for the reply. We have the new floats and did check them at 200 hours and they were fine. We did not check them again at 400 hours. We will check them now and the gascolator also to see if there is any debris or contamination as well as weigh the floats.

Bob

I believe the 200 hour check Scott speaks of is a complete disassembly/rebuild of the carbs. The float check is every 25 hours. While I may not agree with a 25 hour check every time, all the time, it sounds like you are going waaay over what would be prudent.

https://www.rotax-owner.com/en/rotax-blog/item/34-912-914-float-inspection
 
I believe the 200 hour check Scott speaks of is a complete disassembly/rebuild of the carbs. The float check is every 25 hours. While I may not agree with a 25 hour check every time, all the time, it sounds like you are going waaay over what would be prudent.

https://www.rotax-owner.com/en/rotax-blog/item/34-912-914-float-inspection

I just checked the latest version of "SB-912-065 UL_SB-914-046 UL_R3_Periodic inspection of the float buoyancy for ROTAX® Engine Type 912 and 914 (Series)" from the Rotax/BRP website (flyrotax.com) and it included a note in the Compliance section that doesn't appear in the above link so that link might not be to the latest version of the SB.

The note was
"1.5) Compliance
NOTE: The installation of new floats as per SB-912-067/SB-914-048 “Exchange of floats” (latest issue) supersedes and cancels the requirement to comply with SB-912-065/SB-914-046 (latest issue).
- Before the first installation in the aircraft and/or the initial start-up. ..."
 
I just checked the latest version of "SB-912-065 UL_SB-914-046 UL_R3_Periodic inspection of the float buoyancy for ROTAX? Engine Type 912 and 914 (Series)" from the Rotax/BRP website (flyrotax.com) and it included a note in the Compliance section that doesn't appear in the above link so that link might not be to the latest version of the SB.

The note was
"1.5) Compliance
NOTE: The installation of new floats as per SB-912-067/SB-914-048 ?Exchange of floats? (latest issue) supersedes and cancels the requirement to comply with SB-912-065/SB-914-046 (latest issue).
- Before the first installation in the aircraft and/or the initial start-up. ..."

I think measuring the float weight is still part of the 200 interval carburetor inspection though.

One thing I would like to clarify..... the 200 hr inspection interval is not an overhaul interval. There is no requirement to purchase an expensive carb. overhaul kit every 200 hrs. Replacement of parts is on a condition basis at each inspection interval.
The part replacement requirement is on the 5 year replacement interval for rubber parts (diaphragms, carb. mount sockets, etc.)
 
While we're on the subject of 912ULS carb inspections/rebuilds...

We're approaching 400 hours. We suspect that the carbs are in need of some attention, and they certainly need inspection. We can ship them off to LEAF for a $485 overhaul (plus shipping both ways, of course) or buy parts and do it ourselves. I've rebuilt automotive carbs before, but it's been many years. My flying partner has much more recent experience with motorcycle carbs. That said... we know what we don't know.

Is there a compelling reason to ship them off to let someone else overhaul, or is this a no-brainer for guys who have some clue which end of the screwdriver is pointy?
 
While we're on the subject of 912ULS carb inspections/rebuilds...

We're approaching 400 hours. We suspect that the carbs are in need of some attention, and they certainly need inspection. We can ship them off to LEAF for a $485 overhaul (plus shipping both ways, of course) or buy parts and do it ourselves. I've rebuilt automotive carbs before, but it's been many years. My flying partner has much more recent experience with motorcycle carbs. That said... we know what we don't know.

Is there a compelling reason to ship them off to let someone else overhaul, or is this a no-brainer for guys who have some clue which end of the screwdriver is pointy?

My suggestion would be to look through the carb. inspection portion of the manual and see if it all makes sense you you based on your former experience. If it does, then it might be something you could do yourselves.
 
In addition to the RotaxOwner videos, there are a couple of videos on YouTube that depict the servicing of Bing carbs -- not necessarily by the book but they do have some good visuals of the carb's internals.
 
Is there a compelling reason to ship them off to let someone else overhaul, or is this a no-brainer for guys who have some clue which end of the screwdriver is pointy?

No-brainer.

CV Bings are very simple, and have not really changed in 40 years.

Worst case, you throw them in a box later and ship them to a shop.

FYI...We lost a 2 stroke Kitfox here some years ago and traced the fuel stoppage to a small piece of trash hidden in the carb's fuel inlet nipple, i.e just upstream of the the float valve. Nobody ever looks there.
 
Last edited:
Thanks Scott and others for the reply. We have the new floats and did check them at 200 hours and they were fine. We did not check them again at 400 hours. We will check them now and the gascolator also to see if there is any debris or contamination as well as weigh the floats.

Bob

Hey Bob, Were you successful in solving your carb problem? I have the same problem and I am currently working on it........Tom
 
I ran into the same issue in December and did an engine shut down and power off landing. Went through many carb clean/flush with no debris evident. Ended up draining all auto gas and switch to 100LL with fuel system flush. No problem since. Was thinking old ethanol gas had some fuel line deposits.
 
Todehnal - next morning it started and ran perfectly. We did swap out the Ducati Voltage Regulator. We've been flying it and have not had any issues. I'm thinking dirt or debris in fuel line someplace or in the carb that has worked itself through. We're coming up soon on the 500 hour inspection and we'll see what we find. Let me know if you uncover anything.

Bob
 
Hey Bob, glad yours is running well. I'm still in trouble here. I replaced the plugs, pulled the float bowls and inspected. I did adjust the fuel level slightly. Those were the easy things. Next, I plan to pull the carbs and do a tear down inspection. From the replies that I've received, most suggest that the likely suspect is a carb problem. It is hard to diagnose. You would think that a shut down and inspection of the plugs after a rough run situation would point you to the problem. Apparently that is not the case with Rotax. I understand that the rough running shakes the **** out of the carbs, the floats go nuts and flood everything. In my case, all four plugs in cylinders #1 and #2 were totally sooted up, as was the tail pipe. I will post any finds...........Tom
 
Its all better now.

This is Art, one of Bob's partners. The problem with our rough running engine was some contamination in one of the carb float bowls. We think the contaminant was from chaffing of one of the floats. Keith (one of the three owners) found a small initially hard object. After extracting it from the float chamber he was able to crush it between his fingers. We think it would intermittently clog the jet causing the engine roughness. We have flown several hours since without any problems. Also, Lockwood in FL was very helpful in steering Keith in the right direction.
 
Back
Top