Tech Questions/Advice

79121 need carb adjust advice... [Ryan] (Fri Mar 22 14:18:10 2002) 79126 Re: need carb adjust advice... [stephens] (Fri Mar 22 14:56:57 2002) 79137 ok, here we go... tuning DCNF [stephens] (Fri Mar 22 18:37:26 2002)

Message#: 79121
Subject: need carb adjust advice...
From: Ryan Kooken [Ryan] (fiatfanatic@hotmail.com)
Date: Fri Mar 22 14:18:10 2002
Replies: 79126, 79137
I was wondering if anybody could give me some advice 
regarding tuning in a dual weber 40 DCNF set-up.  the two 
screws at the base of each carb are the idle mixure screws, 
right? how does one know if these are set right?  How does 
one know if the two are in sync w/one another. 

Help me please.. this is driving me nuts trying to figure 
these out.

thanks 

Ryan
Tyrone, PA-Located directly 
above the center of the Earth.
82 zagato
76 scorpion


Message#: 79126
Subject: Re: need carb adjust advice...
From: Rich Stephens (Visalia, CA) [stephens] (rich@3Jar.com)
Date: Fri Mar 22 14:56:57 2002
In-Response-To: 79121

Do you have access to a synchrometer? It is very useful to see 
if both carbs are drawing the same amount of air. You balance 
each side of each carb to each other and then you can dial one 
carb in to match the other.

The idle mixture screws don't need to be in sync with each other 
per se - they need to be right for that particular cylinder. 
Reading the spark plug of each one will tell you if one is rich 
or lean and needs to be adjusted. But chances are that with wear 
and such and different temps in different areas of the engine, 
all the screws won't be the exact same distance out from full 
in.  

I'll post more once I'm off work!

-Rich

Ryan Kooken wrote:
: I was wondering if anybody could give me some advice 
: regarding tuning in a dual weber 40 DCNF set-up.  the two 
: screws at the base of each carb are the idle mixure screws, 
: right? how does one know if these are set right?  How does 
: one know if the two are in sync w/one another. 
: Help me please.. this is driving me nuts trying to figure 
: these out.
: thanks 
: Ryan
: Tyrone, PA-Located directly 
: above the center of the Earth.
: 82 zagato
: 76 scorpion
Rich Stephens
Visalia, CA
'71 Dino 2400 coupe #5262 (daily driver, highway warrior)
'64 1500 cabriolet (getting awfully dusty)
http://www.3jar.com/rich/dino/ 


Message#: 79137
Subject: ok, here we go... tuning DCNF
From: Rich Stephens (Visalia, CA) [stephens] (rich@3Jar.com)
Date: Fri Mar 22 18:37:26 2002
In-Response-To: 79121

I've got three of these DCNF webers and the following worked
pretty well. I'm pretty sure it's fairly standard proceedure for
any multi-weber set up.

Drive the car until it is fully warmed up and then go home.
Disconnect the linkage from the carbs. Turn the idle mixture
screws on each carb (total of four - these are the ones you found
down by the base of the carb) fully in and then back out two
turns. This is supposedly the proper range but you will have to
fine tune them later unless you are lucky and two turns out turns
out to be perfect for you.

Next start the car and adjust the idle speed adjustment screws
(not to be confused with the idle mixture screws - there is only
one idle speed adjustment screw per carb and it doesn't go into
the carb - it pushes on the throttle lever) until the car will
idle at the speed you want. Probably this is about 1/2 to 1 turn
in from when they first start hitting the throttle plates.
Depending on your linkage set up, and what the previous owner has
done, you may have only 1 of these screws which is responsible
for moving the linkage and the linkage automatically pulls the
other carb along. But you are going to need an idle speed
adjustment screw in both/all carbs! Hopefully you've got them.
Just make the carb idle and see that both screws are about the
same distance out.

Next, go to the air bypass screws. These are higher up on the
carb and there is one screw per barrel (i.e. two per carb).
You'll recognize them because they have a locknut that you'll
need an 8mm (i think) wrench to loosen and then a flat head
screwdriver to turn the actual screw. Loosen the air bypass screw
locknuts on all four barrels (in your case) and set all the
screws to be lightly in touch with their seats - don't crank them
down but just stop when they seem to be starting to seat. With
the car idling, place a synchrometer into both barrels of one
carb and see which one is drawing the most air. You then adjust
the air bypass screw on the carb drawing the least air to bring
it eqaul to the other one. Without a synchrometer, you can use a
piece of fuel hose or something - stick it into the top of the
carb and listen to the sound of the air and make them match that
way. I had a synchrometer but I assume that the higher the pitch
sound the air makes, the more air is flowing so tune to higher
pitch. When both barrels are the same, tighten the locknuts so
the air bypass screws won't move anymore.

Move to the next carb and repeat. It doesn't matter if the other
carb is drawing less air than the first - all you are trying to
do is get each side of the same carb to match to each other.
You'll match the two carbs together next.

To balance the two carbs to each other, readjust the idle speed
mixture screw of one of the carbs so that it is drawing the same
amount of air as the other carb. Though I've heard/read it the
other way around too, I would adjust the one that is drawing less
air until it is drawing the same as the other which was drawing
more air.

Now both carbs should be balanced - both to themselves and to
each other. All four barrels will draw the same amount of air and
(if the jets are the same, the same amount of fuel). If your idle
is now out of wack, simply adjust the two idle speed adjustment
screws by equal amounts until the idle speed is where you want
it. Small adjustments can make a big differene so I turn the
screw 1/6 turn at a time (i.e. until the next"flat" of the screw
is facing you). If your car has cooled off by now, you may want
to warm it up fully again because if it's anything like my car,
the idle varies widely depending on how hot the engine is, ha!

You can verify the balancing by idling the car and removing one
spark plug lead at a time - if the idle drops by the same amount
in each case then they are all the same. You can further verify
the accuracy of the idle mixture screws by letting the car idle
for like 10 minutes or something and then remove the spark plugs
(might want to let the engine cool first if it's really hot) and
look at them to see if any are too rich or too lean or whatever.
There are various websites with pictures of spark plugs showing
what is too rich, what is too lean, etc. Search google.com for
"reading spark plugs" or something like that.

Then just put the linkage back on the carbs, making any
adjustments necessary so that it does not alter the idle at all.
I.e. don't let it push on the throtle levers at all. With it on,
pull with your hand to verify that it starts moving each carb's
throttle lever at the exact same time - if not, adjust it. This
is a huge pain in the ass on my car!

Oh, before you even start trying to tune the carbs, make sure
that the float levels are set right (i can look up the specs if
you need them) and that the needle jets are in good shape and
that you have no vacuum leaks (with the car idling you can spray
some carb cleaner here and there, like around the base of the
carbs for instance, and if you hear the idle increase, that means
you have a vacuum leak sucking in that carb cleaner and using it
as fuel. You won't be able to perfectly balance your carbs if air
is getting in from someplace it isn't supposed to. I'd also make
sure that the ignition system is all good: good spark plugs,
leads, etc.

I used to be afraid of carbs and all their tiny parts, ha! But
once you take them apart you realize that all the jets are
different sizes and stuff and it's pretty much impossile to put
it back together wrong, especially if you have a diagram, then
playing around with carbs is pretty fun. As far as the balancing
goes, if you are anything like me you'll just get it pretty good
and then drive the car and say, "hey that's a lot better than it
was!" and then maybe another day check it again and make more
progress and then eventually just decide that the damn things are
good enough!

Thanks goes to Pierre Beniston, Pete Angel, and many here at
Mirafiori for teaching me all that stuff I just typed above. It
works. Give it a try!



Rich Stephens
Visalia, CA
'71 Dino 2400 coupe #5262 (daily driver, highway warrior)
'64 1500 cabriolet (getting awfully dusty)
http://www.3jar.com/rich/dino/


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