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Vibration and strange EGT readings at small RPM range. (rotax)

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An interesting issue for those who know Rotax's for the last few hours of flying, the 912ULS has developed a niggling vibration, this vibration shows up when moving from full power to max power in the take off roll, usually at about 3/4 throttle, once at full power, its ok. in cruise at 5100 rpm it is very slight, 5200 RPM is smooth as should be, just noticeable vibration. at 4700 rpm, its as smooth as a rotax should be, between 4700 and 5100 rpm, the vibration is very noticeable..
to try to fix this, i have done the following.. Balanced carbies, balanced the prop, checked prop track and pitch, all good. Leads have been tested anre are within limits, but not new. carbies have been recently overhauled by rotax. engine still starts fine, very quickly, even when cold.
 
so, 2nd last flight, i performed a high power mag check at 5100 rpm results were.
Mag 1 off, EGT's all changed the same amount.. back on.
Mag 2 off, EGT on cylinder #2 steadily fell from 700 deg C to about 300 deg c, back on, all back to normal.
(#3 EGT failed completely and reads 0, and still read 0 for the next testing)
 
so, 1 spark plug change later. (all plugs looked fine), back at cruise power, again, same test, same result. hmmm
, back on both mags, i started experimenting with RPM settings to see at what RPM the vibration was the most noticeable, and the results were interesting.
 
4700 rpm, no vibration,
4800 rpm, small vibration..
4900 rpm, largest vibration, but also, the same EGT drop on #2 Cylinder!! from about 700 to 500 deg C.
5000 rpm, same, still lower EGT on #2.
5100 rpm, #2 EGT comes back up to normal temps around 700 Deg. in line with other readings.
 
Any rotax engine guru, have an idea on what might be happening?
 
Thanks.

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I'm a non-guru.  Worth checking the the throttles open smoothly in harmony through the range idle to full throttle.  I regularly give the wire lock item on the throttle arm a couple of drops of oil to ensure smooth rotating as the throttle to moved through its range.  Also see if any tight spots in the throttle method say inner wire grabbing the outer sleeve or loose rod connections if a rod / lever system.

 

If above happening one carb will lag due to slower opening in that rpm range giving an imbalance; perhaps??

 

Cheers

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How old and how many hours does your 912 ULS have?

 

Why did you have your carbis overhauled?

 

Your carbi balance technique might be worth looking at - please give an action by action description.

 

How long (hrs) since you serviced the air cleaner(s)?

 

Is your carbi balance tube secure?

 

How many hours/years since rubber replacement? & were all rubbers associated with the carbis including the manifolds replaced at this time?

 

Are your fuel delivery in good condition & secure?

 

Approximate fuel consumption at  your usual rpm's ?

 

Prop brand & model ?

 

How did you balance the prop?

 

Is there any unusual movement in the prop?

 

Check all plug leads are secure. (there is a Rotax spec for plug cap security).

 

Plugs - as specified by Rotax ?  Gaps ? Torqued to the recommended weight?

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 There's an instruction to check the crankshaft drive end as some have been cracking. I'm sure it's on the Rotax website as I've seen it myself.  Nev

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1 hour ago, facthunter said:

 There's an instruction to check the crankshaft drive end as some have been cracking. I'm sure it's on the Rotax website as I've seen it myself.  Nev

I am under this SB. I'm starting to get irrate as it's been out for 5 yrs...

Need to check the crank every 100 with special tool.

Rotax need to find a resolution.....Remove the SB or compensate accordingly.....

 

As for the problem at hand. Could it be the transition from neeedle to main jet?

Check carb diaphragm and needles..... clip in needle 3rd slot down as factory setting.

I'm running 2nd slot down......slightly leaner.

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It appears to me that No 2 on mag 1 is not firing correctly between 4800 and 5100rpm. I doubt that the needle could cause that. With both mags on it is still rough so I would be looking for a problem in the plug, wiring or distributor on mag 1, but that should really slow the combustion process and raise EGT.

Do you have CHT figures, they could help in diagnosis. I would expect a drop in CHT when the roughness occurs especially during the rough periods on no 1mag.

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A silly reply

BUT

Have the carbies any play on their "butterfly shafts"

I have Seen a very weak mixture on one cylinder due to the Shaft being in that Zone, letting more air in than when in a different position.

I possess a "Glass spark-plug" that lets me See the firing "colour".

spacesailor

 

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Are your float bowl vent hoses in the correct place, and not kinked or damaged?

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Posted (edited)

The main time for roughness is generally on part  throttle descents and I've always put this down to balance between the carbs. The inlet manifold set up is not optimal for even mixture at varying throttle positions.. Make sure your plug gaps are wide enough.   Is there any possibility of rubber connections being cracked. Nev

Edited by facthunter

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This isn’t happening at part throttle. At the revs it happens at there will be a fairly high pressure in the inlet and little chance of incoming air to dilute the mixture.

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We are not hearing from Ultralights - without his/her input little headway can be made.

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I'll second what Mike said about the carby cables.
Remember that your throttle control doesn't drive the carby position. The springs do that. If there is any drag in the cables at any point, it could cause an uneven actuation of the throttle. It might be worth completely pulling the cable inners out and inspecting them careflly for wear or taipans. And while you have them out would be a good time to lubricate them.

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14 hours ago, cscotthendry said:

I'll second what Mike said about the carby cables.
Remember that your throttle control doesn't drive the carby position. The springs do that. If there is any drag in the cables at any point, it could cause an uneven actuation of the throttle. It might be worth completely pulling the cable inners out and inspecting them careflly for wear or taipans. And while you have them out would be a good time to lubricate them.

Sorry Scott but I THINK YOUR ADVICE REGARDING LUBRICATION IS LIKELY TO BE  WRONG. - most ( not all) control cable of this type are not designed to be lubricate and are likely to react badly to the use of most normal lubricants.

 

Regarding the main thrust of your advice - that is that the cables are operating smoothly throughout their full range of movement - I agree this should be checked. If one or both cables are "sticking" they should be replaced.

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 It affects ONE carburetor (2) or possibly both. It pulls to close. The idea being if it fails, the engine still gives power. This is not a system I fully agree with but it's what it is. You really have to tug on the throttle to ensure it's fully closed at times.  I'm  Not against lubricating cables of this type, (unless there's something I'm missing). They work more freely and wear less and are no different from those used on motorcycles. which benefit from being able to be oiled and some have an oiler in the middle roughly to facilitate this.  Sharp bends should be avoided if possible. Once they start to fray they can jamb. Conventional ones  that don't spring load to open, should duplicate the fixing of the end fittings as coming loose gives a failed engine (back to idle)  Nev

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4 hours ago, facthunter said:

 It affects ONE carburetor (2) or possibly both. It pulls to close. The idea being if it fails, the engine still gives power. This is not a system I fully agree with but it's what it is. You really have to tug on the throttle to ensure it's fully closed at times.  I'm  Not against lubricating cables of this type, (unless there's something I'm missing). They work more freely and wear less and are no different from those used on motorcycles. which benefit from being able to be oiled and some have an oiler in the middle roughly to facilitate this.  Sharp bends should be avoided if possible. Once they start to fray they can jamb. Conventional ones  that don't spring load to open, should duplicate the fixing of the end fittings as coming loose gives a failed engine (back to idle)  Nev

On Rotax 912's  (all 4 cylinder boxer engines with two carburettors) there are a range of carburettor actuating systems, however the most common would seem to be:

  • Each of the two carburettors is controlled by its own cable. The two cables are operated by a single leaver (& sometimes rods). Synchronising/balancing the two carburettors can be a time consuming and frustrating activity.
  • Perfect synchronisation/balance (much aspired too) is unlikely throughout the rpm range - this is accommodated/moderated by having a vacuum balance/compensating tube running between the two manifolds to iron out small differences. In practise this means that small differences between the two carbi settings are shown up at low rpm/high vacuum (rough running &/or failure to idle at low rpm) , not at take off & cruise power.
  • Default open, that is if the cable control fails the carburettor will go the the full power setting. This is achieved by a spring on each carbi pulling against  the closing action of the single acting (not a pull/push system) throttle cable

 

As an obsessive lubricator (I love applying grease/ oil to every moving part) even I know that oiler caps/nipples/etc on cables went out many years ago. Modern cables have a teflon like inner sleeve which is NOT designed to be lubricated and may in fact be compromised by such loving care. If you dont believe me check with your favourite custom cable fabricator.

 

Good advice regarding the carbi cable end fittings - not trusting the single screw clamp fitting to remain secure, I implemented a fail safe second fitting on mine, some 8 years ago. Never had a cable  slip but feel happier that if it does I will still have some control of the carbi, albeit out of  synchronisation.

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On 3/18/2019 at 9:19 PM, skippydiesel said:

On Rotax 912's  (all 4 cylinder boxer engines with two carburettors) there are a range of carburettor actuating systems, however the most common would seem to be:

  • Each of the two carburettors is controlled by its own cable. The two cables are operated by a single leaver (& sometimes rods). Synchronising/balancing the two carburettors can be a time consuming and frustrating activity.
  • Perfect synchronisation/balance (much aspired too) is unlikely throughout the rpm range - this is accommodated/moderated by having a vacuum balance/compensating tube running between the two manifolds to iron out small differences. In practise this means that small differences between the two carbi settings are shown up at low rpm/high vacuum (rough running &/or failure to idle at low rpm) , not at take off & cruise power.
  • Default open, that is if the cable control fails the carburettor will go the the full power setting. This is achieved by a spring on each carbi pulling against  the closing action of the single acting (not a pull/push system) throttle cable

 

As an obsessive lubricator (I love applying grease/ oil to every moving part) even I know that oiler caps/nipples/etc on cables went out many years ago. Modern cables have a teflon like inner sleeve which is NOT designed to be lubricated and may in fact be compromised by such loving care. If you dont believe me check with your favourite custom cable fabricator.

 

Good advice regarding the carbi cable end fittings - not trusting the single screw clamp fitting to remain secure, I implemented a fail safe second fitting on mine, some 8 years ago. Never had a cable  slip but feel happier that if it does I will still have some control of the carbi, albeit out of  synchronisation.

Hi Skippy

 

Just for info you may lube Outer cables with Teflon inner lining and wire inners which are sometimes used on our aircraft. 

 

"As stated, modern teflon line outer cables do not require lubrication. These cables are often designated as being 2P-class cables. 1P cables lack the liner. Not all cables carry such an indication, visual inspection will quickly show if a liner is present. A teflon liner is a thin, plastic layer inside the outer cable, often white or light blue. If you have older, or cheaper cables, without such a liner, lubrication is required. Teflon lined cables with inner cables that have corroded and are stuck should be replaced, due to damage to the inner cable and/or liner. In an emergency however, lubrication can help to unstick these cables."

 

Ultralights may have the usual metal rod type throttle system or a inner outer system so hopefully everyone's comments will assist him getting to the bottom of the matter.  Something I found a few years back on a bing carby set was after servicing there was an 'o' ring shaped build up of crap from the fuel mixtures we get up inside where the main jet in fitted; it was disturbing the air fuel swirl and gave rough running in the higher rpm band above 4,000rpm.

 

I'm keen to hear he solved the problem and what it was.

 

Cheers.

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20 hours ago, skippydiesel said:

I stand corrected

Hi Skippy  No need to stand corrected;  the sharing of information, experience and opinions is what makes this forum and other forum threads a gem and wealth of aviation mates sharing info (Thanks to Ian and his crew).  I've have gained many good bits of info and that's why I share my comments for what they are worth into the mixing pot of information.  Cheers Mike

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Thanks BlueA - I am not totally convinced as I learnt the hard way having managed to "gum up" perfectly functional carbi throttle cables. To add insult to injury, my persistent efforts to improve the situation by trying a wide range of lubricants ultimately damage  cables, that should not have been lubricated in the first instance.

 

I went to a custom cable provider to replace the damaged cables, who informed me that the majority of modern, bicycle/motorbike type control cables do not require and should not be lubricated.

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We seem to have lost Ultralights. Without him we have no idea of the cause of the problem and we also have no idea of CHTs, which would have helped.

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Gearbox could very well need overhaul.

Once the yoke drive gets a bit of wear you will get vibration.

About $1700 from Floods including labour if you take the gearbox off and take it too them, takes about 2 hrs to do.

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Yoke

In a gearboc mine has "selectors "

My HB has a stick,  eggs & American aircraft have yokes.

Goog joke 

spacesailor 

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2 hours ago, spacesailor said:

Yoke

In a gearboc mine has "selectors "

My HB has a stick,  eggs & American aircraft have yokes.

Goog joke 

spacesailor 

yoke

 noun
\ ˈyōk  \
plural yokes

Definition of yoke

 (Entry 1 of 2)

1a: a wooden bar or frame by which two draft animals (such as oxen) are joined at the heads or necks for working together
b: an arched device formerly laid on the neck of a defeated person
c: a frame fitted to a person's shoulders to carry a load in two equal portions
d: a bar by which the end of the tongue of a wagon or carriage is suspended from the collars of the harness
e(1): a crosspiece on the head of a boat's rudder
(2): an airplane control operating the elevators and ailerons
f: a frame from which a bell is hung
g: a clamp or similar piece that embraces two parts to hold or unite them in position
 
yolk
/jəʊk/
noun
 
  1. the yellow internal part of a bird's egg, which is surrounded by the white, is rich in protein and fat, and nourishes the developing embryo.
    "two yolks"
    • ZOOLOGY
      the part corresponding to the yolk in the ovum or larva of all egg-laying vertebrates and many invertebrates.

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