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Idle mixture adjustment

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Idle mixture adjustment

Postby Steevo » Sat Jun 30, 2018 9:34 pm

Hi Everyone, any carb experts out there?

The engine in my J160 has always been a bit lumpy, especially at idle.

Today was a nice hot day but a bit too windy to fly so I thought I would have a play with the idle mixture adjustment screw to see if I could get the idle any better.

There seems to be a problem with mine. I can screw the mixture adjustment all the way in or unscrew it all the way out and even remove it, but it does not change the running of the engine at all. I have taken the mixture screw our, cleaned everything and checked the o ring but it has made no difference. The idle mixture adjustment screw does not work at all.

Before any one asks, yes it is the idle mixture screw I am turning. Done it loads of times before on other Jabs.

Any ideas what could completely disable the idle mixture adjustment screw?
Jabiru J160 & Jabiru SPL-450
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Re: Idle mixture adjustment

Postby skyjeep » Sun Jul 01, 2018 9:45 pm

Is it possible the float valve is starting to leak?
After aroumnd 400 hrs my 2200 would be lumpy at idle for a while after startup and there was signs of fuel staining from the carb air intake - but not a visible drip - I normally left fuel swiched on all the time so this seepage would not be noticed if fuel is turned off after flight (per the Jabiru handbook)

Another issue is whether the air jet has been drilled out per the 'economy tuning kit' instructions and there was another bulletin about drilling out the starting enrichment passage in the float bowl for some engines - which would allow richer mixture for longer during startup / idle
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Re: Idle mixture adjustment

Postby Steevo » Sun Jul 01, 2018 10:58 pm

I know the choke jet is recommended to be drilled out for easier starting especially in cold weather and I think there was a change to the valve seat size but this was only necessary on the 3300.

There are no leaks, or not that I can see or detect. It seems to be very lumpy at idle. Very hard to run at 800rpm. There are also a couple of RPM spots where it seems worse (1200rpm to 1400rpm).

I have also noticed it seems quite rich as the EGTs are very low and the spark plugs are black. I have tried different jets but nothing seems to make any difference. The fact that the idle mixture screw does absolutely nothing sort of says there is some kind of issue.
Jabiru J160 & Jabiru SPL-450
PS Engineering PM3000 Intercom
Funke THT800H OLED/Trig TT22
Funke ATR833 OLED/Dittel KRT2
Flight DataSystems GD-40 CO Monitoring
PoweFlarm Core
PilotAware
Garmin 496 GPS and SkyDemon
MGL Xtreme EMS
TruTrak Autopilot
User avatar
Steevo
Site Admin
Site Admin
 
Posts: 240
Images: 2
Joined: Fri Feb 06, 2015 2:56 pm
Location: Wickford, Essex
Aircraft: Jabiru UL-450


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