more DCNF Weberology - hardcore fans only

While going from the 40 DCNF to the 44 DCNF a couple of intersting Weber issues came up to be passed along for the 3 or 4 folks that read this stuff.

Accellerator pump cams
We've talked some about this before. There are two accellerator pump cams available for the DCNF Weber. The common large displacement pump as found on the 246, 308 and widely available -12 carb is usually black and marked with an '11'; the other is a much smaller displacement usually gray piece, typically marked "10" and found on earlier cars and motorcycles. What is often missed is that the cam is matched to its own lever arm assrmbly: the '11' has a smaller lever armed follower and the '10' a longer lever arm using a different casting. The two are interchangeable, but should be installed as a set Since ALL DCNFs (40,42,44) have the same body casting with different bores, the 4 screw accell pump assembly is easily interchangable.
Just to quickly review - the DCNF accel pump shot is fired by throttle movementsending a volume of fuel determined by am displacement into the circuit, which is then again metered between the sized injector nozzle (typical sizes 35, 40, 45) and a fixed internal bypass tract back to the bowl.
Talking to Mike Pierce, there has often been the desire in many DCNF applications to dial in an intermediate sized pump cam, as the '11' gives such a very large volume. He relates many folks take the commonly available '11' , make a template and a cutting jig, the reduce the ramp size to something intermediate. In the 308 application of the 40 DCNF, the engine is fairly happy with rich mixtures anyway, and does well despite large pump shots taking the A/F down as far as 10.5 or so.

Auxilliary venturi
The aux venturi is that tube that sticks out in the airflow, sometimes out near the the top of the carb that the main jet circuit feeds into. It is primarily responsible for generating the vacuum signal that the main circuit sees. The 40 DCNF uses the larger 4.5 aux venturi to give a stronger signal to the main jet system, I believe more compatable with mid and lower flow levels. The 3.5 is obviously shorter, and in addition to less air resistance, seems more appropriate for very high rates of airflow (i.e. racing or high rpm operation). As example, the 44 DCNFs I am using to upgrade the current 40 DCNFs have the 3.5 sized aux venturi for the original application of a high revving Ducati motocycle engine. As with much DCNF stuff (EXCEPT venturis and butterflies), they are easily interchangeable between the carbs.
In my application, I switched a set of the longer 4.5 aux into the 44 DCNFs as I wanted to be sure that there was a strong signal to the main circuitto bring it in early for street use, empirically somewhat compensating for the fact I have a very large main venturi to provide lots of air for the top end which may dilute the low flow signal a bit. One small note on installation: these are usually staked in at the factory and may be hard to remove. I had to use a drift and a soft mallet to get past the factory staking, and then file the staking off to remove the venturis and then install new components.
We'll see what happens when we start to tune real world and I'll keep you updared as I learn more, and would be very interested in your experiences as well.
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Martin
Genau - now that I have accumulated quite a stash of Weber parts, it seems evident that some parts were subcontracted out to different manufacturers at different times, or there were subtle differences between the Italian and Spanish plants. In addition to your example there are a wide range of different markings on DCNF venturis - some raised letter in the metal, some simply stenciled - suggesting different manufacture places or times.
Each cam I have seen does have the number molded in to the body somewhere.

Best from the New World,

Russ

 

would try the usual sources:
piercemanifolds.com
Redline Weber
and Ferrari UK (FChat sponsor)

Sometimes, volkswagen hot rod shops such as Gene Berg keep a few.

In Oz, not sure if there is a weber outlet, but I am sure Frank Capo at Modena Engineering might be a good lead.
PM me if unable to find.http:

//www.modenaengineering.com.au/

 

My error - former FChat sponsor.
They were always helpful to me when I called - that was a few years ago however when they were the only ones with 36mm venturis.
I believe the much more commonly available 32s could be taken out some, but the shape of the venturi is also important so that would have to be figured in.

 

I seem to remember that they are about $10 - $15 US about 5 years ago. Not sure what that is in Oz dollars, but certainly cheaper than they are over here nowadays with the poopy US dollar.

 

Changing metered components on a Weber DCNF is very straightforward and easy - that is one of the many beauties of these little jewels.

With the carb out of the car:
1. Remove top of carb - 4 or 5 screws depending on version
2. Unscrew and remove accellerator pump - careful of the two small metal washer gaskets on either side
3. Remove aux venturis. This may require some tapping from below with a wooden drift and soft mallet sometimes as some were staked in at the factory
4. Slide out venturi. If the aux venturis were staked in, you may have to lightly file any proud material off the side of the top of the barrel to easily slide out the venturi

Replacement is opposite of removal. I have not found re-staking to be neccesary, and it makes subsequent changes lightning quick.

Gaskets are usually a good thing.

CAVEAT:
If you are doing this on the car (which is ok and also easy if aux venturis unstaked - I've done it many many times), you MUST assume that each small part is ALIVE and will try to jump down the carb throat, so take extra precaution. If this happens - DO NOT MOVE THE THROTTLE LINKAGE as the closed butterflies will usually catch the little bastard. If it is a nut from the top of the carb, they can be removed with a magnetic doofer. If not a magnetic part, one of those little grabber thingees or a long hemostat works well.

 

As I am sure you will see, when you go to a larger venturi, the air velocity and vacuum signal to the main circuit is reduced in proportion across the rpm band. This acts to bring in the main circuit later and open a possible small transition hole (depending on how rich it was to begin with). The reduced velocity will lean the main circuit across the band, and may slightly reduce the throttle response. It will also raise the peak flow and power at high rpms - assuming your head/cam can keep up.
Many folks end up with one or two size larger mains to keep the main circuit coming in at the same time and A/F the same. You can use a larger idle to cover the transition hole, but then your fuel economy goes to poop and you run really rich at cruise -- remember the idle circuit happens below the venturi and is what is most often in play in cruising and daily driving in traffic (for most folks).

If you can use a portable A/F meter like an LM-1, it really helps dial in mixtures.

Have fun! With a little practice, you will find you can re-jet all 4 carbs in the car in 20 minutes, soup to nuts.

 

It may have a small transition flat spot with the 130 mains, but those rich 55 idles may reach up and cover it. I have a bung right before the muffler for my LM-1, and I am considering a dual monitoring system.

When you are using one barrel per cylinder, there is a theoretical peak rpm for a specific cylinder volume that a each venturi size can support due to volume of air. there is a chart in most Weber books that show this - I'll try to post it next week. For example, in my case, with a 400 cc cylinder and a 34mm venturi the theoretical max rpm for airflow is about 6400 rpm. In a 308 with a 365cc cylinder volume it is slightly higher. Again, it's the matching process of how much your carbs flow, verses how much your cylinder heads/valves max flow, air cleaner, etc. In your case, the 34mm venturi will peak flown around 7000 if I remember the graph correctly. I think that a properly jetted 34mm venturi can improve the upper end hp on a 308 if it has the earlier cams - it will certainly raise the available rpms that can be 'fed' from the carb. I am less certain about the later US cams

This gets back to matching components on an engine - a screamer cam in a 308 with stock 32mm venturis is a bit shortchanged from the high rpm gains the cam may provide. Again in my case as example with witch I am more familiar, I am going to run 38mm venturis on a 400cc cylinder, which means the theoretical max flow is about 7500 rpm or so. With the porting it's likely the valve/head will support that flow (the stock qv head will not) so the cam durations were chosen to look at a power band peaking at about 7500 -- everything matches. The headers figure in likewise.

Hope this helps, and look forward to any discussion.