The corrosion was due, I am fairly sure, to the climate I live in. My engine arrived in about October and was installed about March. When I installed the engine I connected up all the hoses to the carbie but it then it sat unused till September. During the time it sat in the box in my shed we had a very prolonged wet season with high humidity. While in the box it had bungs in the openings but while installed on the aircraft it did not. I didn’t open the float bowl at installation (or at all until I had problems when I first ran the engine), so I don’t know when the corrosion started but I suspect it was while the engine was in the crate in the wet season.
When I first ran the engine – it ran very roughly. I turned off the engine and then attempted to restart it a few minutes later. At this time it failed to start and Avgas flooded back up the air intake hose and into the hot air mixer box. It then poured out down the firewall and onto the floor.
I pulled off the float bowl and found a 5 mm or so layer of insoluble white substance in the bowl and clogging up the jets and float valve. When I cleaned it out the inner surface of the bowl was scored, pitted and corroded. Jabiru sent me a replacement bowl and gasket and also new jets. All were replaced and the problem was fixed.
Jabiru said they had heard of a few cases similar to mine and they were all from North Queensland. It makes me think that high humidity may well have been the cause. Which leads me to think that instead of leaving the carburettor empty, which is what they say to do for prolonged storage, it might be better to actually fill it up with something that will stop the corrosion. Obviously this only applies if you are in a high humidity area.
Running too lean.
Although I actually didn’t really notice a problem I was only burning 18 litres an hour. While this sounds good, and given all the temperatures were all OK, this fuel burn rate was all at fairly low altitudes. When I started flying it higher it might prove a problem. The spark plugs were a light tan colour – not quite the ash grey of overtly over-lean.
When discussed this with Jabiru it was advised to change to bigger jets in the carbie. These were then installed and the burn rate climbed to 22 litres an hour. (They also sent me a bigger float valve seat as well. When this was changed it led to another problem.
This happened after changing the float valve seat and seemed not related to the original flooding which was due to oxidized aluminium powder clogging the seat valve. I had thoroughly cleaned the float bowl and valves and replaced the seat so it was hard to believe any more of the grunge was stuck in the seat.
I found a couple of times I forgot to turn off the main fuel tap when I hangered it and when I came to start up again the carbie was flooded with fuel sloshing around in the air intake hose from the hot air mixer box. It got worse with time till it flooded after only a few minutes.
Jabiru advised adjusting the tang on the float needle, then changing the float needle and finally changing the seat again. None of the former two helped (in fact the only time adjusting the float needle stopped the flooding it limited the fuel flow so much that the engine ran dry and stopped after ten seconds.
I sent it back to them and they changed the float needle seat and the needle (for a third time!) and stated it was fixed. When I got the carbie back I set it up on the bench, connected up some tubing with a funnel. I poured fuel in to see if it would flood again. It did and I conducted a whole bunch of tests which showed that I could reproduce the conditions reliably to make it flood and also conditions to make it so it would not flood.
Basically, if you left any bungs on the pipes it flooded all the time. If you took off the bungs and the bowl started off completely empty, it flooded every time reliably. If the bowl started off with more than 12 mm of fuel in the bottom it was OK. Anywhere in the mid range it was variable getting more likely to flood as the level started off lower.
Further testing proved the float did not float evenly, it sank down on one side and it left me thinking that perhaps in some situations it lifted off centre and that caused the needle to not seat evenly.
I sent it back and they gave me a replacement carbie. The new one has not flooded since, so that problem was fixed.
Missing on full power.
I noticed (oddly after 30 or so hours of flight) that I was hearing an occasional miss in the engine on climb out, usually after I had reached nearly 1000 feet.
This was only very intermittent and consisted of a single barely noticeable miss. However one morning on climb out I had a series of about half a dozen in quick succession which caused me to return to the field. I noticed that as soon as I dropped the power back the missing disappeared and the engine seemed to be smooth again. A non eventful landing followed.
I consulted a LAME who happened to be present. He had no experience with Jabirus and suggested it might be a faulty spark plug. On inspection we could not find any that looked a problem. (None with an oily black unburned fuel look about them) but I changed them all anyway.
I then spoke to Jabiru who said to check the altitude compensation tube that runs from the carbie to the hot air mixer box. If it was kinked it would cause the same problem. However there was no kink and that tube was clear.
It seemed like it might be starving on climb as it was only occurring at full power.
As my carby was flooding as well on the ground the whole carbie problem was too widespread so with the help of another mate we convinced Jabiru to take the carbie back and replace it.
The new carbie arrived and the problem seemed to be solved. Until I took the aircraft on a long trip away carrying another passenger. The problem never returned as severely but I could notice an occasional miss, even the passenger another pilot, noticed them.
I kept close watch and noticed that in order for the miss to occur I needed to have all three of the following:
Be in a climb with a rate of over 500 ft a minute
Cylinder head temperatures on the cylinders on the right (Passenger) side less than 95 degrees.
If any one of these were not present it did not miss. While on the trip I kept the parameters out of these ranges and it was fine – but obviously still problematic. I don’t want to fly if I can’t climb out at full power. So we came home carefully watching what we did.
I also consulted a friend whose J 2200 engine was doing the same thing and he said that in his research and talking to Jabiru the problem seemed to be related to the air flow down the intake hose from the hot air mixer box to the carbie.
Now in the manual, Jabiru tells you that the tube must be as straight as possible and that even the sharp edges of the inside of the hot air box must be smoothed to a wide radius. So I pulled out part of the box and checked the edges. They were smooth but a went further and ground out the edges into as wide a radius as possible. My hose was already almost straight.
Still no change!
I had a look at the other Jabiru in town but that hose was very curved – mine was much straighter so that, in itself, was not the problem. But he didn’t have any problem like I had.
Back to Jabiru to discuss the problem. This time I spoke with the chief engineer, Don Richter, who said the problem was from air spiralling down the intake tube and then when it passed through the carbie throat it forced rich air to the passenger side cylinders and fuel poor air to the pilot side cylinders. This explained the lower temperatures on the passenger side cylinders. They had experimented with a baffle in the hose which stopped the air spiralling.
So the answer was to make and install a baffle which stopped the air swirling. The last section of the hose is a solid fibre glass section, called the “Cobra Head”. This is where I installed a lengthwise piece of fibre glass, epoxied vertically, almost full length of the cobra head.
Since this installation it has run flawlessly without any misses at all. The temperatures are now good.
Post note: Although it now does not miss at all I have noticed that the temperatures (CHT ) show a variation from one side to the other (higher on the pilot side). The temps are within the ranges Jabiru say they should be but are not even. On advice from another builder I added a cross baffle as well, but this didn't make any difference in my engine. See the extensive section on engine cooling for more details.)