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Different rough engine problem

yankee-flyer

Well Known Member
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.

THANKS!!

Wayne 120241/143WM
 
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
 
Wayne,

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
 
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.

https://www.youtube.com/watch?v=JdS7OtbcxwY
 
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.

Wayne120241/143WM
 
Carbs?

I may be wrong but I seem to remember the Rotax engine manual recommendation to tear down the carbs at 200 hours.
 
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.
 
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.

Because of an RV-12 builder reporting that he felt that his engine was now running richer (based on spark plug appearance) than it had before he installed the new fuel pump, Rotax was contacted.

Rotax says "that as long as the max. fuel pressure limit is not exceeded, a difference in fuel pressure will not cause a difference in fuel mixture".
 
Don't know if this is your first winter flying season but on my Kitfox I need to drop the clips on the carb needles for cold weather ops or I wil get some roughness typically in the traffic pattern. Colder temps = leaner mixture. You might also have to tweak the idle mixture screws just a bit.

JimS
 
I had a similar problem on a Continental O-200 in another home built. Every time I advanced the throttle for takeoff it initially stumbled. The plugs were sooty so I thought it was mixture. Eventually I tracked it down to running main and backup electric fuel pumps at takeoff. Running just one pump solved the problem.
 
Plug gap OK

I checked the plug gap and it's between .020 and .025. Engine start is fine and I can't reproduce the problem on the ground. It does NOT happen when going to full power for takeoff. ONLY when I;\'ve been at flight idle and the rpm drops below 2500 (usually down to 2100-2200). The the stumble happens as It passes through 2800 or so.

Wayne
 
I'd guess a carb problem. Had a similar problem on a Jab3300 powered IndUS (remember them?) with the same Bing carb - applying power from idle (T&G) resulted in hesitation and rough running for a few seconds. Cleaned carb bowl and jets, problem solved.

FYI, this is what we call a "teachable moment" for a low-time primary student: Putting power in on the go for T&G, engine hesitates, pull power back, and instructor says "what do we do?". Student pulls power off, exits active, heads for ramp. Instructor and student discuss and agree this was the correct decision.

TODR
 
FIXED!!!

Getting ready to do another carb synch, I found that the right carb choke was about 1/3 "on" although the choke was off. A quick look showed that the cable for the right carb choke was out of the knurled fitting on the upper end of the 90 deg turn tube. Sticking it back into the tube brought the choke arm back against its off stop. Pulled the cable out, lubed both ways, put it back in and tightened the knurled fitting as much as possible. Checking the left carb I found that the cable would wiggle in the fitting but wasn't loose enough to come out. Tightened that too. After waiting almost two weeks for snow/ice to melt off taxiways (I LOVE Ohio!!) flew today and engine response was perfect.

My guess is that after 3 years and 200 hours vibration loosened those knurled fitting. One more item for the C.I> and to check every time the cowl is off.

Wayne 120241/143WM
 
Similar problem with badly gapped plugs

if nothing else helps, re gap the plugs to your old setting!

We could never get smooth running at all speeds on our engine--if we synced the carbs at 2200, things go rough at 3200, etc. After about ten tries, we checked the plug gaps (as it came from Rotax), and ALL of them were consistently at 0.017, rather than the speced 0.027. After regapping the plugs, syncing the carbs a 2200 (or whatever the recommended RPM was), and things were smooth everywhere, except a slight vibration that may be the result of not yet dynamically balancing the prop.
 
Just an FYI,

When I have choke or throttle cables not secured I always safety wire them in place so they can not work back and forth as levers are pulled and pushed in the cockpit. Start the safety wire about 1" back and snug it. Then twist it forward up to the knurled adjuster and snug it again. The bring the safety wire forward again and secure it around the forward part of the knurled nut. I call it leashing. Works great and the cable can't move. I also prefer to sync at 3500 because 2200 is too close to idle and doesn't flow enough air/fuel to do a good job. We fly at higher rpms and not down around 2200. That's why if you sync at 2200 and then advance the throttle up to 3500+ the needles many times will be out again. Plug gaps are .023 - .027. The wider gap is good for hotter weather and the narrow gap for cold winter weather. In southern AZ. we use .025 because our winters aren't that cold and if I lived where it was really cold I would use .023. It will make starts easier.
 
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