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Jet Sizing

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Jabiruyank
(@jabiruyank)
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Joined: 4 years ago
Posts: 64
Topic starter  

Sorry about the last reply, it was done at 1:30 in the AM. I ordered part number 4A138B0D-2 from Arion Aircraft. The invoice did not have a "-2" though as the part number. I'm curious to why you asked which part number was ordered. Are there iterations of the Jab needle?

PS: For now, I'm switching back to the original Bing needle so I can at least fly now that the weather has improved in my part of the world. Between me an you,  buying jets and needles is becoming costly - and admittedly, a bit aggravating. I'm sure Bing is the best carburetor for the Jab engines based on Jabiru's extensive testing. However, in my humble opinion, a cockpit adjustable mixture carburetor would be a better choice. A great engine overall though. Keep up the good work mates!

Cheers,

Alex R-


   
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(@jabiru-support)
Member Admin
Joined: 4 years ago
Posts: 190
 

Hello Alex,

We have had a few different variations of needles over the years and I wanted to make sure that I was giving you advice for the right one.   For that needle we run a 290 needle jet and a 235 or 245 main jet.  

I understand that buying jets would add up.  Our reason for changing the needle  was we weren't able to obtain the proper mixture across the range with the standard Bing needle.  

There is cockpit adjustable carburettors available for the Jabiru engine, I believe.  


   
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Jabiruyank
(@jabiruyank)
Estimable Member
Joined: 4 years ago
Posts: 64
Topic starter  

Greetings Jab support. I just received 4A138BOD. The Jab is much shorter than the original Bing needle on the CV94 carburetor? Is that correct? See attachment.

Thanks,

Alex R-


   
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Jabiruyank
(@jabiruyank)
Estimable Member
Joined: 4 years ago
Posts: 64
Topic starter  

Hello Jab support,

I changed the needle back to the original Bing and I still had the same problem; the engine cuts off when I applied more throttle from idle. My thoughts were that if it ran fine before I changed out needles, then it had to be something I've done when changing out the needles. After removing the carburetor, disassembling, cleaning and blowing out every orifice, I put it back together and installed the carburetor - worse. This time, the engine just cut off completely during idle. This had to be a fuel delivery issue. One thing I did try when the engine was quitting was the auxiliary electric pump - it still cut out. I checked all hoses and the fuel filter (just installed less than 5 hours). I energized the electric pump and looked to see if I had any flow and the pump sounded dry at this point as the Facet pump makes a distinct sound when dry (it didn't when I was trying to restart the engine). Again, looking at the hoses, I disconnected the "t"connection to the carburetor and mechanical pump and then I had flow. Air pocket? I then proceeded to look at the mechanical pump. The upper housing screws were completely loose. I disassembled the pump and inspected the overall condition and the diaphragm was intact. After reassembling and reinstalling the pump I started the engine and voila, it worked! It idled well and I was able to add full throttle without any issue. The only theory I had was that the loose housing on the pump was sucking in air that affected fuel flow, even with the auxiliary pump on.

What's your thoughts on this? Could it be coincidental that the mechanical pump was failing just when I was changing out the needle? And what if this occurred during flight? It seems as if the aux electric fuel pump would've been of no help.

A brief history on the aircraft and the engine. This aircraft was built in Romania in 2000 and when I had bought it in June of 2020, it only had 39 hours TT. I have flown t for 44 hours thus far. I think at the very least, this warrants a replacement mechanical pump.

As always, thanks!

Alex R-

This post was modified 3 years ago by Jabiruyank

   
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(@jabiru-support)
Member Admin
Joined: 4 years ago
Posts: 190
 

Hello Alex,

I can't think of any connection between the fuel pump top being loose and adjusting the carburettor, so it is likely a coincidence.  That is very low hrs.  I can see why it wasn't getting enough fuel now. And yes this would have caused inflight fuel starvation.  However if you check out the pump and tighten the screws, maybe with a bit of mild loctite, I don't see why the pump wouldn't still be serviceable.   

And the jabiru needle is shorter than the bing needle by about 6mm. 

 


   
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Jabiruyank
(@jabiruyank)
Estimable Member
Joined: 4 years ago
Posts: 64
Topic starter  

Hello Jab support,

I installed a new mechanical fuel pump, and it looked as if the issue was resolved but the following day, it was doing the same thing - cutting out when applying throttle from idle. By the way, I start the engine with choke applied. If I attempt to remove choke, it cuts out. It is only when the engine is warmed up that it finally works well and does not give me a problem during the day flying from one airport to another (full engine stops and restarts at airports).

I agree that it seems like fuel starvation. I will try the 285 or 290 needle with the main at 235 or 245 as you suggested along with the Jab needle.

This issue started after I installed the larger main and needle jets in an effort to reduce EGT. Before that, this never occurred so in my mind, it has to be as a result of carb changes or something in the carburetor that I may have altered or overlooked. I suppose I can revert the carburetor to the original configuration to see if this continues. 

Logic tells me what appears as fuel starvation is most likely a carburetor problem but what puzzles me is why it runs fine after warming up. Is there anything else that I should look at other than the carburetor - valve clearance, manifolds, etc? i did do a leak down test last month and all cylinders checked out ok. 

Always appreciate your help mate.

Cheers.

Alex R-


   
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(@jabiru-support)
Member Admin
Joined: 4 years ago
Posts: 190
 

Hello Alex,

Engine cutting out on advancing from idle is the classic sign that mixture off idle is too lean.  In your case it is only too lean when the engine is cold.  As the engine warms up it can burn leaner mixtures. As a test, if you leave the choke on as you increase revs through the problem rpm range it should fix it.  Increasing the size of the needle jet will usually fix this (however it will also effect your cruise mixture). From the sound of it you won't need much.  You may be able to fix this by adjusting the idle mixture screw but that is not for sure.  

I have no idea why this happened when you put a larger needle jet.  Is it possible the fuel level in the bowl has changed?

 


   
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Jabiruyank
(@jabiruyank)
Estimable Member
Joined: 4 years ago
Posts: 64
Topic starter  

@jabiru-support

To use your expression, crikey! The soot accumulation using the 290 with a 235 was super thick in the exhaust pipes! This occurred after test flying in the pattern, three takeoffs and landings. I’m going with a smaller jet. Should I continue with the Jab needle or revert to the original Bing? And I’m still having issues with throttling up from idle when it’s cold. After warmup, it runs perfectly. I need to look at the enricher to make sure I didn’t screw up the assembly after having it apart for cleaning and inspection. 

Alex R-


   
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(@jabiru-support)
Member Admin
Joined: 4 years ago
Posts: 190
 

Hello Alex,

I think you have some egts sensors fitted to your engine.  The test for cruise and full power mixtures is the exhaust gas temperatures.  

The soot could be coming from being overly rich in decent or idle (taxi).  If the egt sensors are positioned 120mm from the port face on the head you can directly compare your egts with the numbers we get.  We aim for a full power around 690C on the hottest cylinder and in the 700 to 710C range for cruise at low altitude (eg 1000ft).


   
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