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Rough running under load at high RPM

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Rough running under load at high RPM

Postby SEQFTA » Fri Apr 05, 2019 9:03 am

Hi All
Any idea's ?
Jab160D Hydrolic lifters 2.2 4 cylinder
I suffer a slightly rough running motor between 2700 to 2900 RPM , but only under load not in descent
So
Below 2700RPM totally smooth , 3000RPM and above totally smooth , All altitudes .
So Far I have
Changed Bing Carby with overhauled kit and really cleaned the needle jet and the main jet , added the fluted insert from Jabiru at the carby end of the induction pipe to stop the air from becoming turbulent .
New air filter , new NGK D9EA x 8 gapped correctly
Glued rotor buttons
Had ohms checked with auto electrician on leads (Yes , I know "not under load") but still good ohms reading
Leakdown 74'PSI all round
Checked the distributor caps
After all that , still the same
No , I haven't individual EGT's reading's
Jabiru say's a possible fix is to cut a hole in the cowling directly beside the air box and fit a scat from the cowling to the air box to stop turbulent airflow but my aircraft never used to have this problem and it's always had the normal induction on the side of the cowl , so why now ?
Anyway , any idea's anyone ?
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Re: Rough running under load at high RPM

Postby PKnight » Fri Apr 05, 2019 9:18 am

I would normally have suspected an electrical issue but that would have usually meant that the rough running continued at all revs above a certain level (when the spark was sufficient to cause an electrical breakdown) rather than cease at higher revs.
Is the rough running independent of the mags? If you have a duff plug that is breaking down at a certain point it is very obvious when you switch the mags off in turn.
Do you get the rough running when you do your power checks or only in the cruise? This may give an indication of temperature dependency of the effect.
Another check that can be used is to apply carb heat when you get the rough running to see if there is any change. I am not suggesting the carb ice is the problem but an application of carb heat will change the mixture and may help narrow down the cause.
The last time I detected a hesitancy in my engine it was very difficult to find the cause. It was certainly electrical as it only happened with the LHS mag. I changed all the plugs and the problem disappeared. Close inspection shown all the plugs were perfect so I never really knew the cause.
Incidentally has this effect suddenly happened or slowly crept up on you?
Good luck
Peter
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Re: Rough running under load at high RPM

Postby SEQFTA » Fri Apr 05, 2019 12:45 pm

Thanks for replying Peter
Yes , the fact that it resolves under "full power" 3000RPM is confusing .
It's funny , On take off I get 2800RPM and thrust generated , i can feel , is normal even on initial climb out at 2800 full power , no problem .
lower the nose when re-configuring into cruise tacho climbs up to 3050RPM , reduce the throttle back to 2800 and it runs rough ????
I'll try a static full power "mag" drop check tomorrow , might even have a quick go up stairs in cruise , i'll make sure i'm over an airstrip first ;)
I will try the carb" heat in cruise as I haven't tried that yet , I don't think that it will make any difference but I might even disconnect the scat pipe from the cowling to the air box and try another static run .
This issue is a bit of a long term-er , Iv'e even tried balancing the prop not dynamic just knife edge , wish there was some solid indicator that I could sink my teeth into .
I will give you the results of my tests tomorrow Peter .
Thanks once again
Wayne
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Re: Rough running under load at high RPM

Postby PKnight » Sat Apr 06, 2019 8:39 am

P.S
One more possible check to try. When you get the rough running, switch your electric pump on and see if there is any effect.
Peter
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Re: Rough running under load at high RPM

Postby Steevo » Sat Apr 06, 2019 8:14 pm

One suggestion that may sound stupid but I have seen before is engine mounts. I have seen a Jab that seemed to run rough in a certain range of RPM. It felt rough in the sense of a vibration like feeling but really did feel like a rough running engine. Turned out to be engine mounts had sagged and just the resonance in that range felt like a rough running engine.
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Re: Rough running under load at high RPM

Postby Ican » Sun Apr 07, 2019 7:47 pm

I had a very similar problem on my Jabiru.
After a lot of searching, including having the prop checked it was discovered that my engine was running rich at around 2700 to 2800rpm.
I believe that this is where the main jet takes over from the needle jet.
On take off the throttle is fully open meaning the main jet supplies the fuel even though the rpm is around 2800.
I changed my needle jet from 2.82 to a 2.78 and suddenly l have the smoothest engine I could have hoped for.
This may or may not be your problem, but it sounds very similar to the symptoms my plane was showing.
Good luck, hope you have it sorted soon.
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Re: Rough running under load at high RPM

Postby Kai Lyche » Mon Apr 08, 2019 6:42 am

All,

Just to make sure everybody thinks along the same lines here- there is no question about ‘the main jet taking over’. The main jet is always part of the game.

The jet battery in the Bing is built in such a way, that ALL fuel that goes to the engine once past the idle/low load stage, HAS to pass through the main jet, after this the needle jet, and finally the diffuser. Of course, the vertical movement of the needle in the needle jet is controlled by the venturi depression diaphragm.

As stated above, at 2700-2800 rpm, the diaphragm is at its top position- it can go no further: the jet needle is lifted almost completely out of the needle jet. The orifice in the needle jet available for the fuel to pass through, is huge- much bigger than the orifice in the main jet, which as mentioned is situated directly below, and which will happily supply all the fuel its orifice will admit. So what increases fuel supply to the engine for more power once at 2700-2800 rpm?

Indeed- the throttle plate. Your beautiful constant depression Bing is suddenly a CD carby no more. It has reverted to function as an ordinary of the shelf Solex or Zenith or whatever carb: the position of the throttle plate controls the amount of fuel that is supplied by the main jet.

At the end of the jet battery, at the venturi inlet, is the fuel diffusor. Its task is to atomize the fuel in such small particles that fuel gas is formed (no engine will run on raw fuel, but loves fuel gas). Unfortunately the Bing CD carb diffusor is not a perfect thing- tests have shown that at full tilt no fuel gas but a jet of raw fuel enters the venturi, passing on to the manifold plenum. There is a shaped bar in there that is supposed to atomize the fuel when hit by the raw fuel jet. The intended function of this component is giving no cause for rejoice either. The main proof of this failure to function are all the grey hairs you have because of uneven EGT’s.

Thanks
Kai
Last edited by Kai Lyche on Wed Apr 10, 2019 6:27 pm, edited 3 times in total.
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Re: Rough running under load at high RPM

Postby Ican » Mon Apr 08, 2019 7:10 am

Thank you for the explanation, Kai.
I agree with your explanation. I could have worded my post better.
The idea that l was trying to convey was that the needle and needle jet does not control the fuel into the carb after a certain stage. (Around 2800 rpm on my plane)
In my case l could also have changed to a different shape needle. It was easier and fortunately effective to just change the size of the needle jet.
The problem as explained in the original post seemed exactly like what l experienced and l thought that my experience may provide an avenue to explore.
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Re: Rough running under load at high RPM

Postby mikew » Tue Apr 09, 2019 7:34 pm

Been really interested in these series of posts, I wasn't really aware of the problem until I flew last weekend and on climb out realised that there was a slight roughness to the feel of the engine, nothing serious but different to the normal smoothness. I found that if I throttled back just a small amount the smoothness returned. This may be the same as others have experienced and so will try the smaller jest to see if that improves things, will keep you updated.

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Re: Rough running under load at high RPM

Postby MikeL » Thu Apr 11, 2019 3:19 pm

Is there any chance that these problems might relate to Jabiru Service Bulletin JSL022-1 08/02/2019?
Fuel pump drain tube getting squashed.
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