AIR FUEL MIXTURE RATIO

[vk3awa]

Aviators,

 

As most of you would be aware, Jabiru have recently released JSL002-2 updating the recommended carby jetting for 2200 & 3300 engines. In the service letter, recommended air/fuel mixture ratios (Lamda) are given for various engine RPM. I decided rather than "play" musical jets, I would purchase a Lambda meter and measure my air/fuel mixture ratios. I made up a proforma using the Jabiru recommended Lamda values as my reference, fitted the Lamda meeter and went flying. Fig 1 shows the readings with the original jetting of 250 Main and 285 Needle jets. (3300 engine) If you look at the graph in fig 1, it shows the engine running lean in cruise. I then fitted the mid range jets as recommended in the Jabiru Service Letter (255 Main & 290 Needle) and the results are shown in fig 2. I now burn 3 l/hr more and find CHT much lower on the ground, in cruise and climb.

 

Hope this is of some help!

 

CHEERS

 

Daryl

 

Jabiru SPT-3300

 

 

 

 

[Guest J430]

This just highlights a point I have made several times before, do not pusyy foot around by running at low fuel flows.......2600RPM is not the go.

 

I came back from LRE yesterday, well Rocky in my plane, and I was at 2900 rpm and maybe a touch more and about 23-24LPH in the cruise, and overall average of probably 22LPH.

 

Drive them how they should be driven. Do not try to cheap scape on fuel burn.

 

J

 

 

[Guest Teenie2]

Daryl are you going to leave the O2 sensor fitted all the time? I think the life of the probe will be very limited if running avgas.

 

 

[Guest JRMobile]

J430

 

"This just highlights a point I have made several times before, do not pusyy foot around by running at low fuel flows.......2600RPM is not the go."

 

I'm a little confused by your comments here in this thread and some of the other forums. My trusty old Jab (SP 6) originally was burning 21-24 L/Hr at the then recommended cruise setting of 75% power which equated to 2600 - 2700RPM.

 

After installing the carby upgrade last year fuel burn dropped to 16 - 17 L/Hr, 2600RPM, Ias- 100Kt. All Good!:big_grin:

 

Doubled checked the figures on the weekend, two up still air:

 

16L/Hr, 2600RPM, 102Kt

 

18L/Hr, 2800RPM, 110Kt

 

:;)5: Question is am I pussy’n around and damaging a good engine?

 

Any other Jab fliers with similar fuel burn figures?

 

 

[Yenn]

The GA aircraft which use mixture control can reduce fuel flow whenever there is less than about 80% power required. They don't suffer from detonation running lean of peak and still can produce the same power as rich of peak while using less fuel and maintaining better heat. When reducing altitude it is a good idea to keep the temps. up by running leaner and the only problem arises if you attempt a go around and then you risk detonation or more likely misfiring.

 

Some pilots lean aggressively when taxying to reduce lead fouling and if they forget to enrich the mixture they soon know about it as the engine will be very rough.

 

 

[Guest J430]

As the name suggests....J430 is not a 2 seater.....and I expect the speeds may vary, due to prop pitch and a bulkier aircraft.

 

The swept volume of the engine at a certain rpm is fairly constant subject to leaks etc, and hence the fuel mixture will be about the same given certain throttle settings, the only variable is the needle position, and it leans out or richens up depending on where you are on the step of the needle. Those who manage to sneek back onto the leanest of the needle settings in an attempt to use less fuel, are doing the same thing as a GA pilot leaning beyond the ideal running mixture, of course in an old 172 you can lean her out till she runs rough, and the Jab you cant, its not that lean, but why when the factory recomend a certain RPM and fuel flow would you want to run further up the lean side of things. The correct thing in a GA a/c is to run a bit rich of peak EGT, not at peak EGT.

 

I have rambled on a bit here, but the bottom line is fly them as recomended. The next issue is more critical during running in, and that is glazing of bores through not pushing it hard enough. The engine will not bed in properly if its babied. This is all engines not just Jabiru's. Read the Jab manual about loading them up, backing off, loading up again. Same as your outboard motor and car engines. Do the right run in procedure and you have far better long term results. Cars especially as they are not loaded all the time. best to get in and drive it like you stole it (within limits).

 

Another critical point is make sure the tacho is calibrated correctly. I had an over reading tacho straight from the box by 200 RPM. Others I know of have been spot on, others out by similar amounts.

 

One other point, I refer to "fuel flow" as it is in the cruise at any point in time, not my average LPH, which is often less, climb is usually 32 - 35 + LPH and during decent and landing with reduced RPM its quite a lot less than 20, so the average fuel burn per hour on the clock, allowing for taxi time etc is more like 20-22, where the fuel flow in the cruise is 23-24LPH.

 

I am running the 285 jet by the way, we fitted a 295 and a 290 and she was rough at low RPM, Don Richter even suggest leave the 285 in (I had the first economy jet at 278) as it was fine provided you dont have any induction leaks and did not try to fly too lean. I am in QLD also, those in colder and denser air regions need the bit more fuel apparently.

 

OK enough for now, hope that made sense and if in doubt, ask Don Richter.

 

J;)

 

 

[Guest brentc]

The short of it is that when around 2 years ago Jabiru released their 'lean burn kit' and everyone said 'BUT WON'T IT RUN MY ENGINE TOO LEAN AND DAMAGE MY EXHAUST VALVES?' to which Jabiru replied 'NO, it won't we have tested it.'

 

The short answer is that the engines ARE now running too lean and HAVE burnt out exhaust valves (mine included) and a resolution is required- being on the new needle. Not only valves are burning, other issues occur too.

 

I get exactly 20 litres per hour at 2,800 rpm in general day-to-day operations. I believe that this is too lean and causes my engine to run too hot and is likely burning out my exhaust valves and causing head movement. This will be confirmed on Saturday when I rip the heads off for a closer look.

 

 

[Guest JRMobile]

Hi brentc - "The short answer is that the engines ARE now running too lean and HAVE burnt out exhaust valves"

 

Thats the sort'a response i was chasing,;) my only clue to date was checking the colour of the plugs and keeping an eye on the temps (which have been good). Could you please post the results and observasions after you "rip your heads off"

 

Cheers

 

JR

 

 

[Guest brentc]

Will do yes. I'll take photos and post if they show anything up that's noticeable. Apparently plugs aren't the best thing to go by, but rather exhaust residue and temps as you have been doing.

 

Based on Daryl's post with the graphs I should be in the safe zone as I run at no less than 2,800 on all occasions. We'll soon find out.

 

 

[Yenn]

The correct thing in a GA a/c is to run a bit rich of peak EGT, not at peak EGT.

 

Is that so! Current opinion seems to be pointing the other way with lean of peak once below about 80% power. This dates from the glory days of the radial engined airliners and Charles Lindbergh when fuel burn could make the difference between arriving and not.

 

With Jabiru carburetteed engines the problem is different because there is no way of leaning or enriching the mixture with a Bing. You are stuck with the needle and jets available and they control fuel flow right through the range so you have little control yourself. An EGT will give some idea of wether or not the mixture is correct but unless you are very skilled at the job of setting up a carby you will still be guessing.

 

My personal opinion is to listen to the manufacturers and their experts. Remember when 2 strokes were failing all over the place and then we learnt to leave tham as per the experts advice and now they are very reliable.

 

 

[Galpin]

Is the Sonex Aerocarb a viable option for the Jabiru engines?

 

http://www.sonexaircraft.com/orders/accessories.html#aeroconversions

 

 

[vk3awa]

No the probe is fitted for testing only and the system is completely portable.

 

Probe life between 50 - 500 hours on leaded fuel and replacement cost around about US$80.00

 

 

[vk3auu]

Jets

 

Darryl, for those of us who are not completely up with it, can you give us a more detailed interpretation of the two graphs.

 

My impression is that Brent should be using at least 22 litres. My 80 HP 912 uses around 17 litres at about 70 percent power. I worry when I see people claiming very low fuel consumption.

 

Ian Borg says " My personal opinion is to listen to the manufacturers and their experts". Unfortunately, they don't seem to be experts. Sorry.

 

David

 

 

[vk3awa]

Darryl, for those of us who are not completely up with it, can you give us a more detailed interpretation of the two graphs. My impression is that Brent should be using at least 22 litres. My 80 HP 912 uses around 17 litres at about 70 percent power. I worry when I see people claiming very low fuel consumption.

Ian Borg says " My personal opinion is to listen to the manufacturers and their experts". Unfortunately, they don't seem to be experts. Sorry.

 

David

David,

Jabiru have produced, through a service letter, the optimum air/fuel mixture ratio for given engine RPM. The two graphs have a grey area which depicts the optimum air/fuel ratio as listed by Jabiru. The first graph shows measurements (blue line) outside the optimum and running too lean. (My original jetting) The second graph shows the blue line inside the grey shaded area after rejetting the carby.

 

CHEERS

 

Daryl

 

 

[facthunter]

Sonex aerocarb.

 

Galpin, the only aerocarb I have seen used was on a VW. (Geared) toothed belt. I think the power output was quite high.(not because of the carb) It didn't appear to want to run well at idle or low revs and fuel ran out of it all over the place prior to start. (The carb does not have a float) The throttle is a slide. I wasn't very impressed overall, for what that's worth. Nev....

 

 

[jetboy]

Daryl and Brent,

 

any updates? I am running a 2200 #1680 this is a late solid lifter build, 195 hrs. It had the first economy tunining kit version - the thouroghly tested one- yeah right - fitted @ 50 hrs and fuel use went from 18 L/hr to 15L/hr

 

A couple of side- effects were the oil temp increased 5 degF meaning I had to fit a cooler and the muffler colour changed to a darker shade it doesnt look like stainless anymore

 

The full power EGT has gone up by 50 deg F and low cruise 2700 by 100 deg f.

 

Currently running a tailpipe EGT of 680 deg C full power and 695 deg C @ 2800

 

(the probe only fits the 2" single tailpipe I use so I dont know the exhaust port EGTs)

 

I'm planning on rejetting to the later spec. depending on what I see with leakdowns and borescope at 200 hrs check. The EGTs are still rich of peak but it might be better to move them a bit further from the "edge of destruction"

 

Did Jabiru change the valve / valveseat alloy used in some engines , my heads are engraved "Welltite" if I remember correctly?

 

Ralph in ZL (ZL1TBG)

 

 

[Guest Andys@coffs]

Well, the scare factor got to me and I ordered a leakdown tester from the US yesterday. $60US (ebay) for the tester (10mm, 12mm, 14mm and 18mm adaptors) and $50US for the transport ???? Still when compared to the rebuild costs if ignored its nothing.

 

I'll let you know what I find on my 140hrTT 3300 solid lifters motor, hopefully bypassing the eyebrow surgery that Brent felt the need to perform :black_eye:

 

Andy

 

 

[Guest JRMobile]

A good ,simple leak down tester - check out: http://www.xs11.com/tips/misc/misc3.shtml

 

I finally got around to having a chat with Don at Jabiru today re the JSL002-2 update.

 

He is advising that the needle jet for the 3000 be drilled out to 2.85mm or do a replacement thru jab. I opted for the later.

 

 

[facthunter]

valve seats.

 

Jetboy , welltite is a proprietary brand of valve seat, which I use always ( if that means anything), for best performance. Nev..

 

 

[jetboy]

Nev,

 

Thanks, I presume if seats of that composition alloy erode there must be something else very wrong to cause that.

 

The real question is when did Jabiru change to it? Has a head failed with these seats, or were they all a previous material?

 

Ralph

 

 

[facthunter]

Valve seats.

 

Jetboy, sorry for the delay in reply. I would imagine that the welltite is a later fitting hence the stamping on the head. I would have thought that wear of this seat is very unlikely as it is extremely hard, (I am unaware of anything readily available that is better). In all the discussion on this matter the valve seat recession phenomenon hasn't been specifically narrowed down.

 

Is it 1. valve seat wear? ( the actual valve itself)

 

2. Valve mushrooming. ( valve deforming, bending up at the edges due to softening when overheated).

 

3. Wearing/erosion of the insert.

 

4. Insert impacting into the aluminium head, due to inertia & spring forces.

 

All of these will be reflected in reduction of valve clearance in service, which will not be obvious with hydraulic lifters. N...

 

 

[Guest Andys@coffs]

So I did the leak down test today and was able to avoid the eyebrow surgery, but unfortunatley the wallet surgery seems to be unavoidable.

 

Cyls 2 and 4 are marginal, holds 70% (min is 80/60=75%), Cyl 1 cold was 22% improving to 35% at standard operational temps. Ive rung the local Jabiru engine guy and well look at it next weekend..... I checked the tappet clearance and it was 9thou. mini violence with the wooden hammer handle to the valves didnt improve anything and the hissing was so loud that I could work out where it was coming from, definately some in the exhaust, and some back up around the solid lifters. The cylinder heads all have Welltite scribed on the back of the largest fin and some form of serial number/lot number

 

It would seem that the $130AU leak down tester with some luck may have prevented a future scary moment.

 

After next weekend I'll advise what the outcome was, and how poor as a result I have become.

 

Andy

 

Well, the scare factor got to me and I ordered a leakdown tester from the US yesterday. $60US (ebay) for the tester (10mm, 12mm, 14mm and 18mm adaptors) and $50US for the transport ???? Still when compared to the rebuild costs if ignored its nothing.I'll let you know what I find on my 140hrTT 3300 solid lifters motor, hopefully bypassing the eyebrow surgery that Brent felt the need to perform :black_eye:

 

Andy

[Guest brentc]

That's both good and bad news Andy and I'm glad that a potential disaster may have been avoided.

 

Everyone else should follow our advice and do the same on both 4's and 6 cylinders!

 

Operating temps are absolutely critical and need to be quite low so as to avoid damage. There are others on this forum that know more specific figures that are recommended, however as an example, mine rund CHT's at around 250 deg no more and usually around 80 oil, sometimes up to 90 which can be a tad high. The damage is done with the lean burning and the high head temps, so care must be taken to check exhaust colour.

 

 

[Guest Teenie2]

So I did the leak down test today and was able to avoid the eyebrow surgery, but unfortunatley the wallet surgery seems to be unavoidable.Cyls 2 and 4 are marginal, holds 70% (min is 80/60=75%), Cyl 1 cold was 22% improving to 35% at standard operational temps. Ive rung the local Jabiru engine guy and well look at it next weekend..... I checked the tappet clearance and it was 9thou. mini violence with the wooden hammer handle to the valves didnt improve anything and the hissing was so loud that I could work out where it was coming from, definately some in the exhaust, and some back up around the solid lifters. The cylinder heads all have Welltite scribed on the back of the largest fin and some form of serial number/lot number

 

It would seem that the $130AU leak down tester with some luck may have prevented a future scary moment.

 

After next weekend I'll advise what the outcome was, and how poor as a result I have become.

 

Andy

Mate just double check with the instructions that came with the tester, because in the past I've used 90% (10% leakage) as the limit.This was in automobiles not A/C.Maybe the clearances are larger on the aircooled beastie.But you could be right ,from CASA http://casa.gov.au/airworth/aac/PART-6/6-032.HTM

 

Have fun.

 

 

[Guest Andys@coffs]

I couldn't wait until next weekend and started to pull off the head on Cyl 1 to try and work out what was wrong. In the end I noticed a couple of things and will post photos of the various bits to illustrate the findings. Some are OK photos, due to the fact that they were done at home with a reasonable point and click digital camera, and the others are the best you can hope for with what the manufacturers optimistically call cameras that fit into phones.

 

When I pulled the head off the fist thing that struck me was that there appeared to be a problem with the piston. At the prop side (almost exactly) there is an erosion at the edge. There is nothing on the cyl wall to suggest that its a big problem, however I cant see the rings and as such I semi suspect the expert will want to take of the cyl to have a look, (note that I removed the lead/carbon build up around the eroded area with a screwdriver blade so don't think that its odd that you can see piston surface for an arc of 60 degrees or so around the erosion)

 

[ATTACH]3168.vB[/ATTACH][ATTACH]3169.vB[/ATTACH]

 

in any event thats what I want him to do, unless you guys can tell me this is normal. If that occurs, based on the previous conversations that have occurred, the through bolts will be replaced (not reused) as I don't have, and I suspect the expert wont have with him,appropriate NDI equipment to tell me if they are flawed in any way.

 

The head didn't show any significant signs of any leakage between head an cyl, though the tensions in the various bolts definitely were not equal.

 

[ATTACH]3170.vB[/ATTACH]

 

The valves look OK, and I asked a LAME Sumpie that was at the airfield today how they test and he told me refit the plugs and set so that the head/cyl cavity is facing upward and carefully pour in kero. poor in enough that the valves and plugs are fully covered, but no so much that kero runs down the head bolt holes. If the valves leak the leakage will be evident in the inlet/outlet ports.

 

[ATTACH]3171.vB[/ATTACH][ATTACH]3172.vB[/ATTACH]

 

In my head, in both ports kero ended up dripping out so the valves do need to be looked at.

 

So all in all, perhaps more work than I had first anticipated. A replacement piston, if needed, at 140hrs is a bit rough me thinks.

 

Teenie, in the jabiru 3300 motor book the acceptable leakage is defined as, 80psi in, no less than 60psi held (75%). The leakdown tester I have, because the 2nd gauge is not marked in PSI, rather as a percentage, requires a base pressure of 100psi, which then translates, no leakage, to a show of 0% leak on the 2nd gauge (obviously 100psi). The LAME I spoke to today told me that in the GA world, the same 80/60 is used to asses serviceability, less than 60, means the aircraft maintenance release isn't signed until something is done to lift it up to, and beyond 80/60.

 

Regards

 

Andy