Idle Jets and tuning (1984 Turbo)

[Geeman]

I am working on getting the best idle jets and tuning for my US spec 11984 Turbo.  I have removed all the emission equipment and fitted  a catalyst bypass exhaust from SJ.  I have a new 123 distributor fitted courtesy of Barry Spencer too.  The carbs have been ultra-sonically cleaned and rebuilt with a new rebuild kit.  Factory idle jets were 52 and everything I've read suggests these are a little too lean and contribute to the off idle/ 2000-3000rpm 'stumble'.  Often folks over come this by going with large idle jets.

So here is my 'dumb' question.   What is the difference in carb/mixture performance if you fitted say 54 idle jets with the idle mixture screws turned out 4-1/2 full turns vs 58 idle jets with the idle mixture jets turned out 3 full turns and both gave a good smooth idle?

 

[jonwat]

I'd recommend you read Des Hammill's book starting a chapter 4

If the link doesn't work try this

http://foxed.ca/rx7manual/manuals/carb_book.pdf

Edited April 27, 2023 by jonwat

[drdoom]

Just go ahead with the larger idle jets, adjust the mixture screws accordingly. Fuel today is not quite the recipe it was back in 1984. Seems there is consensus on the need for re-jetting these cars.

Cheers

[910Esprit]

Progression takes place prior and independently of the idle mixture screws,  so the larger jet will significantly enrichen progression.   But as you say, you could achieve  a similar mixture at idle by manipulating the idle screw - until you touch the accelerator (although there will be some extra 'bleed' from the progression holes, as the throttles are still cracked open slightly at idle - which can be counteracted by turning in the idle screw).   I personally wouldn't recommend going from a 52 to a 58

 

Edited April 27, 2023 by 910Esprit

[drdoom]

What advice does Barry offer? He'd surely know something of value.

[jonwat]

12 hours ago, 910Esprit said:

 I personally wouldn't recommend going from a 52 to a 58

I agree & would be looking to get all of the jets out in order to give them a thorough cleaning, removing any deposits that have built up over the years. Modern fuel has all sorts of stuff added that can clog up the holes in the jets.  

[Geeman]

Barry recommended size 54, but I have also read of 58 being a good size.  I have 54, 56 & 58 brand new idle jets to try.  Currently I have 54 fitted but they are turned out about 6 full turns to get what I think is a good smooth idle.  Not sure if that's too many turns out as a rule of thumb.

I bought a Gunson CO gas analyser and with the 54 at 6 turns out it gave a reading of about 4.7%

Barry had advised a CO of about 1.4%, but that was with emissions equipment still fitted.  So with mine all removed he said not to worry too much about getting a CO of 1.4% and go with what the engine sounds happy with.

I am just concerned about it running too rich.  Right now after trying 54 and 58 jets, the plugs are more black than brown.

I think it's just a case of me fine tuning all around several times.

[drdoom]

Yup, too rich as things stand at present. Barry speaks wisdom.

[Geeman]

I am following up.  Over the weekend I spent time adjusting the idle mixture screws again with the 54 idle jets.  I drove the car to put it under load, and concentrated on gentle throttle progression to see if I could feel any hesitation or stumble coming off idle.  I kept turning the idle screws in a 1/2 or 1/4 turn (from a 6 turns out baseline) after each run until I just felt a hesitation, then turned out a 1/2 or 1/4 turn.  Checked the idle was still good etc.  Then checked the CO with the Gunson CO analyser.  I got around 2.1% CO.  Reading the Des Hammill book, that CO% seems acceptable and the idle screws are screwed out 4-3/4 turns, so again within an acceptable range.  I am going to maintain those settings for a few miles and see how it performs.  I expect to see the plugs be less black for sure.

Thanks for all your input.

[ekwan]

On 01/05/2023 at 20:37, Geeman said:

I am following up.  Over the weekend I spent time adjusting the idle mixture screws again with the 54 idle jets.  I drove the car to put it under load, and concentrated on gentle throttle progression to see if I could feel any hesitation or stumble coming off idle.  I kept turning the idle screws in a 1/2 or 1/4 turn (from a 6 turns out baseline) after each run until I just felt a hesitation, then turned out a 1/2 or 1/4 turn.  Checked the idle was still good etc.  Then checked the CO with the Gunson CO analyser.  I got around 2.1% CO.  Reading the Des Hammill book, that CO% seems acceptable and the idle screws are screwed out 4-3/4 turns, so again within an acceptable range.  I am going to maintain those settings for a few miles and see how it performs.  I expect to see the plugs be less black for sure.

Thanks for all your input.

You can almost always achieve the percentage CO level you require at idle by adjustment. The big difference is at progression.

Edited May 4, 2023 by ekwan

[mike.griese]

I have found that the factory jets are a good option unless you are having a hesitation when transitioning off of the idle circuit (around 3200 rpm).  Also, have you balanced the carbs with a manometer?  That can make a huge difference for driveability.

[Geeman]

17 hours ago, mike.griese said:

I have found that the factory jets are a good option unless you are having a hesitation when transitioning off of the idle circuit (around 3200 rpm).  Also, have you balanced the carbs with a manometer?  That can make a huge difference for driveability.

It was originally experiencing a transition stumble, and yes I have a Morgantune 4 way manometer.

[mike.griese]

The next step larger idle jets are usually sufficient.