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Rough Engine at Mid-Throttle

Catbird

Well Known Member
I don't usually post technical questions to VAF. But when I do, it's only after exhausting every diagnosis that I can come up with. So, here's my problem:

I'm flying a first-generation RV-12 that I completed in May, 2012. Hobbs is up to 306 hours now, and every hour has been totally enjoyable. The Rotax 912ULS is equipped with the original Bing 64 carburetors and has not been modified from the factory setup. The airplane is based in north Georgia at a small field 1,790 feet above sea level. The weather in the mornings here is mild with temperatures in the mid-70's.

At the last annual maintenance and condition inspection, I removed the carburetors, disassembled them, cleaned them thoroughly, and replaced all rubber components in accordance with Rotax requirements and guidelines. Upon completion of the engine maintenance, I carefully vacuum-balanced the carburetors and made final adjustments. So far, so good, right? This is where the problem arises.

In the morning, before the afternoon heat builds up, I'll fire up the Rotax, let it idle until the oil temperature reaches 120F, and then slowly feed in the throttle. As the engine reaches ~2,700 RPM, it suddenly begins to run rough, as if one cylinder is missing. Further throttle application reveals that the rough operation continues until ~4,200 RPM. At that point, the engine smoothes out and develops full power. After operation for awhile, the problem goes away, resulting in smooth operation throughout the entire throttle range.

I've gone through both carburetors several times now, searching for anything that might compromise midrange fuel management, and found nothing.

If anyone out there has an idea, I'd sure appreciate your input. Thanks!
 
Did you disassemble both carbs at same time, or separately? Sometimes the enricher (choke) gets assembled on opposite carb by mistake if parts for both carbs are accessible at same time.
 
I'm by no means an expert on those engines, but I've watched some YouTube videos on them! Not as good as staying at a Holiday Inn Express, but it's all I have to offer.

One of the things I've heard as an issue with those carburetors as they age is the floats start to absorb fuel and get heavy, which throws off the amount of fuel in the bowl. Rotax has a very specific weight that the two floats should weigh--if it's more than that, they'll need to be replaced.

Looking in Vic Syracuse's Maintenance Handbook for RV Aircraft, it says, "The floats in the Rotax carburetors should be removed and weighed annually. I recommend they be done more often if using auto fuel. The spec is that together two of them cannot weigh more than 7.0 grams. We routinely find them weighing much more."

Not sure if that helps any or if that's one of the things you already looked at when you went through the carbs. If I'm wrong, I feel sure someone will tell me pretty quickly.
 
You may be getting fuel vapourising in the carb bowls. If you are running Auto/Mogas drain all the fuel out and replace with about 2-3 gallons of 100LL avgas and see if it happens again.
If it does, as stated above remove the carb bowls and weigh the floats. There is bundles of information, about this, on the web.
If you live in a high humidity area and are operating after a heavy dew, you may be getting carb icing and the increased warmth of the engine bay may be clearing it.

I've experienced all of the above on a Rotax.

Have a look at the above and report back.

John.
 
FLOATS!!! Now, why in the world did I not think of this? I'm still running the original floats installed by Bing probably 13 years ago. I've followed much of the float discussions throughout the years, but always took a sort of narcissistic pride in 'knowing' that MY floats were unaffected. I tried to weigh them during the last condition inspection, but the kitchen scales wouldn't go that low. So I threw them back into the float chambers and called it a day. I've ordered a replacement set from Lockwood this morning and will report back upon arrival, installation, and operation.
 
I installed Marvel-Schebler epoxy floats after many iterations with poor quality "Bing" floats...

You can buy very accurate weigh scale on Amazon for <$10
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FLOATS!!! Now, why in the world did I not think of this? I'm still running the original floats installed by Bing probably 13 years ago. I've followed much of the float discussions throughout the years, but always took a sort of narcissistic pride in 'knowing' that MY floats were unaffected. I tried to weigh them during the last condition inspection, but the kitchen scales wouldn't go that low. So I threw them back into the float chambers and called it a day. I've ordered a replacement set from Lockwood this morning and will report back upon arrival, installation, and operation.

If the problem wasn’t present before you took the carbs apart during the condition inspection, my guess is that replacing the floats will make no difference (and I wouldn’t spend the money on new ones without properly doing a weight check and confirming they were heavy).
The carbs have 3 operating stages.
Idle
Mid range ( becomes active at near the rpm your engine begins to run rough)
High power

My guess is one of your carbs has contamination in the midrange circuit.
 
Take off the air cleaners and raise up the vacumn slides to see if they go up and down smoothly, they raise and lower a metering pin. Could be one of them is sticking.
 
Alright, in the interest of full disclosure and hoping to prevent someone else from making the same mistake, I hereby submit my discoveries regarding rough engine operation at mid-throttle.

First of all, the floats all weighed in at 3 grams each. Four of them together on the scale weighed a total of 11 grams. So, we can disregard the floats as an issue.

I pulled the carburetors off the engine Monday morning and took them home for close examination on the workbench with good light and a magnifying glass. What I found was shocking. One of the throttle plates had been installed backwards. At first glance, it looked fine. But closer inspection revealed the problem. The leading and trailing edges of the throttle plate are cut on a bevel since the plate fully closes at an angle to the bore or throat of the carburetor. When reversed, the bevels don't even begin to seat up with the bore. This occurred when I had removed the throttle plates and shafts during the last annual maintenance and condition inspection for the purpose of replacing the O-rings on the throttle shafts.

Now, I will admit that I cannot determine the root cause of why a reversed throttle plate would affect mid-range performance. But I do know that, with the throttle plate fixed, the engine runs as smooth as silk throughout the RPM range, from idle to full throttle.

I do sincerely hope that someone can learn from this most frustrating mistake.
 
Its a good habit to take plenty of pictures before tearing anything down, also to do one carb at a time so you have a reference just in case, glad you fixed your problem.
 
Thanks for closing the loop by sharing your findings. Problem found and fixed...and fortunately, nothing really bad happened.
 
Thanks for the info. Mine also runs rough mid-range (has since I bought the plane) and I have not been able to figure it out. I've checked float weights,
Idle screw adjustment, mechanical sync, vacuum sync, carb to intake manifold o-ring, carbs overhauled, all with no joy. My ignition system is fine. Keeping my fingers crossed...
 
Screw missing from throttle plate

Found this last night on LH carb. Screws in other carb were tight. Removed screws, applied blue loctite and reinstalled. Borescoped cylinders, no signs of damage or missing screw.
 

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Jack those screws should be peened over to lock them in place. It takes help to do it and I used a steel rod in a vice to back the screw. And a punch to peen the threads.

Saw it done in a Rotax video and just followed their technique.
 
I agree with funflying in that we staked those screws when we rebuilt the Bings on our legacy RV-12. However, the latest MMH edition that I could find states:

"Install throttle valve with 2 new slotted head screws M3x6 secured with LOCTITE 603, ensure that it is reinstalled in the same position as marked at removal."

73–00–10
Page 16
Edition 2 / September 2022
 
Details count.

Kudos to the OP for not only finding the issue, but reporting the cause!! Two thumbs up.

Pictures are helpful for low memory, but correct throttle plate position is clear as day. The plates are machined at an angle so they will fit the cylindrical bore when closed. Once this is learned, one should find it easy to install properly. Back-in-the -day, I used to leave the screws touching but loose, snug with the plate closed, then tighten and stake.
 
She runs smooth now!

After 2 years of on and off troubleshooting, I inadvertently discovered my problem. While comparing another RV12 to mine, I noticed his choke arms are 180 out from mine. I confirmed in the manual that that his are installed correctly. Flipped mine 180 and all is well with the world. I'm embarrassed that I did not catch this sooner. Dog leg should be pointing down not up. 1st pic is wrong. 2nd pic is correct.
 

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I’m missing something in the translation…. I see your photos and understand that the actuating plate was installed incorrectly, however… it appears that the rotating brass shaft is still in same position when the plate is installed correctly. This doesn’t explain any difference in operation of the enrichment circuit (choke).
 
I’m missing something in the translation…. I see your photos and understand that the actuating plate was installed incorrectly, however… it appears that the rotating brass shaft is still in same position when the plate is installed correctly. This doesn’t explain any difference in operation of the enrichment circuit (choke).
Jim,
I don't what to tell you. Here's what I know: I flipped the arms over and I gained 100 rpms at full throttle static as well as an increase in my idle speed. I had to redo my mechanical rigging and rebalance the carbs pnuematically. As previously stated, it runs very smooth now.
 
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The brass shaft is in the same position in both pictures. But it might not stay in that position when the engine
is running because the spring has little tension in the first picture and greater tension in the second picture.
 
it appears that the rotating brass shaft is still in same position when the plate is installed correctly.

It may appear to be but it is not in precisely the same position.

The plates are not symmetrical in shape so when reversed and the plate is in contact with the stop, the shaft is at a slightly different position of rotation.
 
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