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Clive J
Joined: 03 Nov 2007
Posts: 340
Location: UK
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 Posted: Wed Apr 30, 2008 12:38 am Post subject: Grooves |
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RE: the amount of grooves on the needle. This is my understanding. The
early simple needles were gradually tapering and as such could be moved
giving a richening or leaning of the mixture. The later needles look to
be 'profiled' that is the diameter change isn't constant with length.
Moving one of these needles by a groove may actually put the mixture out
by a real step change rather than a tweak hence a single groove so it
couldn't be done. Changing the needle jet size achieves the 'tweak'.
Reference the 'profiled' needle. With a carburettor the whole business
of air fuel mixture is a set of compromises, compromises which go along
with the ones that come with our aircraft/propeller combination. There
maybe a couple of speed/rpm setting that are optimum and there maybe a
couple of speed/rpm/mixture settings that are optimum. Generally though
things will always be less than the 'ideal'.
When Jabiru in their wisdom started the 'Economy kit' and profiling
needles there was one element that wasn't included and set the idea up
for failure (this is in my opinion only). As the Jabiru engine is used
in many planes the Aeroplane/propeller combination can be very
different. In the Jabiru planes themselves different props and pitches
(same numbers different pitches with Jabiru props) mean that the idea
mixture requirements will be different. With different airframes
different once again, in some cases and order of magnitude.
So Jabiru produce a needle that runs the engine in cruise on the edge of
too lean for plane X, great, we stick that engine on plane Y, Z, W and
with prop this that or the other and it's not right...why should we be
surprised. Anyhow just my sixpence worth (5 cents?)
We had a strange one in the UK which I can't really understand. It is
around the same issue as I mention above. A Jabiru J400 is built in the
UK with a 3300 engine 'as supplied' (whatever that was at the time)
three and a half years ago. It is flown happily for those three and half
years until one day there's a running problem. Starts in the air and
smacks of carb ice. Pilot circles over large airfield trying to isolate
the problem. Carb ice suspected as problem began when entering a misty
shower. Eventually nothing found and problem seems to go away. Continues
to base. Two days later when flying the problem comes back and nothing
will get rid of it, slight uneven running. Landing out at an airfield we
check the plane over and by manipulating the throttle we can make it run
like a bag of spanners and even stop. We get after the usual fault
finding nothing found, certainly carburettor but what. Discussion with
one of the UK agents engineers of the symptoms and he immediately
suggests the mixture and a new needle jet. Other suggestions are tried
'in case' but it all points to a mixture problem. Eventually I drive
across and get a selection of jets. Drive back and fit the 'next one
up', and bingo as predicted, smooth as silk and I'm soon flying the
plane back to base.
So, my question is why, after 3 1/2 years (300 hrs same prop) does this
problem manifest itself? Wear would richen things up, filters
eliminated. ALL the checks show nothing. Just the jet to fix it......
The agent has been over and given it a dose of looking at and found
nothing else. The jet needle combination has been matched correctly. One
thing that is noted is it's running much cooler so was undoubtedly lean.
But what changed to bring the problem on.
Ideas?
Regards, Clive
List of what was checked
Fuel filter
Air filter
Induction leaks
Carburettor stripped completely and blown through and rebuilt.
Float level
Fuel, the same as another Jab that was flying over the same period
Loads of other stuff I can't remember.
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Lynn Matteson
Joined: 10 Jan 2006
Posts: 2778
Location: Grass Lake, Michigan
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| Posted: Wed Apr 30, 2008 3:15 am Post subject: Grooves |
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Maybe the faulty jet or passageways leading up to the jet got a
buildup of varnish or other contaminants over the years, and finally
something dislodged and caused the problem. That's a strange one
alright.
Lynn Matteson
Grass Lake, Michigan
Kitfox IV Speedster w/Jabiru 2200
flying w/490+ hrs
On Apr 30, 2008, at 4:34 AM, James, Clive R wrote:
[quote]
<clive.james(at)uk.bp.com>
So, my question is why, after 3 1/2 years (300 hrs same prop) does
this
problem manifest itself? Wear would richen things up, filters
eliminated. ALL the checks show nothing. Just the jet to fix it......
The agent has been over and given it a dose of looking at and found
nothing else. The jet needle combination has been matched
correctly. One
thing that is noted is it's running much cooler so was undoubtedly
lean.
But what changed to bring the problem on.
Ideas?
Regards, Clive
List of what was checked
Fuel filter
Air filter
Induction leaks
Carburettor stripped completely and blown through and rebuilt.
Float level
Fuel, the same as another Jab that was flying over the same period
Loads of other stuff I can't remember.
--
| | - The Matronics JabiruEngine-List Email Forum - | | | Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:
http://www.matronics.com/Navigator?JabiruEngine-List |
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_________________
Lynn
Kitfox IV-Jabiru 2200
N369LM |
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