Jab Fuel

[Kenlsa]

With Covid19 having an impact on our flying (not in SA though for private owners) and when we come out of it the financial environment may not be too friendly to flying (I am down 15% on Super) I am most pleased to have a Jab in the hangar.

It is so frugal that I plan on 12 litres per hour. Further to this I run 98, with a saving of 50 to 80 cents per litre depending on the cycle. Today I filled up and saved $1 per litre, $12 per hour!

I see that the latest Jab 2.2 are to use about 15L/H but everybody I have spoken to with a Gen 1 use a similar amount to me. My in flight usage is actually 11.7lt but 12 is my plan number.

My Gen 1 solid lifter engine was the last iteration of that model but it was still in need of some fuel delivery work to the jets. The SP model is fairly slippery so it does sip fuel. I normally accelerate slowly and in the early days I found that the mid-range transition would lean out and the plane would almost die through lack of fuel on the needle jet, until onto the main jet.

I lived with it for a while as that was what was supplied in the kit so all must be kosher right?

In the end I swapped out the needle jet for one larger, a 2.80; mid range fixed! Climb-out temps were a little high but still in the green. The factory released a SB indicating that the main jet should be increased to 235 or 245 (if my memory isn't failing). So as a good little Vegemite I replaced the 220 main for the larger size. I use 2800 rpm as my cruise revs so everything is worked out around those revs and this gives me a plan speed of 100kts; easy to plan around. With the larger jet it was running harshly at 2800; very hard running is the best I can explain it. It just wasn't smooth.

In the end after going through a few jets I settled on a main of 225 (2 sizes larger than stock).

Results are a smooth transition through the mid range and a smooth cruise. With only me and half fuel, I climb out at 3000rpm, 85kts @1150fpm to 1500 feet AGL. Temp on climb is 275F, I then transition to a cruise climb of 2800 and 250fpm until at the desired altitude, then maintain 2800 @ 100 true. Temps are now 250F, right in the middle of the green.

It must be difficult to those of us that use the Jab engine on a different air frame, particularly a draggy one, to get the jetting and cooling right, but I guess they call it "fun".

Ken

[kgwilson]

The Jabiru manual quotes 16.5 lph for the 2200 at 2800 rpm & 23.5 lph for the 3300 at 2800 rpm in a Jabiru aircraft. I get much better than that with my 3300A in the Sierra at low level (around 18-19 lph but at high altitude (7-9000 feet) it is up around 23-24 lph. I have never altered the jets so the carb is as it was out of the factory.

[Red]

I run Esso super unleade in my jab as its the only Mogas around here with no ethanol in it, its quite inexpensive now......... fat lot of good that is to me

I havn't had the jab engine long enough to get a consistant LPH Figure but it's about 13LPH, certainly nowhere near what jab says it is..I run about 2600-2700rpm cruise

 

Oh forgot to say its in a draggy airframe (Easy Raider tail dragger)

[Kenlsa]

Our club's current L2 and our new L2 maintainer who both seem to be serial builders have just recently finished an Aeropup with 21" bush wheels and with a 2.2 Gen2 up front. Early figures suggest 80kts true at 2800 burning 16lts. They recon they might be able to improve theses numbers a little. They both have to skills to eek out a bit more, time will tell.

Ken

[Kenlsa]

Btw that Aeropup has a bolly prop and they have spent quite a bit of time getting 23" manifold pressure while testing various pitch settings to get the above numbers.

Ken

[coinz]

The Jabiru manual quotes 16.5 lph for the 2200 at 2800 rpm & 23.5 lph for the 3300 at 2800 rpm in a Jabiru aircraft. I get much better than that with my 3300A in the Sierra at low level (around 18-19 lph but at high altitude (7-9000 feet) it is up around 23-24 lph. I have never altered the jets so the carb is as it was out of the factory.

So what am i missing here?fair enough speed would be up but why has fuel usage gone up at 9000ft?

[kgwilson]

In theory fuel burn should be lower at high altitude and in an aircraft with mixture control you can lean the mixture at high altitude. The air is thinner so more air is required and if the mixture stays the same the engine will run rich. The Bing carb is supposed to be pressure compensating so should automatically lean as the air gets less dense but I don't think it does this very well. I don't have a fuel flow meter and just work out fuel use by dipping the tank before & after the flight. Climbing to altitude obviously uses more fuel but I also cruise at a higher speed & a few more RPM 2850-2900. At low altitude around 2000 feet I fly generally somewhat slower as the thermals can be quite uncomfortable so while 2800 is normal I throttle back a bit when it is rough both from a comfort and airframe stress perspective

[Paul davenport]

I have now flown for some hours with a mixture control on my 230. Because of an EGT problem at full power I have to run a larger main and needle jet, at altitude it runs too rich and far too cool EGT wise, but with the mixture adjusted and EGT set to 710 I use between 19 to 21 lph . You are correct in saying the Bing doesn’t manage mixture very well and my experience confirms that up to 4000 ft it’s does the job, above this however it just can’t cope

[Red]

I wonder if the 32mm Bing as fitted to very early Jab Engines might do better at altitude? anyone still have the smaller Carby fitted?

[jetjr]

Are you watching EGT?

Chasing economy in Jabiru engines is a slippery slope

Was a series of low time failures from a Jabiru supplied economy kit

Fuel is used to cool Jab engines and also remember fuel distribution within the engine can be terrible. Some cylinders cool with others at destructive temps

[Paul davenport]

Always adjust mixture to the HOTTEST cylinder and let the rest look after them selves. So in my case through good luck or great technical ability ( I. Would like to think the latter, but that’s debatable ) my mixtures are within 30 degrees of each other. I monitor all cylinders, one Jab driver with similar leaning systems has 200 plus hrs experience and recommends leaning above 4000 ft. In my experience cruising at altitude can produce EGTS of lower than 600 c.

[Bruce Tuncks]

my old ( serial 425 ) Jab engine still has a 32mm carby. The EGT spread was awful and so were the cht's. They are now very good but it took a lot of tweaking to get them there.

As far as the old carby goes, I was surprised when they upped the size to get more power. Remember the old VW beetle? It had legendary reliability until they upped the power.

Once I complained that my engine was left out of most of the AD's. " no history of trouble with those serial numbers" was the reply. That's a pic of the cht's at the top of the climb a few days ago.

[Bruce Tuncks]

Kensla, I used to flight plan on 15 l/hr and one day I swear it used more like 17.

So these days I flight plan on 17 and it uses more like 12.

There was a Jab forced landing where he ran out of fuel and he reckons it must have used 17. My point is to not underestimate Murphy and be aware that if you rely on 12 it will use more.

[Flightrite]

I operate a 2200 solid job wth a carbon fiber prop in a small single seater. 2900 rpm I get 12.3 LPH, I flight plan 15LPH, fuel is the cheap part of flying!

I also keep 1 hr Fixed res.

A FF gauge is a must in any aircraft, well for me anyway.

Edited May 25, 2020 by Flightrite

[Old Koreelah]

Kensla, I used to flight plan on 15 l/hr and one day I swear it used more like 17.

So these days I flight plan on 17 and it uses more like 12.

There was a Jab forced landing where he ran out of fuel and he reckons it must have used 17. My point is to not underestimate Murphy and be aware that if you rely on 12 it will use more.

I agree with Paul: the carb doesn't adjust much for higher altitudes. I routinely see 11 lph or less at 85kt, but climbing over bad weather can burn heaps more. I once had to land to refuel because, on top of the 23 lph at WOT, there's the extra rich burn because of thinner air.

As others have said, it's a petrol-cooled engine, so don't go overboard trying to get the best economy; it may end up costing you heaps.

[kgwilson]

The cost of fuel is the cheapest part of flying.

[Bruce Tuncks]

Thanks old K. I didn't know about extra usage at altitude. I reckon this will need some flying to investigate. And I agree about a gauge. I find it harder these days to turn and see the last 20 l of fuel in the tank.

[Paul davenport]

On a flight to Swan Hill today at 6500 ft these are my temps with the mixture adjusted before adjusting the mixtures my EGT was 660, my adjusted limit is 710 c . I might say it was very cold up there . The way my carburettor is jetted and at altitude I can get 19 lph 106/110 knts 2600/2700 rpm. Oops file to big but EGT 710 cht 130 highest temps

[Flightrite]

I run 90-100 Cyl temp 640 EGT, runs like a Swiss watch the old 2200, 485 hrs? I run a Navman 2100 fuel flow gauge, from the marine industry, best thing since they invented GOS?