Hi All,
My trusty old Sigma GJ has never failed me in the 30 plus yeas.
But ever since the introduction of ULP it has not been the same gem viz,
poor fuel economy, running with too rich a fuel mixture and resultant black sooty sparkplugs.

Last week first time ever the car refused to run on all fours and I had to clean the soot of the
sparkplugs to get it going again (as it happens I was in a hurry!).

Would love to know what things have been tried to these old Sigmas to improve the performance with modern fuels.

Things like:
Timing changes? (ULP burns slower than LP but RON 91 burns faster than RON 97 - so is the net effect a retarding or advancing one).
Hotter sparkplugs?
Increasing sparkplug gaps?
Adjusting those factory sealed screws for fuel mixture ratio?
Different air filter?

It would be interesting.

Does anyone use lead replacement lubricants in these cars?

I owned a 2.6L GJ years ago, and you are right, they run rich. Terrible fuel economy for the power.

You might be looking at oil burning and a slow to release choke as well. They were also well known for piston rings losing tension and excessive bore wear. Mine had an engine that was pretty well worn out at 180,000 km.

Just use hotter sparkplugs. N9YC were standard if I remember correctly.

I had a GH sigma it was my first car and never used lead replacement additives, only used 91 unleaded. Couldn't tell you about fuel economy as all I wanted to do was go fast and run people lol.
I was running a 32/36 weber off an XF falcon without a choke before I turbo'd it which the stromberg would play up during summer and run rich but was fun to drive. I miss it some times :( check the air filter. You could try iridium spark plugs as the electrode is smaller and helps with plug foul but for around 4x the price it not worth it.

It's not loosing any oil but the oil is black even after a flush and change.
The auto choke I am sure is part of the problem, the car often travels short distances like about 10 KM and half of that distance its on choke and fast idle.
Not so good for fuel economy (btwn 6 and 7 KM/L - more like a thirsty V8 than a spritely 4!) and it sure helps with sooting up the sparkplugs.
Always planned to get a manual choke kit for it but never got around to doing it.

Yesterday bought a new air filter for it (repco A299) and see if that helps, the old one looked okay but there was some black oil contamination on it.

Still like my sigma and some tweaking (I hope) might just avoid having to pull the sparkplugs out for a clean at the most in-opportune time.
Only last month I put in a relay to feed the starter moter solenoid as it sometimes wouldn't engage (because of increasing volt drop in the igintion switch).

The spark plug you are running is too cold. If you use a hotter plug it would definitely reduce, if not eliminate the soot build-up. Is the soot evenly distributed along all 4 plugs? If one is copping it worse, then as above excessive bore wear or piston ring damage could be evident.

On a sigma that age, I would use a higher heat range copper plug and try changing it every 20,000km. Just as insurance against fouling. Plugs are cheap enough. An extra $20 per year won't hurt.

You haven't mentioned km on the engine.

Auto choke should come off after 1 or 2 km.
It's adjustable.
They get lazy as they age and stay on too long.
Manual choke kit is a good idea, if you can source one.

You've probably got a high fuel level in the carburettor too, if the carb has a small window on the front of it, have a look.
Should be on the dot with the engine running.

Oil turning black quickly is a sign that the engine is running rich, and carbon is being washed off the pistons and combustion chamber into the oil.

As above, hotter plugs and change them often. Keep a spare set and clean them up when the engine starts to misfire.

Oil on the air filter is an indication of blowby past the rings, ie a tired engine.

ULP is fine without additives, alloy heads on the Sigmas have hardened valve seat inserts that don't need lead replacement. Ran ULP on earlier Sigmas without problems.

@ dreggzy -
Couple of years ago I went up 1 "heat step" with bosch plugs, at least I think I did. Will pull a plug out and check the number.
Don't mind going to even hotter plugs but have no method as to which heat range will suit.
All 4 plugs have the same sooty look and it happens quickly. I cleaned the plugs a few weeks ago when I first started driving the sigma again.
I am guessing the car has done 400 to 500 km of short trips since then (half of that on choke!).

@ MadMax -
Km done in 32 years is a mere 145,000 and I've only just started to drive the sigma again after 6 months (because my Magna TP also has problems
- see http://www.aussiemagna.com/forums/showthread.php?t=100243).
Fast idle from the auto choke is all over the place on first start but settles quickly (1/2 km or so). When I look at the choke flap its fully closed and spring loaded (amazed any air gets past at all!).
My service manual has no info on how to adjust the auto choke mechanism but it would be great if I can experiment with its settings so it opens a bit quicker.
However, I have experimented with the fast idle adjust from no fast idle to 2000 RPM (to try and find some fuel saving here) I left it at about 1700 RPM (bit below spec).
Fast idle starts to reduce after about 3 km (as soon as the temp gauge needle creeps up) and the engine is at normal idle after 5 km.
Will act on your hint of checking the fuel level in the carburettor and also get a set of hotter sparkplugs (then I can keep the current ones as quick replace spares).

Thanks All.

Fuel float level in the carburettor is just below the centre of the dot mark when the car is idling.
It rises to about 1 mm above the top of the dot mark when the engine is not running.
I think that is okay but not sure.

Sounds ok. One less potential problem to worry about.

Definitely get hotter plugs and keep a clean pair handy.

@ MadMax
Will get hotter plugs for sure.

I guess I will have to do some trial and error on the timing as there doesn't seem to be much info on using standard ULP with these old cars.
Since standard ULP has a lower octane rating (91 vs 97 for LP) it seems reasonable to try retarding the timing a couple of degrees.
Hopefully I'll see a bit of improvement in consumption and the engine might even run a bit hotter (in turn reduce the rapid soot build up on the plugs).
If anyone has experimented in this manner I would love to hear about any outcomes.

My wagon is one of the last leaded petrol models, December 1985. It has no problems regarding unleaded fuel. If the choke is all OK, check the needle valve and the float level. I would say get the carby overhauled, but finding someone that knows their way around them is another thing. Mine has just ticked over 150k, and I still love driving it.

The auto choke is a pain and I should have replaced with a manual choke long ago when the kits were easily available.
I stopped using the lead replacement additive after finding out the valve seats were hardened ones and just use ULP now.

After adding hotter plugs the carbon fouling eased sufficiently for me not to have to clean the plugs every few weeks.
They are still completely covered in black sooty deposits but still fire enough to start the car every morning.

I changed the timing up and down to see if there was some improvement in fuel economy.
The "sweet spot" is 7 degress BTDC as per manufactures spec.
Fuel consumption is still about 15 L /100K on very short drives 5 km to 10 km trips (a bit better on longer trips).

And I still like driving this now rusty bucket that has no sheen left.

Pity so few people are now interested in this old car. Over 25 years makes it a vintage car?