Ferrari Mailing List: 40DCNF rebuilds, fit the last

[Rick Lindsay (rolindsay]

Hello Friends,

Below is an account of the rebuild and subsequent tuning of the four 40DCNF 
Webers on my '79 308GTB.

I have read the workshop manual, various Weber manuals and your excellent 
posts. All have led me in the right direction. Here's the process I used.

For access to the carbs, the air cleaner assembly had to come off, of course. 
Sounds easy but like everything on a Ferrari, it has to happen in the right 
order. First the cover, then the filter element, then the connecting bellows, 
then the trumpets...

Once the trumpets were off I disconnected at the tank, the hose from the oil 
tank (dry sump) to the bottom of the air cleaner. I then carefully lifted the 
lower part of the air box, making sure that the metal spacers stayed on the 
carb studs. I did this with a tiny little screwdriver, poking at the spacers, 
guaranteeing that they really did exit the rubber gasket/adapter/vibration 
isolator/whatever. I then carefully removed each spacer and stored them away 
for cleaning.

Next step was to disconnect the choke cable and thread it back from the front 
carbs. I then disconnected and removed the links between the chokes (enrichment 
devices). This left the carbs connected only by bolts, linkage and fuel lines.

Rob convinced me to replace the fuel lines with new, modern alcohol-safe fuel 
line, so I simply disconnected the banjo bolts and left the old fuel plumbing 
in place, awaiting new.

I disconnected the linkage and removed the bolts holding the carbs to the 
manifolds. The sealing rings were copper, not fiber as is supplied in the new 
kits. I saved everything, of course.

Removing the nuts from the manifold studs is a nasty job. All wrenches are too 
long and sockets won't fit. I ended up following the advice of a kind poster by 
cutting an older wrench to fit! Yet another specialized Ferrari tool, now in 
the box.

I carefully removed the carbs and rebuilt them one at a time starting with the 
left rear. It lifted right off of the studs and the heat insulator. The 
manifold throats were spotlessly clean and bright aluminum. Next off came the 
insulating spacer. I plopped it into a bath of hot soapy Dawn solution and left 
it there as I worked on. I later reused the spacers with new gaskets.

With the carb on the bench, disassembly began. I've posted pictures of that and 
discussed it in detail but suffice to say, all parts went into the ChemDip bath 
for 45 minutes, followed by a bath in hot soapy water. I then blew everything 
dry with compressed air.

I cleaned every jet, tube and corrector as well as cleaning every pathway in 
the carb body. Nothing was gummy or dirty but everything did have that old-fuel 
smell. Anyway, the first carb went back together just fine. I set the float 
level carefully to the factory spec. and put the completed unit is a clean 
storage box.

The second carb, the right rear, came off without much hate, discontent and 
cursing. I had by then learned a bit about getting hands in, around and under 
the various bits in the 'V'.

Upon disassembly, the fuel drained from this second unit was dark brown. I 
suspect it had been trapped in some well, port or the accelerator pump. And of 
course, all these parts went into the ChemDip bath. Upon reassembly I 
inadvertently let a finger slip and under the influence of the spring, launched 
a couple of choke parts into low earth orbit, never to be seen again. That put 
a halt on carb number two's rebuild. It too went into the box while I went 
inside to order replacement parts.

The third carb, front left, was the most difficult to remove. One of the 
mounting nuts was all but impossible to reach with mortal tools. I ended up 
making that stubby wrench, as described earlier. It worked, if painfully.

Back on the workbench, the rebuild went without a hitch, so I moved on to the 
final unit. It too went without problem AND the replacement parts for carb 
number two had arrived. With the installation of the new parts, that unit too 
was finished, thereby completing the rebuilds.

Re-installation of all four carbs went easily. With the threads cleaned on the 
studs and nuts, everything went together finger tight without much fight - 
using new gaskets, of course. I then snugged the mounting nuts down evenly 
completing the physical mounting.

I reconnected all the fuel plumbing, using new fiber crush washers, and after 
installing a new battery, pressure tested the system. No leaks! It was about 
then that Rob shamed me into replacing the fuel lines. :-P Thank you Rob. And 
being susceptible to Shipwright's Disease, I replaced all the hose clamps too. 
Only the lines down to the tanks remain unchanged - but that is because I could 
not access them with the car in the third bay of garage. I'll replace them 
shortly. They are quality line, being about 10 years old.

Today, I disconnected the linkage and decided to do the balancing act. My first 
discovery was that the air-flow meter wouldn't fit the Weber trumpets! A trip 
to the hardware store provided a PVC plumbing elbow that fit my gauge on one 
end, and the other end, being male, fit into the velocity stacks! I added a big 
o-ring for a better seal and set about making measurements.

The linkage was disconnected so I could make the idle settings. The first thing 
I measured was the depression (or airflow) from side to side on each carb. They 
all balanced one another verifying that the throttle shafts were not twisted. 
That was a relief, even though I was extremely careful in cleaning and 
evaluating afterwards. What I discover was a rather large (20% or so) 
difference in air flow between the front two carbs and the rear two. The front 
two, side to side, were nicely balanced right from the start, indicating that 
the between-carb-linkage adjustment was good. The rear two were off by 10%-15%, 
the right rear exhibiting notably lower airflow.

I set the idle to about 1000 RPM using the front left-hand carb's idle screw 
and re-verified the right front's air flow. They were beautifully matched. A 
good start. I moved on to the left rear carb and using the idle speed screw on 
that carb, brought the airflow up while feeling the linkage to verify that it 
wasn't contacting - but it was. I therefore disconnected it and went back to 
balance the airflow on the left rear to match that of the front pair. Pretty 
easily done. And of course, the engine speed increased.

I set the idle back down with the front left idle speed screw and then matched 
the left rear's airflow. Things were getting better. I then measured across the 
rear pair again and adjusted the right rear air flow up to match using the 
between-carb linkage. Everything was quite close. This side-to-side step could 
be done in either order. I believe I did it backwards from the manual's 
recommendation.

Another couple of iterations verified that the airflow on all carbs and all 
throats was equal and the idle speed was (much more) stable at about 950 RPM. 
Of course, the engine was hot. I reconnected the linkage and verified that it 
had free play while at idle. My paranoia made me check the idle airflow again. 
It was beautifully balanced and the exhaust note was a thing of beauty.

Next step was to advance the throttle to a stable 2500 RPM, and check the 
airflow balance. It was pretty far off too - from front to rear. The 
side-to-side balance was already set via the idle balance adjustments.

With the engine off, I unscrewed the locking nut and popped one end of the 
between-banks linkage off. The rear bank was low so I added two complete turns 
of the linkage head and remeasured. It was too much. I went back to one turn 
and remeasured. Fortunately, the balance was right on so I didn't have to 
disconnect the other end of the link and make incremental adjustments.

I verified the balance on all four carbs at 2500 RPM then allowed the engine 
back to idle at 950 RPM and rechecked the airflow at idle. Both settings are 
now beautifully balanced. I just need to replace the locking wires on the 
linkage and the tuning will be complete.

Actually, that's almost right. I do want to rotate the cold coolant cam to 
position one and set the cold idle speed to about 1200 RPM or so. That should 
finish the settings. It will then be time to reassemble the air cleaner and 
road test it for hours on end!

Thanks for reading. Having now been through this, like the others who have 
experienced that process, I am happy to assist anyone needing a coach.

-rick