Looking for a steer on tuning DCNF's

[ladaspeed]

Long story short, I bought this car back having built it up 15 years ago. At the time I didnt have the funds for an AFR or rolling road tuning after I put the DCNF's on so I took a punt and left it at that.

After the rust work and a scratch rewire it has an AFR and I would like to get the jetting bang on.

So this is the state of play;

2x 40 dcnf 12
At the correct angle for an X

Tipo 1600
Deshrouded valves, everything lightened/balanced etc
Fast road cam, 421 headers

Venturi = 33 (roughly, someone machined these out)
Idle jets = 45
Mains = 125
Emulsion tubes = F30
Air correction = 200

Carbs are balanced.

At idle I have set the screws to give about 13:1.

The idle circuit is basically running at 12:1.

Running at higher revs is a bit leaner, it moves around a bit but is sitting at 13-14.

Any acceleration at all gives a little bit of hesitation and when it picks up it leans it out to 15.5 or 16.

I imagine smaller airs would be the first port of call, but basically looking for any obvious steers to get that acceleration figure back into range.

It pulls pretty hard as it is up to about 8500rpm.

TIA

 

[dllubin]

Any reason why you are using F30 emulsion tubes? I'm not familiar with the Tipo 1600, but with the stock 1300 and 1500 motors most seem to be using F24 or F36 emulsion tubes. If the hesitation is through the transition region you might want to go up on the idle jets. Using 50 or 55 is common. I'm running 130 mains and 220 air correctors with F36 tubes.

 

[ladaspeed]

The F30's are just what are there when I got the carbs. I will be ordering some parts so maybe I will include F24's.

Tbh I am slightly unclear on the difference an f24 will have, I know it means less fuel in the well and less holes so probably a bit richer, but the finer details are still a bit vague in my head.

Today I checked the float levels, they were set at 50 - 58.5, I have tweaked them to 48 - 58.5 for a bit more fuel. I will check to see if that has any effect on the next drive when it has stopped raining....

 

[ladaspeed]

The 1600 is essentially a longer stroke 1500, but with a few detail differences.

I suspect the hesitation is coming from too little fuel in the main stage rather than too little in the idle given the 12:1 afr down low.

 

[Ulix]

I think I would step up the mains to a 135 before I‘d touch the air correctors.

 

[ladaspeed]

Can you explain the logic behind going to 135 mains?

I am not being difficult or combative in any way by asking, I just genuinely want to learn the why of the thing...

 

[dllubin]

The 1600 is essentially a longer stroke 1500, but with a few detail differences.

I suspect the hesitation is coming from too little fuel in the main stage rather than too little in the idle given the 12:1 afr down low.

Hesitation going through the transition region is often caused by not enough fuel from the idle jets as the mains have not kicked in yet.

 

[ladaspeed]

Ah so they run out of capacity before the mains kick in?

Back when I put them on I collated as many jetting setups as I could find and most used 50 idles.
The hesitation isnt bad, but noticeable, I am more worried about how lean it is under acceleration.

 

[Ulix]

Any acceleration at all gives a little bit of hesitation and when it picks up it leans it out to 15.5 or 16.

This description sounds to me like the lean condition is between the transition from idles to mains and the point when the air correctors really kick in.
Also, while I ran a 117 main in my 40DCNF, many people seem to end up around 135.

55 idles are good for improving the transition leanness that X1/9s with DCNFs always seem to have.

 

[carl]

You choose your idle jet (probably a bad name, they should be called transition jets) to provide proper transition from idle to the main circuit. Your idle "Jet" is really your mixture screw which can be thought of as an adjustable jet. You adjust your mixture screw for optimum idle mixture. I wouldn't set my idle using the air/fuel gauge, just set the mixture screws for best idle. Save the air/fuel gauge for tuning the transition and main circuit.

Your initial main jet/air corrector combo does seem a bit lean compared to what I ran on the dual 40 IDFs I had on my X, but the air/fuel gauge knows all.