Shudder

[Jim Meade]

What different symptoms, or are you guessing?

[FlyingMonkey]

Carb ice in most (single carb) engines causes loss of RPM and engine power eventually leading to engine stoppage. In a two carb setup like the Rotax, I could see the shuddering as a symptom if one carb iced first, since you'd get loss of power in just HALF of the cylinders, leading to uneven forces on the engine.

 

Yes, I am just speculating. Is that allowed?

[Roger Lee]

Speculating, NO because when you ask a question it goes 9 pages of post. Just kidding

Variable throat carbs (Bing 64) can form ice, but it's harder to form than a standard throat carb.

[Tom Baker]

Jim, I have never had carb ice on a Rotax that I know of. What Chris is saying makes sense, because if the carbs form ice at a different rate then the engine will be out of balance due to one side producing more power than the other.

[Roger Lee]

The UK's weather is optimum for carb ice if it is going to form in a Rotax.

[Jim Meade]

Jim, I have never had carb ice on a Rotax that I know of. What Chris is saying makes sense, because if the carbs form ice at a different rate then the engine will be out of balance due to one side producing more power than the other.

 

Tom,

I accept that theoretically one could have differing amount of ice on each side, but it's hard for me to see it as likely. First, as you say, Bings don't ice up well. Second, I don't think his symptoms necessarily reflect carb ice, though I'll grant that differing carb icing might have unusual symptoms. Thirdly, it's hard to imagine why one side would ice up and the other not.

 

Well, I'm not saying it can't happen, only saying that it doesn't seem like the most likely or obvious cause. To me. It will be interesting to see how the conversation develops.

[207WF]

Do you have a fuel pressure gauge? Any fluctuations in that? I had similar symptoms, eventually traced to a fuel pump that had failed in such a way that you had no symptoms unless at high power and high altitude, where you are demanding a lot from the pump. When it happened, I reduced power everything seemed normal. There was sort of a "thud" when the engine surged. This was so intermittent that it took a long time to figure it out (like, 2 years). I don't have fuel pressure indicator but am now thinking about adding one. WF

[mocfly]

No pressure gauge. The thud was an interesting comment. I experienced a thud that felt like something hit the back part of the fuselage.

 

Is there a written procedure to check the pump?

 

 

 

[Roger Lee]

Hi Chris,

 

Yes in the Rotax manuals.It is just a pressure gauge inserted in the fuel pressure side out line. The gauge needs to read down low between 0-10 psi.

[mocfly]

Roger,

Thanks for that.

[Roger Lee]

You can buy those parts at Ace Aviation for about $15-$20.

[FastEddieB]

Using one I already had to confirm (actually dis-confirm) low fuel pressure readings on my Sky Arrow:

 

[207WF]

Can you see that from the cockpit in your sky arrow, Fast Eddie? WF

[Brucegoose]

sorry, I have corrected my posts! I meant vent tube and not balance tube.

 

Please re-read, hope my experience makes more sence now.

 

Bruce

[mocfly]

Bruce,

Can you expand on your reasoning on the vent tube?

Thank you

 

 

[Runtoeat]

I've also recently had vibrations similar to Chris' at mid 4,000 rpm. running @ <2,500 or at WOT @ >5,000 rpm the problem goes away. I experienced this during a very cold flight in 8 degree F. Question: Would a dislodged balance tube (running thru the airbox) or unbalanced carbs cause rough running in the mid 4,000 rpm range and not at >5,000? In other words, does uneven balance of the carbs stop being a factor when the high speed portion of the carb circuit comes into play? Are there any other areas like stuck slides, problem with floats, that might cause poor mid-range rpm running? I will be looking inside the carbs for any problems and will try to find warmer temps to go back up and see if I might have experienced icing but this has never been a problem with my plane.

[sandpiper]

I've also recently had vibrations similar to Chris' at mid 4,000 rpm. running @ <2,500 or at WOT @ >5,000 rpm the problem goes away. I experienced this during a very cold flight in 8 degree F. Question: Would a dislodged balance tube (running thru the airbox) or unbalanced carbs cause rough running in the mid 4,000 rpm range and not at >5,000? In other words, does uneven balance of the carbs stop being a factor when the high speed portion of the carb circuit comes into play? Are there any other areas like stuck slides, problem with floats, that might cause poor mid-range rpm running? I will be looking inside the carbs for any problems and will try to find warmer temps to go back up and see if I might have experienced icing but this has never been a problem with my plane.

 

Mine has done this every winter since new. If I really get ambitious I balance the carbs and it mostly goes away - until the next winter.

[mocfly]

I am curious what each carb temp is when we all experience the shudder.

The only times I have experienced this shudder is with -32f OAT.

 

[Runtoeat]

John, What engine speed(s) do you get the vibration? Am I correct that you can clear up your vibration by balancing the carbs? This happens during winter season but not during warm temps?

 

John and Chris. Let me throw another variable into this. Have you measured the "torque to turn" past your gearbox clutch? My torque is around 24 ft.lbs. Low side of spec is 22 ft.lbs. I've been told that this low torque has caused vibrations in other CT's. Wondering if low temps might make this worse? Do you guys know what your your torque might be? Easy to measure if you've got a fish scale.

[sandpiper]

Dick - the temps I am talking about are probably warmer than yours. Here in the Willamette Valley I'm talking between 35 and 50 degrees F where I notice this on a typical winter day. It's most noticeable between 4400 and 4800 RPM and is subtle but noticeable. When I check carb balance after experiencing this, it will be a little off. Doing a fresh balance make it almost unnoticeable. Sometimes I don't bother.

 

I do the torque measurement each annual. I am not where I can check my logbooks right now but I remember the reading to be mid range.

[Runtoeat]

Thanks for the reply John. I've noticed the vibration you mention. The vibration I've got is very noticeable - similar to the vibration resulting when the carb balance tube gets dislodged from the airbox or becomes disconnected from one of the carbs. I'll have my carbs inspected and rebuilt/rebalanced. If the vibration remains, I think I'm headed for a gearbox teardown and rebuild.

[Rich]

Once upon a time in a far off land, much different than what we know today, floats were made of bronze.

Imagine, no need to worry about a critical part of the fuel system flaking off and causing a reduction in power and engine vibration.

 

Even the corn we use for fuel today couldn't eat it up.

[Doug G.]

Gotta save weight somehow. :-\

[mocfly]

I will check the torque and advise.

 

[Roger Lee]

A little vibration can be fairly normal. Remember your engine was designed to run at 5000+ rpm all of the time (unless you slow to sight see a little or land) and rpms in the 4000's can generate a slight vibration. That said many that experience this is because the carb sync is off a little especially if you only did a sync at 2500. This is why i always do my high rpm sync at 3500+ rpm. The other cause usually for higher time engine can be the gearbox needs a re-shim.