Rough Engine at Mid-Throttle

[Catbird]

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!

[Piper J3]

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.

[D-Dubya]

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.

[G JWTP]

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.

[Catbird]

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.

[Piper J3]

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
-

[rvbuilder2002]

Originally Posted by Catbird

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.

[bobg56]

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.

[Piper J3]

Brian Carpenter @ Rainbow Aviation Services explains the inner workings of the Bing 64…

https://electricmotorglider.com/wp-c...V-Carb-Web.pdf

https://electricmotorglider.com/wp-c...Part-2-web.pdf

[Catbird]

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.