Idle and CO2 Adjustments

[Bercilac]

Now I've got my 280S running properly I'm trying to get the idle and mixture set up. I'm using a Gunson CO analyser, timing is spot on, unleaded heads, pretty much new injection and ignition parts throughout.

My 1990 Haynes Fuel Injection manual covers every popular car from 1979 to 1989 including the TVR and it suggests that the idle should be set first then the mixture.

If I set idle to 900 rpm and then set mixture to 1.30 (1.25 plus or minus .5) then the engine hunts on idle when cold (the Aux Air Valve is working perfectly). If I wind the idle out to 1000 rpm it idles fine when cold, but when warm idles at 1300 rpm.

Cold or hot starting is perfect first turn of the key and there is full power right through the range, no popping or banging on overrun.

Anyone seen this before?

[KelvinatorNZ]

Two things come to mind, check your valve clearances are correct, and that the fuel pressures cold/hot are also correct. The hunting idle is indicative of the mixture being incorrect.

It would be worth having a check through the Capri 2.8i manual, as that's more likely to be accurate than the Haynes imo. http://files.capri.pl/library/manuals/capri-2.8i-1...

Edited by KelvinatorNZ on Monday 13th April 11:26

[Bercilac]

Yep valves spot on, I've got the Capri manual so I'll cross reference all the numbers against the Haynes data.

I just set the hot idle to 900 rpm after running my eldest lad to work (key worker so not a joy ride honest!) and she drives spot on so I will start her up tomorrow when everything has cooled down and see if it still hunts.

[Bercilac]

Incidentally, I don't know how you V8 lads get on with all that power - the 2.8 is enough for me!

[AlfaSpider]

When setting mine up I went for around 2.4- 2.6 on the CO, this is a little rich but runs well. I can’t remember the correct setting exactly.
I think 1.3 is too lean for the 2.8 engine which could be causing it to hunt.

[KelvinatorNZ]

Just checked the TVR 280i (wedge) manual, and its asking for 1.25 + 0.25% CO at 900RPM (which seems the same as the Capri manual). I did mine by feel and sound instead of CO as its not tested here and just set it to the sweetest feeling and sounding mixture at that speed.

Just FYI the manual also states that an irregular idle on the warmup phase isn't the idle mixture, its either cold pressures, AAV not working correctly, start valve leaking, injectors leaking or a vacuum leak. Idle mixture should always been set at operating temperature.

Edited by KelvinatorNZ on Tuesday 14th April 00:41

[Bercilac]

I'll go through the cold pressures again after checking it later this morning. We have a light frost overnight so it should be good and cold today.

[KelvinatorNZ]

Remember that the cold pressure is on a sliding scale, so the lower the ambient temp, the lower the cold pressure.

Details are over here if you arent familiar with the system https://www.tasteslikepetrol.net/2020/02/bosch-k-j...

[v8s4me]

Have a read of THIS. When I had my Wedge I found the best way to tune the tick-over was by ear using the old "best lean" process as you'd use on a carb. The CO seemed a bit high but within limits and it ticked over and ran well and passed the MOT, so that method worked well for me. Try it

[Bercilac]

Cheers that's a cracking guide. She was running sweet this morning but I'll check it all again at the weekend.