Ferrari Mailing List: Re: Spark plug reading?

(continued)

Hi Peter,

Here you go - usually these formulas are needed when the history of the 
Carbs is not fully know - if they were purchased used for instance -  Let me 
know if you have any trouble with the formulas - I'll be happy to help.  I 
may have the precise engine capacity and rpm/peak hp numbers for your 
Ferrari.

(I may have repeated some of the info below)
To calc the size of the main Venturi's (yours are probably sized properly 
but it's a interesting thing to check -

20? V/1000*N/1000
Where:
V = the size of one cylinder in cc's.
N = the RPM where peak HP is reached.
Then, it's ? of V/1000 X N/1000 *20.
For example, a 3.0L Porsche 911SC would be:
499/1000=.499
5900/1000=5.9
.499*5.9=2.9441
? of 2.9441=1.715838
Times 20=34.31676 or a 34mm venturi is called for.
Next,
To calculate a starting point for the Main Jets, use the Main venturi Size 
/0.21 such as 34/0.21= 162.  In this case the 160's or 165's should be tried 
to determine which works best for this particular engine.
The Air Correction Jets can be calculated by using -
Main Jet Size X 1.1
So, a Main jet of 135 would suggest a AC Jet of 135 X 1.1 or 138.5 making a 
140 Air Correction Jet an appropriate starting point.
The rules of thumb still hold true. The base line settings are only the 
starting point. The example would be:

If your mixture screw is out more than one  - like 11/2 turns - then your 
idle jet is too lean - go up one half size on the Idle jet not main jet.

If you mixture screw is not out one full turn something like only 1/2 turn 
out from seat then your Idle jet is too rich. This is all based on the 
important fact that your speed screws are not open more than ½ turn if they 
are then that is also an indication that you have a lean Idle circuit. You 
are cheating by opening the throttle plates and exposing additional 
progression holes in the transition.
be sure when ever balancing multiple carbs,  bring the high carb down to the 
low carb, then bring them both up to proper idle speed demands.

Here are some synchronizing tips for DCOE's -
http://www.piercemanifolds.com/Sync_And_Idle_Mixture_Adjustment.htm
Here are some tips about tuning the carbs and knowing when you have the 
right idle jet sizes.
http://www.carburetion.com/Weber/adjust.htm

The above should help get you very close -- good luck -
Larry T (67 MGB, 74 911, 78 240D, 91 300D)
www.youroil.net for Oil Analysis and Weber Parts
Test Results http://members.rennlist.com/oil
PORSCHE POSTERS!  youroil.net
Weber Carb Info http://members.rennlist.com/webercarbs
.

----- Original Message ----- 
From: "Peter Pless" <ferrarilist [at] pless.com.au>

To: "Larry Turner" <l02turner [at] comcast.net>

Cc: "'The FerrariList'" <ferrari [at] ferrarilist.com>

Sent: Friday, October 05, 2007 6:35 AM

Subject: Re: [Ferrari] Spark plug reading?
That would be awesome...thanks
-----Original Message-----
From: LarryT [mailto:l02turner [at] comcast.net]
Sent: Friday, 5 October 2007 11:00 AM
To: Peter Pless
Cc: 'The FerrariList'
Subject: Re: [Ferrari] Spark plug reading?
Hi Peter,

   I have no idea what your knowledge of Weber operation is, so excuse me

if this is something you already know.   The Webers run on the Idle jets 
90%
of the time.  Until the pedal linkage reaches the point where they start 
to

engage the acceleration pumps, the Idle Jets are in control.  Once the

Acceleration Jets are engaged the Main Jets are brought into play.   It 
may

seem odd, but you can be running near redline and still be on the Idle 
Jets.
   So, as you can see, the Idle Jets are critical for not only good

running, but also for economical running as well - even though that may 
not

be your primary goal.
   When I tuned my Webers I ended up trying 3 or 4 sets of Idle Jets as I
tried to zero in on the best jets.
   There are some mathmatical formulas for calculating the sizes of the
jets and venturi's - I'll dig them out and send them along - unless you
don't need them?
Take care -
Larry T (67 MGB, 74 911, 78 240D, 91 300D) www.youroil.net for Oil 
Analysis

and Weber Parts Test Results http://members.rennlist.com/oil PORSCHE

POSTERS!  youroil.net Weber Carb Info 
http://members.rennlist.com/webercarbs

.
----- Original Message -----
From: "Peter Pless" <ferrarilist [at] pless.com.au>
To: "Larry Turner" <l02turner [at] comcast.net>
Cc: "'The FerrariList'" <ferrari [at] ferrarilist.com>
Sent: Thursday, October 04, 2007 7:03 PM
Subject: Re: [Ferrari] Spark plug reading?
Thanks, Larry....but I had it on a dyno to check the full throttle stuff
and
ended up with 135 mains.
And this....
http://www.pless.com.au/cars/gt4/rebuild/images/308dyno.JPG
I'm getting a portable analyser in the next few days, so I'll be able to
check what the actual mixture on cruise is.
It was 12.8:1 with the .55 idle jets...but I didn't get to fiddle with
those
while I was on the dyno.
-----Original Message-----
From: LarryT [mailto:l02turner [at] comcast.net]
Sent: Friday, 5 October 2007 2:54 AM
To: Peter Pless
Cc: 'The FerrariList'
Subject: Re: [Ferrari] Spark plug reading?
Hi Peter,

   They *do* look good.  My only concern would be driving hard.   50 
seems

a touch lean to me but I'm no expert.
   Perhaps a 20-30 minute run with rpm's covering the range from idle to

redline.   But be careful with temps, especially at WOT - too lean at 
full

throttle can cause high temps on pistons tops.  Would hate to hear you

burned a hole in a piston.

\

   I know some say when you examine SPs you;re seeing only what happened

immediatley before the engine was stopped - but I believe it takes a

little

time to get a SP to change color/texture a meaningful amount.
   Good luck -
Larry T (67 MGB, 74 911, 78 240D, 91 300D) www.youroil.net for Oil
Analysis
and Weber Parts Test Results http://members.rennlist.com/oil PORSCHE
POSTERS!  youroil.net Weber Carb Info
http://members.rennlist.com/webercarbs
.
----- Original Message -----
From: "Peter Pless" <ferrarilist [at] pless.com.au>
To: "Larry Turner" <l02turner [at] comcast.net>
Cc: "'The FerrariList'" <ferrari [at] ferrarilist.com>
Sent: Thursday, October 04, 2007 11:31 AM
Subject: Re: [Ferrari] Spark plug reading?
http://www.fcavic.com.au//308/index3.htm
I installed .50 idle jets in my 308 and cut the engine whilst doing a
light
cruise.
Haven't stuck the analyser in yet, but this looked pretty good to me.
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Re: Spark plug reading?	<– Date –> <– Thread –>
From: LarryT (l02turnercomcast.net)
Date: Fri, 5 Oct 2007 05:20:40 -0700 (PDT)