so much job and bad result...?

[antonio]

hi
i've just finished upgrading my engine with ligthed flywheel, dcnf 42 and CSC exhaust, but i'm not satisfied with power improvement.engine is balanced (cranks,pistons, flywheel..) and runs very well, but it suffers from poor throtle in spite of lighted flywheel. i suspect that my fuel pump (electrical one) gives low pressure (0.4 bar with king regulator).but i also suspect bad dcnf tuning, but i don't know what a good jetting would be.if anyone can helps....
ps: dcnf's are 42, but a 40 dcnfs' jetting would be good too
regards
antonio

[Geoff T]

Antonio,
DCNF's generally only run 4-6 psi (.4 bar). Any more and you risk flooding.
I am running twin 40 DCNF's with 32mm chokes down here in New Zealand with the following jets:

Mains 125
Air 195
Idle 65
Emulsion Tubes F22
General performance is excellent but on initial acceleration on a light throttle I get a small "flat spot".
Not really a problem, just take off with a bit more rpm! Must get it sorted one day.
Larger air jets will give you a leaner mixture at the top end, but one size either way doesn't make a descernable difference. IE a 190 or 195 air jet.
Best method to sort out jetting is to run on a rolling road with an exhaust gas analyser connected. That way you will know if you are either running rich or lean right through the rev range.

Geoff T.

[Wallace]

More juice through the accelerator jets ? Made a big difference on the Dellorto's I had fitted . .

[antonio]

thanks guys
i ask guy croft about this, here is what he adviced me;
main 130/135
air 170/175
emulsion F21
idle jet and pump 45/50

i've alerady bought main and air, but it's very hard to find emulsion, idle and pump jets!!
if someone knows...
regards
antonio

[FiatFactory]

DCNF series carbs ... lots of air cooled VW places carry parts. (a lot of parts are also interchangeable with DMTR/DATR carbs)

DCNF Air corrector basic weber part number is 77501. followed by the orifice size, so for example a 1.50mm air corrector will be 77501.150.

A larger air corrector weakens the mixture more at higher than lower RPM.

As a rule of thumb 3 progressions of air corrector (say 1.50 down to 1.35... so 3 x 0.5mm jumps) equals (roughly) one step up in main jet size (say from 1.50 to 1.55)

Increasing the main jet orifice size enriches the mixture uniformly at high and low engine speeds. Weber basic part number for DCNF main jets is 73405. followed by the orifice size, so for example a 1.50main is 73405.150.

Both these metered parts are linear in their numbering... so a bigger suffix = a larger orifice... however when it comes to emulsion tubes, this goes out the window and you need to consult reference tables to decide what is richer/ leaner... as E tubes doesn't follow a numerical progression... just to make life interesting.

Emulsion tubes have more influence at small throttle angles and during accelerations, significant factors that influence it's operation are the outside and inside diameters of the E-tube, which alters the amount of fuel that is displaced in the jet well, generally a thinner tube is richer as it will leave more fuel in the jet well for the engine to draw on.

The location, size and numbers of holes that it has along its length control how the fuel is emulsified as the fuel is drawn from the jet well thru the primary venturi...

If you need more info or want the parts, contact me thru my ebay store.

SteveC

Visit my eBay store at http://stores.ebay.com.au/The-Fiat-Factory

[Darren]

Hi Antonio, as mentioned earlier in this thread, and also on the Montecarlo consortium pages where this same question came up - the settings given will only really allow you to get the engine running, and the best settings will be dependent on your actual car and more importantly engine. The only way you're going to get the correct settings is to take your car to a rolling road speicalist who knows how to tune twin carbs and spend your dosh there.

My twopenneth

Darren