Carburettor idle mixture adjustment
I presently have a Jabiru J160 engine number 2954. For a while, the engine has been a little lumpy especially at idle.
Today I thought I would have a play with the idle mixture screw to see if I could get it any better.
‘It seems that my idle mixture screw does absolutely nothing. I can screw it all the way in and it makes no difference to the running. I can unscrew it all the way out and even remove it and it still makes no difference.
What could be causing this? I have taken the idle screw out, cleaned it and the hole it goes in and checked the o ring but it has still not made any difference.
The carb currently has an idle jet of 35, needle jet 2.90 and a main jet of 245.
I am going to order an overhaul kit, new jets and idle screw. I am a little confused over the needle as there seem to be so many versions.
Is the correct needle part number 4A138AOD-5?
Many thanks in advance.
That screw isn't the most powerful adjustment in the world. If it's not having an effect then the chances are that it's being overridden by something else in the carby. If you're a little lumpy then checking float level would be a good start, as if the fuel level in the bowl is too high it can over-richen everything. There are a lot of other things mentioned in the maintenance and overhaul manuals too - making sure the sense pipe is fitted correctly, that there isn't any SCAT hose blocking or affecting airflow into the carby... Have you had a chance to look around in there much already or was the idle jet the first place you checked?
Thank you for replying.
I have not had a great look around at it. I have taken it off and brought it home with me. I did notice that the spark plugs were very black which says it is a very rich mixture. I also noticed that in cruise, my EGTs are lower than specified by a fair bit. They are fairly even but in cruise the highest I saw was very low 600 degrees centigrade.
I have now received an overhaul kit from the UK Jabiru dealer plus new jets etc. I have taken the float bowl off and inverted it and the float does seem a bit high. The tab is horizontal with no bend in it at all.
What should the float height be? I have not been through the manual yet but I assume you measure from the casing (where the gasket is) to the bottom of the floats while it is inverted.
When taking it off, I did notice a liquid that looked like oil on the SCAT hose side although there was nothing in the SCAT hose itself. There is a Cobra head fitted.
According to the manual... "Remove the spring clip and detach the bowl from the carburettor. Measure the distance from the fuel surface to the top of the bowl – it should be approximately 12mm"
It does sound like you're too rich. Is this something that just started happening one day or has it always been like that? Depending on how the symptoms presented it could be anything from carby heat being stuck on to float level to fuel pump over-pressure to sense tube issues or just carby settings. If the float needle isn't sealing you can get fuel leaking out of the engine during ground running - are you seeing anything like that?
It's not totally unusual to see a little bit of oil in the intake or in the plenum - particularly if the engine was inhibited recently - and it usually isn't anything sinister. Too much can be a symptom of valve guide/top end issues but your other maintenance checks (leak downs etc) monitor for that sort of issue and if they're coming up ok I wouldn't do anything more than continue monitoring it.
I have only had the J160 a few months. When I first got it, the idle speed was set to 1100rpm so didn't really notice how lumpy it was until I put the idle speed to 800rpm.
I am guessing that the previous owner turned it up to get over that issue.
I have not noticed any fuel leaking/dripping.
I have now noticed that one of the floats is cracked and coming away so over the weekend I will be overhauling the Carby and fit new floats in the hope that will solve the problem. The Aircraft has only flown 106 hours from new (2009 model). It was inhibited so I guess that is where the oil has come from.
Sounds like you might have found the Gremlin, hope it behaves for you!