Ferrari Mailing List: Re: Found 308 no-start problem (unbelievably simple)

[Robert W. Garven Jr. (rgarven]

Hans,

I know about those I was asking about the ones in the specs I posted the "slow running air correction jets"?   @ 1.70mm  I am not sure I know what this is even after taking all my carbs apart reading all the books and looking at my diagrams am I missing something?

Thanks for all the info!

Rob

Attachment: 1975 GT4 OM Carb data.pdf
Description: Adobe PDF document

Robert W. Garven Jr.

rgarven [at] gmail.com

 "The Ferrari is a dream - people dream of owning this special vehicle and for most people it will remain a dream apart from for those lucky few." Enzo Ferrari

On Sep 9, 2014, at 2:46 PM, Hans E. Hansen <FList [at] hanshansen.org> wrote:

Rob:

The air corrector jets are on top of the emulsion tube.  The emulsion tube

assembly is essentially 3 jets:  1) the tube itself  with its assorted holes,

2) the main jet at the bottom, and 3) the air corrector jet on top.

Carbs are an inherently nonlinear device.  If you had a simple venturi-type

carb, when you flow twice as much air, you get more than twice as much

gas.  To compensate, designers use all sorts of Rube Goldberg methods

to alter the mixture.  Rochester Quadrajets used a needle (metering rod)

that moved up and down inside the jet to block or release fuel.  Strombergs

use a piston apparatus that moves with manifold vacuum.  And Weber

uses emulsion tubes.  Note that the A/C jet lets in air.  So if you 

want a richer mixture, you actually use a *smaller* A/C jet to restrict

the air flow into the emulsion tube.

I don't have my jetting info handy.  Actually, I can't find my notes.

Here's how I ended up tuning mine.  (warning - long boring content.

Most of you should save your time and hit delete right now)

When I got the car it had been tinkered with a bit.  The back end

of the car would rapidly get covered in soot.  It turned out this

was caused by the idle mix screws turned way too rich to compensate

for idle jets that were too small.  Not an optimum solution.  I had

read that in an independent runner Weber setup, the car will run

on the "progression" circuit at a surprisingly high load and rpm

before transitioning into the main fuel circuit.  Thru experimentation

and dumb luck, I figured out just when that happens on my car.

The car had a faint "pop" or "snap" sound at steady highway

speeds.  It was worst at around 70mph, and cold weather

aggravated it.    If I tipped into the throttle even a little, the sound 

would stop.  Also, it went away at a steady 80mph.  On a hunch, 

I moved the choke lever a bit.  Sound went away.  It was running too 

lean.  It appeared that the car was transitioning into the main

circuit at a steady 80mph.

It seems that OEM Weber idle jets are in increments of 5 - 45, 50,

55, etc., but most suppliers (like Pierce Manifolds) sell 1/2 sizes as

well (52, 57, etc.)  I went up just 1/2 size in the idle jets and readjusted

the idle mix screws.  No more soot, no more popping.  Now I had

to figure out the mains and air correctors....  The easiest way is dyno.

After strapping the car down, I explained to the dyno operator that

all I was interested in was plotting A/F vs. rpm.  Didn't really care

that much about hp.  We did a couple of pulls, and found that it 

wasn't too bad.  A bit lean, but not bad.  This was 12 years ago, so

I'm a bit fuzzy as to the exact numbers, but this is representative.

It was in the low 14:1 area at 2500, and gradually richened to the

mid 13:1 area as rpm increased.  A step richer main jet dropped

low rpm to the high 13:1 zone, and high rpm was now near 13:1.

I'd like the high rpm number just a tad richer, so I after I got home

I ordered the next smaller A/C jet to richen that up a bit.  

The dyno guy mentioned that it hit 245hp, but I would take that

with a grain of salt.  Good quality engine dynos are probably

quite accurate, but chassis dynos can vary some.  Many use

various adjustment factors, like driveline loss, atmospheric conditions,

etc.  So one guy's dyno likely will give a different result than the

shop down the street.  I once heard of a dyno test where someone

accidentally place a cold Coke can near where a temp sensor

was on that particular dyno.  Apparently it affected the hp

readings noticeably.  The important thing to look for in a chassis

dyno test is to see if your changes are going in the right direction.

The other factor in my hp reading is that it was a cold wet winter

day in Portland.  The shop doors were wide open to vent exhaust

fumes, so the engine was breathing 40deg air.  Probably helped

a lot.  Other cars that I saw dyno'd that morning seemed to have

hp readings a bit high also.  This dyno did not have any sort of

temperature compensation.  If I took it Phoenix, it might only

show 210hp.  Who knows.

As an aside, the dyno guy said, "I've driven this car."  Huh?  It

turns out that in the 1970's he worked as Bill Harrah Motors, which

was the West Coast distributor for Ferrari.  He did all the PDI 

work.

Hans.

On Tue, Sep 9, 2014 at 12:03 AM, Robert W. Garven Jr. <rgarven [at] gmail.com> wrote:

I am running the stock settings. BTW where are the slow running air corrector jets? Is that the idle screw? Would be willing  to try another setting but mine seems fine

Robert W. Garven Jr.

rgarven [at] gmail.com

 "The Ferrari is a dream - people dream of owning this special vehicle and for most people it will remain a dream apart from for those lucky few." Enzo Ferrari