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Old 11-25-2013, 07:28 PM
yankee-flyer yankee-flyer is offline
Join Date: Jun 2009
Location: Dayton, OH
Posts: 838
Question Different rough engine problem

After finishing the 200-hr inspection, C.I., and pump replacement (in that order) I now find that when I've been at Flight Idle (throttle closed. RPM 2100-2600 depending on airspeed) that when I add power again the engine "stumbles" noticeably for something LESS than a second-- rough enough to shake the panel. Goes away when power is back on and isn't really noticeable on the ground. I can get a little bit of roughness (less than the "stumble" at power settings of 2300-2800 in flight at about 60-65 knots. Full power is normal as is the rpm at 75 kt. climb.

My first guess is that the carbs are slightly out of synch at low power settings. Don't know how it could be ignition since it's not a steady thing. I did not pull the carb bowls after installing the pump (flushed the lines, though) but here again I'd think anything in the carb would be continuous, bit momentary and at one rpm range..

I'd really like some thoughts from those who know and understand the Rotax.


Wayne 120241/143WM
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Old 11-25-2013, 08:04 PM
RFSchaller RFSchaller is offline
Join Date: Oct 2010
Location: Phoenix, AZ
Posts: 3,161

Wayne, is your electric fuel pump on continuously? Just wondering if your fuel flow burps.
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Old 11-25-2013, 08:59 PM
yankee-flyer yankee-flyer is offline
Join Date: Jun 2009
Location: Dayton, OH
Posts: 838
Default Yes it is

I haven't been watching the fuel flow readout or psi

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Old 11-26-2013, 05:46 AM
KALEWIS's Avatar
Join Date: Nov 2009
Location: Jackson, OH
Posts: 491

Do you run avgas? I have heard the blue dye in it may affix itself to the carb float stems and gunk them up causing roughness. MitchL has details on that
Airspeed, Altitude, or need at least two

RV-12 #328
At 630 Hrs on the Hobbs
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Old 11-26-2013, 07:19 AM
BigJohn BigJohn is offline
Join Date: Nov 2008
Location: Gloversville, NY
Posts: 1,588


You say you suspect a carb sync problem, but you don't say if you checked the carb sync at the CI. I would suggest re-checking with a vacuum sync instrument at the low idle position before digging into the carbs etc.

If this behavior started coincidentally with the work you mentioned, it is most likely caused by something that occurred during that work. Double (triple?) check everything you did.

If the carbs are in good sync, then check your fuel pressure. There have been reports of high pressure readings from the newer pumps. You could also try to see if the stumbles occur with the electric fuel pump off, as well as on. Pull the fuse if you don't have a switch.

Good luck with the detective work!

John Peck, CFII, A&P, EAA Tech Counselor, Flight Advisor.
Master Pilot Award, UFO Member.
RV-12 N37JP 120176, built 2011, sold 2020.
One Week Wonder Build Team, OSH 2018.
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Old 11-26-2013, 08:41 AM
roger lee roger lee is offline
Join Date: Sep 2013
Location: Tucson, AZ
Posts: 322

Hi Wayne,

John has some good ideas and I'm not thinking electrical here either. I would add these two and they are easy to do. First make sure when you operate the throttle in the cockpit that both carbs are opening equally and at the same time. Some cables do have a hitch in them. Check the float bowls. It can be done with only a screwdriver. Watch this short video. It is for a different plane, but has the technique that is quick and easy to do and not have to get too deep into a project. So check the bowls for debris, but also fill the bowls with fuel up to about 1/4" from the top and put your floats back in and see if one may be sinking. The brass pins that stick out the sides of the floats should be even with the fluid level and not down under the fuel level. Then I would check the sync again. Use gauges and not an electronic sync instrument. You can't diagnose with the electronic ones. You need gauges to do a diagnosis and see which carb may have an issue and they can discern between several issues.
I don't think this is a sinking float, but since you have the bowl off you might as well check. Do the sync at idle (1700-1800) and up around 3500 rpm. John's test with only one fuel pump is another good idea. Personally I'm not a fan of running both pumps at the same time. If things all come together at the same time just right with both pumps it could have a slight flooding issue and stumbles as fuel is added until the engine runs faster to use that extra fuel. Not all duel pumps cause flooding, but is something you need to check and rule in or out during your diagnosis.
Roger Lee
Tucson, AZ
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Old 11-26-2013, 09:21 AM
yankee-flyer yankee-flyer is offline
Join Date: Jun 2009
Location: Dayton, OH
Posts: 838
Default Thanks guys

I'm not running avgas so that's not the problem but I will do the other checks as soon as we get a warm enough day to stand in the propwash. Haven't been watching the fuel pressure since I'm typically getting this on very short final as I add power to adjust flight path. The carbs were synched at the 200-hour (with gauges) but the new pump was added after that.

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Old 11-26-2013, 09:26 AM
Ueli N Ueli N is offline
Join Date: Oct 2013
Location: Blaine WA
Posts: 28

if nothing else helps, re gap the plugs to your old setting!
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Old 11-27-2013, 07:57 AM
Peterk Peterk is offline
Join Date: Mar 2008
Location: Texas
Posts: 1,378
Default Carbs?

I may be wrong but I seem to remember the Rotax engine manual recommendation to tear down the carbs at 200 hours.
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Old 11-27-2013, 08:42 AM
unclelar unclelar is offline
Join Date: Jan 2013
Location: Johns Creek, GA
Posts: 20
Default RV-12 Stumbles

I have over 800 hrs behind the Rotax on a Titan Tornado. I've never used an aux pump so I don't know about that issue. You don't need to tear the carbs down unless you have elminated other things. Make sure that you have the carbs synched at your lowest idle. As you go up in RPM the butterfly opens more and more so that they pretty much have to be synch at 75% power or so. It definatley sounds like the carbs. Make sure that your choke is not partially open too. Check all of the springs on the carbs. Make sure that the air filters are tight and not letting air in. Maybe try the forums on the Rotax Owner website. Good luck.
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