Fiat Dino Forum (FDF)

Thanks Rob,

But i'm sure distributor is 100% right. I rebuilt it completely, new weights, bearings, points and everyting.
I have a distributor testmachine here at home so i'm sure everything (including the advance graph) is as good as when it left the factory.

Hi Stefan,

I've been through the same problem as you - but on a Maserati Ghibli Coupe that has 40DCNL carburettors - similar, but definitely different to the DCNFs on our Dinos (and also on later Ghiblis) The problem is also there on my Dino (2400 Coupe) but not quite as annoying.

The bad news is - I have never completely resolved it. But I do have two remaining ideas that I am going to try.
- 1) modern fuels make the advance curve sub-optimal. I understand that you have the perfect factory advance curve calibrated - that's great, and not easy to achieve - but maybe it's not right for the gas we buy today (especially where I live in California). I want to try a more aggressive advance curve, i.e. one that reaches the maximum advance more quickly - so that in the "transition" phase the advance is increasing faster as revs rise.
[Note: I think Adrian, our Dino ignition expert, would agree that he has had good results from setting a slightly more aggressive timing curve]

- 2) Weber DCNF carburettors have no provision to calibrate the IDLE air correction circuit. On a DCNL the idle air can be corrected via the idle jet holder. Look at this diagram, item number 23 "Jet Holder"
http://www.piercemanifolds.com/category_s/243.htm
it is a calibrated part with different sizes you can choose.

On the equivalent drawing for DCNF...
http://www.piercemanifolds.com/category_s/242.htm
there is no such part.

With a DCOE carburettor you can choose idle jets with same fuel calibration but different idle air correction. Thus - a 50F8 is "leaner" than a 50F9 with the same fuel drilling because it has a larger air hole. It works via a cross hole drilled through the jet at right angles to the fuel drilling. Supposedly the idle air correction dominates the fueling at the upper end of the idle circuit's influence - in other words where it transitions to the main circuit and where we feel a hesitation.

Then...a while ago when talking with the guy who calibrated my Dino distributor (he has a spring winding machine!...) he told me that on DCNFs on 308GT4s and other cars he has soldered the air corrector bushing, then re-drilled it a different size to re-calibrate it. That's drastic, but he says it works - and he is a good machinist.

So maybe that is the answer. You can try to check whether your hesitation is a lean or a rich hesitation with the well know procedure of opening the idle speed screw so that the rpm is at the transition phase (and maybe hesitating/stumbling) then open and close the idle jet to see which direction improves the running.

Good luck, please let me know what you think - and especially report anything you find.

Thanks!

-Richard

Hi Richard,

Thank you for sharing your thoughts!

On my Dino i have the DCN carburettors which are again slightly different than the DCNF's but than also not that different really. I think it uses different emulsion tubes, doesn't have a provision for balancing the individual throats, doesn't have choke tubes etc. but in reality quite similar.

Do agree with you that the fuels now a day will have an impact on the adjustments of our carburettors and also ignition.

- 1) I can assure you first hand that your not really gonne get rid of the problem with another advance curve. I made a programmable 123 ignition for my Dino (it was the cheapest and easiest way to fiddle around with my curve without having to disassemble and most likely ruin my original distributor ) It runs fine and it does affect the 'flat spot' by programming a more radical curve. But i think what it does is it covers some of the edges or narrows the band where you really feel your running lean, but your running lean anyhow and no ignition is gonne cover a 1:20 AFR or something that i assume you have when your in the middle of the flat spot. Quite interested in Adrian's opinion/thoughts about this.
So what i need to do now is install a wideband A/F sensor (luckely my exhaust needs to be replaced in the nearby future so i can easily drill a plug for a sensor in there) and just measure what's going on exactly. Maybe i'm running lean even at higher rpm's which would mean i can just install a slightly larger main jet and bring down the point of transition or many other variaties but i just don't know. If i can measure i'll know and than see what i can do about it.



- 2) Your right about the idle air correction, also at my DCN's this is a standard diameter hole and no adjustment possible. As you say drilling that bushing is a bit drastic and i hope i won't have to go there but it might ultimately be an answer to the problem. I think in that case i would chose to put in some 30mm venturi's instead of the 32's which would cost you some top end power around 7000/8000 rpm (an rpm range where i might drive 3 times a year ) but i expect would be a great benifit for your driveability and i'm quite sure it will fix the flat spot.

I think i will install a wideband next week and see if i can find some good knowledgeble oldscool guy around here with a dyno and see what has to be done to sort this problem. As soon as i know more i will let you know.

Thanks!

Stefan

Hi Stefan,

Oh that's great that you've tried a 123ignition distributor - I didn't know they existed for the V6 engines, very interesting. Not good news that adjusting the curve didn't fix it, but still very good to know.

On the fuel side - yes installing an O2 sensor is a good idea and very useful for tuning at steady throttle/cruising situations. I've used one for tuning on Alfa Romeo engines and it's great to have that certainty that the fast-cruise settings are bang-on.

But...one place I've never got it to work is when diagnosing hesitations - when the car starts to stumble, the reading goes all over the place. I imagine it as something like "misfire, misfire, misfire (now super rich) and then BANG (super lean) then misfire again"...and so the O2 sensor never gets time to stabilize and read accurately. But then again, you do get to see what's happening either side of the hesitation, and that's definitely useful.

I'm looking forward to hearing what you find.

Regards,

-Richard

I think officialy they don't make them for the dino but i just took a tunable version of a 6 cyl Mercedes or something and fabricated the flange, bushing and drive-carrier myself.
I'm quite sure though, if you have a really small flat spot that you can 'tune' it out with optimizing the advance curve, or i'll might say cover it so that it won't be noticible anymore.

Trying to diagnose, did you use a wideband or a normal O2 sensor? I can understand that with a normal O2 sensor it might be a problem but i would assume a wideband would be capable of diagnosing excessively rich and lean conditions and most of all display to what extend it's lean or rich.
But if you've been using a wideband and had a hard time diagnosing i'll might reconsider going that path.

I would think that the flat spot (if it is there) is less noticable on a 2400 compared to a 2000 because they both use 32mm venturies, which means that on a 2400 you have the main jets kickin in earlier because of higher airspeed and vacuum through the venturi at the same rpm. That's why i wouldn't be to scared to switch to 30mm venturies on a 2000 because it would still be a somewhat equal setup to a 2400 with 32mm venturies.
What is the displacement of your Maserati and what venturies does it have? Just curious...

Regards,

Stefan

re: distributor - ahh I see - very nice work to adapt that one for the Dino then! Not much room there before you get to the radiator.

No I agree that a steeper advance curve wouldn't mask a bad jetting-induced flat-spot. I mean that if it was caused by modern fuel formulation, then maybe a steeper curve could help a bit. But yeah, if it bogs down and really stumbles then it's probably jetting/mixture.

The Maserati is a 4700cc V8 - but I can't remember the venturi size, sorry. I can look at the book later. It's interesting that you're so focused on velocity - I've always concentrated on mixture, but I see what you mean. The Ferrari Dino 246 used very slightly different jetting - I have it written somewhere but maybe you've seen it? I always assumed it was due to the transverse engine installation and problems in cornering - but not sure.

Have you experimented with float-height? That controls the time when the main circuit kicks-in. That could be another thing that needs adjustment due to modern fuel?

O2 sensor - hmm I'm not sure - I got it from these people http://www.innovatemotorsports.com/products.php but it's an older model - I think it's a wideband.

-Richard

Stefan,

here's the table of DCNF setups. I think this came from Tobi originally. Note different idle jetting for 246 cars.

type car main air corrector emulsion idle pump
40 DCNF 12 Fiat Dino 2400 125 220 F24 50 45
40 DCNF 19 Dino 246 125 220 F24 55 50
40 DCNF 45 Dino 308 GT/4 135 220 F26 55 45
40 DCNF 3 206?/Dino 2000? 120 220 F24 50 45
DCNF 40 Standard Tuning 130 210 F24 50
40 DCNF 12 ES-D-246 130 210 F24 55 45

sorry, table looks good when I'm editing it but all over the place when submitted. Hope it's readable.

-Richard

I did not experiment with the float hight yet, was planning to do that but just ordered some new needle valves before i'm going to experiment. Might make some difference yes but i think i need to stay close to original and it's pretty close now.
I'm not sure if i can really compare these values with my DCN's, the original setting for the DCN looks quite rich compared to the DCNF. I think it is 130 mains, 185 correctors, 55 idles and not sure about the emulsion tubes.
But the diffence in values compared to the fiat and Ferrari DCNF could have something to do with the exhaust system i would guess... but also not sure.

Interesting how small the Dino community is

I once wrote an article about the Dinos DCNF carbs - out of this is the table with the different jettings.
As I have mounted a slightly bigger idle jet (0,55 instead 0,50) to get rid of a small flat spot when slightly accelerating.
Es-D-246 is the License plate of my Dino

maybe its worth a try...

Thomas

That's funny Thomas - yes I wondered what car was "ES-D-246" but now we know, small world indeed. Maybe you could publish the whole article here - might be very interesting!

It's difficult to plan an approach to resolving these hesitation problems - you have to weigh several factors.
- How much work is it to try something?
- how much do I believe it's the problem?
- how expensive are the parts just for an experiment?

If it were my car (and I might do this myself soon, but I'm on other projects right now) I think I would do it in this order:

- work out whether it's a lean or a rich stumble - then assuming it's LEAN...
- adjust float height higher (or fuel depth deeper) by 1mm - replace needle valves while you're in there (I don't think they're your problem)
- check for idle jet size and try 55 if not already
- try the F24 emulsion tube if it's not in there already (but..very expensive) - that also assumes DCN and DCNF emulsion tubes are the same, I'm not sure
- then reduce venturi size.

just my opinion and nothing more - whatever you do I would be very interested to know the results.

-Richard