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Discussion Starter · #1 ·
Short story: WAY WAY WAY RICH

Short story: WAY WAY WAY RICH throughout the whole range.

Took the car for its first dyno run today as I tweak the carb settings. I succeeded all too well in avoiding a lean situation (no wonder it starts so great on snowy mornings).

During the entire run the A/F was usually well below 12, peaking at 12 at 6000, then falling off to 10 at 7000. Did not get a downloaded graph, but the curve was very smooth (4th gear used). The curve leveled off at 6000 rpm and held close to peak hp through 7, only dropping a little. I think I can find a few more ponies leaning out the mixture, as well as going to a 36mm venturi for more flow.

Carb: Weber 40 DCNF; 34mm venturi; 150 mains; 190 air correction; .6 idle; stock FI cams







With the 17% fudge puts me at 260 hp SAE (270 DIN) with suboptimally rich mixture and before any tweaking. I'm told that peak hp occurs around 12.5 to 13.5 A/F, so have a lot of room to improve. Sure explains the sooty tailpipes.
Current plan:
More air, less gas
upsizing the venturis to 36mm for more flow, keep 150 jets in for first run, but have a set of 145 mains ready. Probably should have some 200 A/C as well. This jetting stuff is pretty cheap.

Once I get the new distributor on will roll in a bit of advance as well.
Cams probably at the first part of the year or so.

Considering these first settings were a WAG on my amateur part, I'm pretty proud it runs at all. If I can get the mixture better with more flow, will have much improved response and peak hp. The response and drivability is already way ahead of the FI.

As a side story, the dyno tech at this Mustang hot rodding place says it was an extremely smooth pull and a very smooth and flexible engine - and these numbers are about what he sees from a stock 5.0 liter mustang.

Will be ordering the new jetting tomorrow. Damn, this is fun.
All advice welcome, and thanks to all the great help from jrv, David and many others over the past months.
best to all

Discussion Starter · #2 ·
Hi RT,

others definately ne

Hi RT,

others definately need to chime in here, because when it comes to jetting Webers it generally takes all the help one can get

My guess goes like this:

34mm Venturis
145 mains
220 AC
45-50 idles

34 Venturis to avoid low end bog
145 mains because very little street use is WOT
220 AC's to offset the fat 145's w/your present cam set up.
50 idles to get a lean snappy off idle, but enough fuel for cruising speeds without popping and bucking. (the idles also depend on the size of the idle air bleeds you're currently running).

Just my stab in the dark so no offense taken for disagreements

Regards, JRV

Discussion Starter · #4 ·
jrv - thanks a bizillion


jrv - thanks a bizillion

I am so enjoying setting these up - so much more romantic than duty cycle and pulse width.

I had not thought about it, but will back off on the idles as well as you suggest. It's creamy butter smooth on the 34 mm venturis, so my plan is to try the 36s for drivability - if it is unacceptable will go back to 34s with the 145s as you suggest.

Something interesting is that while even at this not good jetting, I matched claimed Ferrari stock hp, and when corrected, the torque was higher than stock. Weird. I'm hoping to find 15 - 20 hp.

Will be ordering tomorrow; expect next dyno run after the holidays.

best to all, and thanks again

Discussion Starter · #5 ·
I've got to run to

I've got to run to a meeting shortly...back about 2:00 pm today. I've got a few thoughts to help you dial this engine in. Nice torque curve, BTW.


Discussion Starter · #6 ·
Hi Russ,

Although a 34mm ve

Hi Russ,

Although a 34mm venturi is a good choice, a 36mm likely would provide more power up top with little loss in bottom end power. Venturi size is a mathmatical relationship between single cylinder displacement and peak RPM HP desired. Venturi size can have a significant effect, in that it shifts the HP curve...

First step-Dial in idle mixture with Colortune or your method of choice. 45 idle jet great starting point.

Second-Select venturi size. Trade off between low end pull versus high end power. Too big of a venturi can ruin the driveablity of a great engine.

Third-Select main jet. 145 or smaller, even with bigger venturi. This should get the overall mixture, above the transition point, very close.

Four-Select air corrector. Only provides "tweak" at last 20% or so of throttle angle. Bigger AC number means more leaness up top...A single AC jet size change affects the main jet by 1/3 of a jet size. So, it is possible with the AC jet to either "pull" the main jet up or down slightly in mixture strength.

And if you really, really want to get into this...we can talk about the function of the aux. venturi...

Best of luck!


Discussion Starter · #7 ·
Hi Russ,

to add to what Dav

Hi Russ,

to add to what David has says....

once you get close to the general mixture desired you can play with Emulsion Tubes also...(might have to actually with the bigger Venturis to keep bottom/midrange drivable).


Discussion Starter · #8 ·
As usual, thanks to jrv and Da

As usual, thanks to jrv and David for terrific support!!

Got a Weber lesson (for me at least) on proper venturi sizing from Mike at Pierce manifolds as I was ordering the new jets, so thought I would pass it on.

You should use the smallest venturi that allows you to spin to the rpm you are looking for. In my case, moving up to a set of 36mm venturis would be no improvement over the 34s which already allow me to spin to 8K+. Smaller venturis keep the velocity up and crispin the response. A too large venturi will not add to the top end and only flatten the response. A venturi is too small if the engine goes flat at high rpm and will not pull to the desired rpm, it will just get stuck at a certain rpm. That all being said, I am curious about trying the original 32mm chokes that came with the carbs once I get the mixture better sorted to see If I can pull up to 8K with the 32s and improve the response.

It's a little at odds as I understand it to David's experience, but I ordered a set of 36mm venturis as David suggested and will see for myself - Venturis are the easiest things to change.

The jets I ordered were
.5 idles
140 and 145 mains
200 and 220 air correction

This gives me quite a few combinations to try, especially if I swap venturis. He said to shoot for 13.2 or so.

Fun fact: the difference between paper airfilter/airbox and just air box was 8 horsepower !!!! Unlike the Forza article, when I took the airbox off I gained 14 horsepower over baseline!!!!! Of course, no ram effect was in play on the dyno, but interesting numbers, don't you think. So where do I get a 308 K&N?

As usual, any and all advice/experience/corrections welcome.
This is great fun.

thanks to all
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