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possible carb rubber failure coming up

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possible carb rubber failure coming up

Postby Garyo » Mon Feb 29, 2016 11:46 am

had a tick over of 1K yesterday, normally have 800 or lower if I want. nothing changed bar oil change and tappet set. so found it unusual, mag check ok. I pulled the choke out while running, the motor dipped then ran quite rough. I would assume it should of cut instantly if there was no air leak, so I assume there is a leak on the inlet, poss a small 1 to increase the tick over. 2 years ago I changed the rubber that had a split, the symptoms for this were quite severe.
so wondering if the rubber is on its way out again, any thoughts
Gary
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Engine serial 1041
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Re: possible carb rubber failure coming up

Postby Garyo » Thu Mar 03, 2016 12:39 pm

found a small AGE slit on the inside of the rubber but not all the way through and age cracks starting around the dia of the middle. I have ordered a new 1, but I don't think this is the cause.
this rubber has lasted 2 yrs or 60 hrs, BP ultimate 97 RON, the last 1 was 18 months and a complete failure at 70 hrs, RON95 and BP ultimate the original 1 went rock hard but 10 years and 500 hrs AVGAS
Former Owner of
G-OMER
Jabiru 450 UL
Engine serial 1041
1 CHT sensor on No 4 under plug
No Exhaust temp sensors
No glass panel
No xponder
Microair radio
No bells and whistles
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Garyo
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Re: possible carb rubber failure coming up

Postby PKnight » Thu Mar 03, 2016 6:14 pm

I have just changed mine after 250 hours and two and half years. A split had started to form around 120 deg of the middle section. It was not quite all the way through but well on the way.
It does surprise me that this component is prone to splitting like this. After all there is not much room for movement as the carb is reasonably tight against the firewall (J430).
I suspect that over enthusiastic tightening of the retaining clips may exasperate the problem. As it is difficult to see the start of the split I have taken to changing the carb rubber every 250 hours as a precaution.
Peter
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Re: possible carb rubber failure coming up

Postby diablo » Thu Mar 03, 2016 7:38 pm

my one probably has 150 hours and is over 6 years old. I dont over-tighten as in my world, all it need is to be firm. That dampens vibration and does not stress the rubber so damaging it. If rubber bulges, it is too tight. When it is too tight, it just seems to make it weak and vibration, as always gets its way and cracks the rubber.
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Re: possible carb rubber failure coming up

Postby Garyo » Wed Mar 09, 2016 8:50 pm

The new socket arrived, quite soft, on fitting, the carb wants to keep popping off, so it has to be nipped up to prevent this. Not tested yet due to weather, inspector coming friday for permit, hope to test fly then or the w/end
Former Owner of
G-OMER
Jabiru 450 UL
Engine serial 1041
1 CHT sensor on No 4 under plug
No Exhaust temp sensors
No glass panel
No xponder
Microair radio
No bells and whistles
User avatar
Garyo
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Posts: 323
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Tips for coupling longevity and carb support

Postby LSAGuy 104 » Thu Oct 04, 2018 7:52 pm

Just a couple of tips on carb couplings and rubber parts in general that I picked up years ago in Rotax classes.
On any rubber part for which there is not a factory specified mounting procedure you can increase its life by taking a little dab of lithium grease and rubbing it into the part until its a nice shiny black. The explanation the instructor gave is that it helps keep the rubber supple and offers some protection from ozone. My own experience is that does indeed increase the life of rubber parts. I even use it on the rubber diaphragms of the Bing carburetors. How big an increase? I'm coming up on 10 years for the carb couplers on my HKS engine and they show no signs of cracking inside or out.
To prevent carb sagging you might want to try a trick that Rotax uses to support the Bing 64 carbs in the 912 engine. Rotax uses a longer screw on the forward (coupler side) dome attachment with a grooved spacer. The groove allows a spring to be attached that stretches from it to a tab on the forward end of the carb coupler (Rotax uses a flanged coupler similar to the original Jabby carb coupler). For the current Jabby engines you'd have to fabricate this tab. It might be a PITA to do, I haven't looked over the 2200A I work on so call this tip a PBI (partially baked idea) It might take a bit of adapting to do it on the Jabby but the spring and spacer are readily available from Rotax dealers.
Spacer part no. 847910 (Rotax calls it a distance sleeve; weird but then Rotax calls Scotchbrite lapping fleece, go figure)
Spring part no. 838242
Screw part no. 640251 (Oval head screw 5mm X 20)

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