Weber DCNF jetting
- Thread starter andreav
- Start date
Joe F
Hi Miles, Lo Maintenance
I also have dual 36 DCNF's I run:
Main jets: 125
Idle jets: 55
I strongly recommend eliminating the startup circuit as this makes adjusting the idle much easier.
https://www.ebay.com/itm/COLD-START...-45-DCOE-40-44-48-IDF-DCNF-EMPI-/302788615337
And, it looks like they are even cheaper on their website: https://www.carbparts.eu// (Thanks to MikeHynes).
I love 'em.
Main jets: 125
Idle jets: 55
I strongly recommend eliminating the startup circuit as this makes adjusting the idle much easier.
https://www.ebay.com/itm/COLD-START...-45-DCOE-40-44-48-IDF-DCNF-EMPI-/302788615337
And, it looks like they are even cheaper on their website: https://www.carbparts.eu// (Thanks to MikeHynes).
I love 'em.
carl
True Classic
Over my Fiat life I have had many sets of 40 IDFs which use the same cold start device as the DCNFs. Never used them and always blanked them off. No problem starting my X even on days in the upper 20s. May have to tap dance on the gas pedal for the first mile but after that you are good to go. Can't help with the jetting but there is a chart someone made years ago that was collected data on what DCNF jetting was used.
andreav
True Classic
Thanks, do you know what chokes sizes,emulsion tubes and air jets you run more?
Thanks, I had same experience here with cold start mechanisms, very rarely we used them back in days, so okay I will blanked them.
Dr.Jeff
True Classic
Sorry I can't help with the jet/tube/choke sizes. But the discussion about the "cold start" portion brought back memories of running dual Webers on my off road vehicles many ages ago. They had the old air-cooled VW engines and were strictly offroad use. But all of them had the cold start removed and blocked off. As Carl said, they were a bit cold blooded at first. Typically I'd let them sit running for a minute before driving to help. And starting was no problem if you pumped the linkage a couple times. However with other single Weber applications strictly for street use I found the electric choke (for models of Weber where that was an option) was a very nice addition. Made daily driving the vehicle just like a factory induction; instant start, no warm up period needed, smooth running right from the beginning, never noticed the transition as the choke opened gradually, and never had to give them a thought. So it might depend on the intended application and use (i.e. the target market).
dllubin
Don
This is how my 40 DCNFs are configured:
2 - Weber 40 DCNF 47 7L, Sprint Manifold
Choke: 32mm
Auxiliary Venturi: 4.5mm long
Emulsion Tube: F36
Main Jet: 1.30mm
Air Corrector Jet: 2.20mm
Idle Jet: .50mm
Idle Air Orifice: 1.6129mm (measured)
Pump Jet: .45mm
Pump cam type: 11
Float Level: 50mm
Starter Emulsion Tube: 60F6
Needle Valve: 1.75mm
I actually use the cold start devices. They are connected to the stock 1974 choke cable and work well.
Fiataccompli
Chris Granju
I have 36 DCNFs on a 1500 with European head & an unknown “hot” cam (sorry, all done by a friend and he didn’t document the cam & I haven’t measured it). I eliminated the cold start mechanism and have not found that to be a problem. I should have the jetting written down and I’ll look around for it. I’ve not checked it with a wide band sensor/gauge so while I can say it’s comfortably tractable from 800-6500 rpm, I can’t say whether it’s ideal. It would, however, help establish a “ballpark” on suppose.
andreav
True Classic
I dont have exact spec, as I m working on one dual 36 DCNF kit for market only and I have ideal to put at least some good starting point jetting inside of them.
Ulix
True Classic
Oh, I see.
I agree with dllubin‘s list above.
Additional comments:
The F24 emulsion tube is mostly used and is what I use with good results.
Main jet can range from 115 to 135, air corrector 190-220, depending on the car, but the sizes stated are good starting points.
A 55 idle jet will improve the transition from progression to main circuit.
I agree with dllubin‘s list above.
Additional comments:
The F24 emulsion tube is mostly used and is what I use with good results.
Main jet can range from 115 to 135, air corrector 190-220, depending on the car, but the sizes stated are good starting points.
A 55 idle jet will improve the transition from progression to main circuit.
Last edited:
andreav
True Classic
Thanks will use then as you said.
fiatfactory
Steve Cecchele
As a general "rule of thumb" I like to go a maximum 75% of butterfly size (27mm in 36's) for street driven applications, a maximum of 80% of butterfly size (28.8mm, so max 29mm chokes for 36's) for a car that see's aggressive / spirited driving most of the time and a maximum of 85% of butterfly size (30.6mm for 36's) for purely race conditions.
Too large a venturi will result in poor atomisation of the fuel and lack lustre performance until the air speed is sufficient thru the carb
Required jetting will be based on the choke size chosen, as the carby meters the AIR that the engine can consume, and we pre determine the amount of fuel that will be mixed with the airflow with our jet choices, again a rule of thumb is main jet size = 4 x venturi size (as a good starting point)
I really like F24 E tubes for a car that gets driven aggressively / spirited driving often or for race conditions, F36's seem to work well for street driven or milder tunes.
start with 45 idle jets and work up.... remember stepping up from 0.45 orifice size to 0.50mm orifice size isn't a roughly 10% jump, it's considerably more as it's the CROSS SECTION of the orifice that interests us (pi x radius squared) so having fractional sizes (47 and 52 idle jets) handy will make dialing in the idle / progression phases of the operation (about 80% of the time you're driving unless you're racing)
an often overlooked metered part is the auxiliary venturi size. most of the time these are 4.5's.... this number refers to the cross section area in mm's squared of the orifice that the fuel pulls from, this affects the timing of the main circuit i.e. a smaller orifice size will bring the carby onto the main circuit earlier
SteveC
fiatfactory
Steve Cecchele
it depends on the application, but for a sohc with two carbs, 4.5's seem to work well.
but if you were finding the progression circuit was going very lean, and that couldn't be rectified by a larger idle jet (or if then using a bigger idle jet created a far too rich idle condition) then using a smaller aux venturi size (4.0 or 3.5) to pull the main circuit on earlier would be an option.
SteveC
italianfrank
Daily Driver
I would love to see a pic of how your choke cable is run and hooked up if you could..id appreciate it