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pbatters

Jabiru lacks revs on climbout

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I have an early 3J with 2.2 lit engine. I have noticed recently that the motor is only developing 2600 revs on climbout. The engine has done 350 hrs and has been serviced at 25 hr intervals. The engine is running smoothly, the plugs are clean and a good colour. The fuel filter is new and a compresion test at last service was fine. A friend with a J160 tells me he gets 2800 revs. Does anyone have any ideas/similar experience on what may be causing this?

 

cheers

 

Peter

 

 

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My mechanic husband asks, Peter did it have a compression test or leak down / differential test ?

It was a compression test, as far as I know

 

Cheers

 

Peter

 

 

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I have an early 3J with 2.2 lit engine. I have noticed recently that the motor is only developing 2600 revs on climbout. ......

I take it you used to get higher revs until recently. Are you climbing at the same speed? If so and you haven't changed anything operationally such as propellor it indicates a reduction in power output. I would conduct a leakdown test, it is different to a compression test. At 350 hrs, I would suggest you are up for a top overhaul.

 

 

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Cruise prop ??

 

If it did get 3000 before.....then forget above suggestion....( prop.)

 

Check carby to see when you apply full power on the throttle, you are actually getting full power at the carby. ( May need to adjust cable / stops etc )

 

Mine......shade over 3000 always.

 

 

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Check your static rpm. 2600rpm on climb would tend to suggest that that your static may be below specs. on a factory prop. If the prop has been varied - ?

 

 

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If all other 'normal' tests don't show anything untowards, as a long shot I'd be checking the airbox to see that everything is ticketty-boo. I have an ST1, early production number aircraft, and the airbox is, to be polite, not the most inspiring thing.

 

 

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our guess is more major issues. a compression test won't show what you really need to know. At 300+ hours, rings and cilenders may be the cause. It os the same reason why most jabirus are advertised with top end overhauls at 350 hours. May be look at the other threads on jabiru engine in specially the one about cooling.

What is a cilender ?

 

 

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I'd check the basics first, ie throttle has full throw, cables OK etc, choke ok, I assume from the colour of your plugs and exhaust that it is not running rich for any reason (float level for example or is the needle in the correct notch). Next I'd check the prop has correct pitch for the engine and jab model (specs in POH). Any change will make a difference to top end RPM. Density altitude will make a difference to performance also. Good luck.

 

 

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Long story short

 

I had noticed a similar drop in RPM with my 3300 at similar hours (but I have a good idea why it was caused now), with all the same indications that you have stated, good plug & exhaust colour, engine running smoothly, the only thing I did notice on a couple of occasions in the few hours previous was the odd miss at altitude in cruise (had put it down to a bit of carby ice at the time) and a slight increase in oil consumption (in fact it was in the catch bottle), leak down test showed 3 cylinders not particularly good (the worst No.1 was 30/80). Pulled the worst one found the rings glued to the piston, the other two were the same (1, 2 & 4). Completed a top overhaul and all was good.

 

Leak down test will take you about 30 minutes, another thing to check is your exhaust make sure the muffler is not blocked up with soot or hornets nests.

 

Anyone want the full story let me know

 

Aldo

 

 

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Check that carby heat cable is working have had carby heat not coming off even with knob in off position. Outer cable may separate from fitting behind dash

 

Mick W

 

 

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Guest Andys@coffs

Compression test vs leakdown test.

 

Compression test to be done when you don't have access to a leakdown tester and you feel that your luck in general and at the casino is significantly above average.

 

Leakdown test to be done if you want to follow the official maintenance manual (you know the one we generally don't have any choice but to follow!) and end up with some results that can lead to a proper diagnosis and just as important to document as a baseline to monitor for change over time.

 

To describe 30:80 as not particularly good is to me the same as suggesting that government in Iraq is a bit below our standard.......... I mean reality is that 59:80 is unserviceable, 30:80 is to me so completely stuffed that to choose to fly with 30/80 without investigation first is going to look really ugly in the event of an engine failure resulting in an insurance claim..... (and I acknowledge that Aldo never said he did that....just me making sure no one is under the misapprehension that 30:80 is only a bit below serviceable)

 

I have seen 30:80 before on a J engine, and depending on the cause, oil usage can be right up there resulting in a really ugly mess on the underside of the aircraft including the elevator.

 

In the RAAus space I cant understand a J owner not owing a compressor and a leakdown tester, unless they use a L2 or better for everything and the L2 or higher of course has those 2 items. . Jump on ebay, there are heaps of leakdown testers available around $120 just make sure you get one with the right adapters for the plugs Jabiru use, For a compressor, get one that has a reasonably high LPM rating, you need 80psi to be constant over the entire test, not dropping as the leak rate stresses the delivery rate. Also try and get one that simply has pressure scales on both gauges, those that have colour coded green yellow red etc on the gauge after the restriction are often coded for an input pressure of 100psi, not the 80 that J call for to be tested and as such aren't relevant for our tests showing something to be poor (yellow) or unserviceable (red) when they aren't at 80psi testing

 

Also, as a previous poster, a few years back was able to point out 80psi can make the prop do rude things to your cranium if it gets away from Top Dead Center for the cylinder under test, you and better yet an offsider need to keep the prop under control as you do the tests.

 

Andy

 

 

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Keep away from the prop is a must. Any drop below good figures is a deterioration and an aircraft engine should really only be operated in Good condition. There has to be figures nominated, so they are out there. With some of the ring/piston condition(s) being found/reported it is obvious that operating an engine like those is dangerous. High blowby rates should not be allowed to persist. These things don't fix them selves and can deteriorate rapidly to something much worse in a short time. Having a GOOD idea of how a GOOD engine feels when you turn it over and do it every preflight is helpful. If you have a really "soft" cylinder , any loss of power or noise or vibration, ground it and find out why. Don't keep flying it. or we will be reading about you.. Nev

 

 

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We know that Jab cooling can be very uneven between pots, if one doesn't have CHT's on all cylinders then it's easy to assume - incorrectly - that temps have all been within limits. Once one pot gets overtemp, it's entirely possible to get an accumulation of effects, and if the dissipation of heat from one exhaust valve goes awry, you'll probably get a build-up of fried oil on that rocker supply every time the engine shuts down and the oil remaining in the rocker supply line stops flowing but the heat-sink effect remains. That will close off the oil supply to that rocker and the effect will simply grow with use.

 

The dissipation of heat - particularly in an air-cooled engine - of individual elements is a highly complex situation and unless you have the diagnostic capabilities of a CAMit, the inter-relationships of things like the cooling airflow, the heat-transference capability of the valve guide and valve-seat material et al is way beyond the appreciation of we end-users of the product.

 

For the cost of installing full engine CHT and EGT monitoring for every pot - preferably with a recording facility that allows post-use examination of the flight conditions and early-intervention remedial work vs. more frequent overhauls - it's a damn no-brainer. As with everything on an aircraft, 'systems' are always a compromise. In the case of Jab engines, the compromise is between light weight (allowing more usable weight within the class limits), cost, and fuel economy. If you want to have your engine Gardner-class unbreakable, it will cost you Gardner-class weight and cost. I'm talking (from personal experience!) 750 kgs engine weight for 140 HP continuous for 40k hours of operation and a price of about $40k... They are magnificent pieces of engineering, but they've never flown.

 

 

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Oh, yes - and generators in the Antarctic, buses, major league trucks... and that's the point. I love them. But you don't get 85 hp continuous from 55 kgs without pushing the envelope.. and people who don't understand that they're pushing the envelope and treat them accordingly, are destined to be disappointed.

 

 

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is it ok to fit 6 x CHT senders to a 24 reg?

Isn't this where the regs get stupid... but if it's a 'C' model, then a Part 21M engineer should be able to approve it instantly under 'nil adverse effects'; and if it's not and Jabiru don't approve, they should be taken out the back of the shed and kicked at 'inappropriate response' level.

 

 

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Oscar, they are a great old dinosaur. A diesel running white-metal poured bearings. I've worked on them a few times. Looked after they might run for a hundred years..An aero engine is very special purpose. IF it would last for a very long time it would be heavier than necessary. Nev

 

 

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... Having a GOOD idea of how a GOOD engine feels when you turn it over ... Nev

Good point Nev. Most of us don't notice a gradual deterioration, whether it's our eyesight, hearing or lubido. The vibration level is another one; without regular reference to another aircraft, who'd notice an increasingly out of balance prop?

 

I tend to write numbers on my flightplan as the engine warms up. Could I monitor prop balance by comparing the wobbly scrawl with previous ones?

 

 

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Compression test vs leakdown test.

To describe 30:80 as not particularly good is to me the same as suggesting that government in Iraq is a bit below our standard.......... I mean reality is that 59:80 is unserviceable, 30:80 is to me so completely stuffed that to choose to fly with 30/80 without investigation first is going to look really ugly in the event of an engine failure resulting in an insurance claim..... (and I acknowledge that Aldo never said he did that....just me making sure no one is under the misapprehension that 30:80 is only a bit below serviceable)

Andy

 

Just not being to dramatic about it, and you're correct I certainly didn't fly it.

 

Aldo

 

 

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Er, the 6LXB in my boat had modern bearings.. and a 400K miles TBO for road use (or 40K hours for boat/auxiliary power use) - and it was legendary that trawlers would exceed that by 10 times or more, running on lard and kicks. I think the shaft alone weighed more than an installed 3300, though I could check - I still have one. When Cummins bought Gardner, they closed down the manufacturing facility, because they considered the engine too expensive to produce.

 

And that's the point: you can't expect - as you say - a cheap aero-engine to deliver that sort of reliability.

 

 

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