Dellorto idle jets

[geeman237]

Maybe someone with superior Dellorto or Weber carb tuning knowledge can answer. With the Dellorto carbs you can fit different sized idle jets and adjust the idle mixture screw (obviously not different sized idle jets on the same car at the same time). So, what is the difference in the performance/running results of the following hypothetical setups.

Assume both setups are made on the same car as follows.

1) Size 52 idle jets and idle mixture screw 4 turns out

2) Size 56 idle jets and idle mixture screw 3 turns out

I think, in theory, both could result in the car running, but which one would be 'more right'?

I asked this on a Lotus forum but I don't think I got an answer that convinced me.

[richhead]

Not sure it would really matter, with carbs ive always gone for getting the jet as close to the middle as any adjustment as poss.

[E-bmw]

Both can be right.

If those sizes/settings result in the same flow then they are both correct.

The only difference being one will allow more adjustment down & one will allow more adjustment up, should either be required.

[stevieturbo]

I presume the idle jets also offer something in transition to the main circuit ?

the only way to see what is optimal or better for your setup, is trial and error, testing for both performance, desired mixtures etc etc

[DVandrews]

There an article on my website on Dellorto jetting, the idle jets and holders supply the idle fuelling *and* the progression fuelling, it’s possible to have the idle perfect and the progression impossibly rich or lean depending on idle jet and holder selection, so the idle / holder selection needs to be correct for progression and then it can be trimmed for idle fuelling.

Dave

[2172cc]

At what rpm is the transition from idle to main jets generally?
My Dellorto's idle perfectly smooth but stumble and has a bit of a flat spot at around 2000 to 2100 rpm .You can push on through it and is fine after all the way up to the red line.

[DVandrews]

2172cc said:

At what rpm is the transition from idle to main jets generally?
My Dellorto's idle perfectly smooth but stumble and has a bit of a flat spot at around 2000 to 2100 rpm .You can push on through it and is fine after all the way up to the red line.

Progression is generally from around 1200-1500 to the main circuit which can come in around 2500 ish depending on choke size, emulsion tube type etc. maybe a richer jet or richer jet holder (smaller air jet) would help.

Here is a snippet from the article below..





Dave

Edited by DVandrews on Saturday 10th May 17:24

[2172cc]

DVandrews said:

2172cc said:

At what rpm is the transition from idle to main jets generally?
My Dellorto's idle perfectly smooth but stumble and has a bit of a flat spot at around 2000 to 2100 rpm .You can push on through it and is fine after all the way up to the red line.

Progression is generally from around 1200-1500 to the main circuit which can come in around 2500 ish depending on choke size, emulsion tube type etc. maybe a richer jet or richer jet holder (smaller air jet) would help.

Here is a snippet from the article below..





Dave

Edited by DVandrews on Saturday 10th May 17:24

That's very helpful....thanks.
I've lived with it for a few years now and learnt to drive round it but must get it sorted at some point.