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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?
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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.
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Re: Idle mixture adjustment

Postby LSAGuy 104 » Sun Nov 04, 2018 11:49 pm

Steevo, Did you ever get this resolved? Would like to know how it all came out. Have you ever tried running some phosphorescent fuel dye? I've had success with this as then you can see what exactly the fuel is doing.

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Re: Idle mixture adjustment

Postby Steevo » Mon Nov 05, 2018 12:03 am

Yes, I did.

The problem was that some previous owner has installed the economy kit on the carb. No idea why they call it the economy kit as on mine it was running very rich. So rich that the mixture screw couldn’t adjust enough to make much difference. I bought replacement jets and needle (still economy) and the problem still existed.

I went back to the standard needle and jets and problem resolved. It now sidles properly and the engine sounds much happier. The plugs are now no longer going black.
Jabiru J160 & Jabiru SPL-450
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Re: Idle mixture adjustment

Postby MrBean » Fri Nov 09, 2018 9:15 pm

Hi Steevo,

I have an almost identical problem to your original post. Would it be possible for you to post info regarding all of the parts you ordered (part nos.) that finally cured your problem. I'd like to try the same with my engine.

Thanks,
Chris.
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Re: Idle mixture adjustment

Postby Steevo » Fri Nov 09, 2018 9:51 pm

It’s an easy swap. Initially, the following components were in the carby:

Needle 4A138AOD-1
Needle jet 290
Main jet 245
Idle jet 35

With this configuration, I had a very lumpy idle that could not be adjusted to make it any better. The Jab came to me like that. It was kind of hidden when I got it as someone in the past had set the idle to 1200rpm. I guess they did that because it would not idle properly below 1200rpm. I only noticed when I set the idle to about 900rpm as it should be. Plus the black spark plugs worried me.

The first thing I did was to completely overhaul the carby with all new parts including floats as the ones in there were the old black ones that were starting to crack. At first I upgraded the needle to 4A138AOD-5 which is the latest version of the economy needle. I also set the float height. After all this I was convinced it would fix the issue but after refitting, I still had a very lumpy idle and the plugs were still going black. All the jets, needle etc were new parts.

I looked at a couple of other 2200 engines in other aircraft with one of them being a lot newer than mine and they had a different setup. I ordered those parts which were:

Needle N2RING
Needle jet 276
Main jet 250
Idle jet 35

When installing the needle, set it to the 2nd groove from the bottom.

After changing to this my idle was much better and now runs perfect at about 900rpm. I am also no longer getting black spark plugs and my EGTs are roughly where they should be. Looking at the jets, it should be richer as they are bigger jets but I guess they work better with that needle and over all produce a better mixture.

I have noticed no change in fuel economy and the engine just feels better.

Jabiru did say that the economy kit does produce a ‘harsher’ idle when I contacted them and they say that is because at idle it is running as lean as possible, but I found it just annoying and made the whole aircraft shake at anything below 1200rpm.
Jabiru J160 & Jabiru SPL-450
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Re: Idle mixture adjustment

Postby MrBean » Sun Nov 11, 2018 4:41 pm

Hi Steevo,

Many thanks for the detailed reply.

My symptoms are pretty much identical to what you had. I can get mine to idle at 900 but it is very lumpy. It will tend to get lumpier and lumpier the longer I leave it idling at 900 RPM. In fact the RPM will tend to drop off slightly and it'll seem to be hunting around. Above 1000 RPM or so the engine is smooth and fine. Idle mixture screw seems to have very little to no effect on the quality of the idle. Plugs are very black. I did similar to you, changed the floats (mine were looking very brown) and also the float needle valve. Adjusted the float height. Cleaned the jets. None of this made any noticeable difference.

I will order the same parts as you and try them. Hopefully I can achieve a smooth idle at 900 RPM!

Chris.
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Re: Idle mixture adjustment

Postby Steevo » Sun Nov 11, 2018 5:44 pm

Does your one have the economy kit fitted?

To be honest, hunting is usually the sign of an air leak, but that would make it run lean and not rich.

If it does have the economy kit, put it back to standard as I have heard of a few people now that had problems when the economy kit was fitted.

Also, the idle jet is almost impossible to clean properly as the aperture is extremely small. This would also make it hunt but again on the lean side.
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
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Re: Idle mixture adjustment

Postby MrBean » Mon Nov 12, 2018 9:42 pm

99% sure it's the economy kit - won't know for certain until I can get at the aeroplane. Most likely this weekend.

The plugs indicate that it is certainly running rich. But other than the rough/lumpy idle, the engine seems to perform fine. However, this is the first Jabiru engine I've experienced so I've nothing to compare it to. There are a couple of Jabiru-powered aeroplanes at my local flying club, but I've yet to see the owners or the aeroplanes fly. They seem to be "hangar queens" at present.

I'll report back when I've done a few more tests. I may try changing the idle jet initially, and go from there.
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