Weber dcnf options
- Thread starter petex19
- Start date
Ulix
True Classic
You ordered the low K&Ns, correct?
Problem is, they are low so they fit under the engine lid, but this makes them too low to fit anything inside them and leave enough room for breathing.
Your best bet is to run a flattop carb in combination with super low 3D printed bellmouths inside those filters.
Or keep it simple and just bolt the K&Ns onto the stock 40DCNFs and call it good. That‘s what most people have done, me included. (I now run the setup described above).
Problem is, they are low so they fit under the engine lid, but this makes them too low to fit anything inside them and leave enough room for breathing.
Your best bet is to run a flattop carb in combination with super low 3D printed bellmouths inside those filters.
Or keep it simple and just bolt the K&Ns onto the stock 40DCNFs and call it good. That‘s what most people have done, me included. (I now run the setup described above).
tigeravg
True Classic
The little "stacks" in the first pic is how a lot of us X geeks have done that mod. But there are many, many choices available, even tiny little stubby stacks. They aren't at all hard to find. You may have to (dremmel or) saw off that top (verticle) part of the casting (pic #2). Minor surgery = the results you are looking for.
Keep in mind clearance between the top of the stack and the bottom of the air filter lid is pretty important.
Keep in mind clearance between the top of the stack and the bottom of the air filter lid is pretty important.
petex19
True Classic
No I ended up buying the genuine k&n filters that are 3.5" high. I've been told they will fit under the engine cover. Are there bell mouth stacks that fit over the carb tops? I have two style of carb tops.
Neither are flat but close to flat.
Yes those are dryer sheets in the picture. Car is still in winter barn storage. Lol
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Dr.Jeff
True Classic
There's two common styles of top covers for the DCNF. One has the vertical cylinders coming up, like in both of your pics in post #4 (more on that in a minute). The other is completely flat across the top. See pics below. The first style (raised cylinders) does not easily fit velocity stacks. I seem to recall seeing some in the past that could fit around the raised tops, but I believe they were custom made and frankly did not look like they would work well. Otherwise there are several styles and sizes of stacks for the flat top carb covers. Some can fit inside the air filters.
Regarding the two carbs in your last pics. The one with the shorter vertical section was likely cut down by someone previously. I've never seen a 'short' version like that before. So it would have started like the other carb in your pictures before being cut. And that is another option for the raised top style; cut off the raised portion to make them flat.
Here's the two styles of top covers:
While this example is not a DCNF, it illustrates how a custom velocity stack could be made to fit over / around the raised style top cover:
Regarding the two carbs in your last pics. The one with the shorter vertical section was likely cut down by someone previously. I've never seen a 'short' version like that before. So it would have started like the other carb in your pictures before being cut. And that is another option for the raised top style; cut off the raised portion to make them flat.
Here's the two styles of top covers:
While this example is not a DCNF, it illustrates how a custom velocity stack could be made to fit over / around the raised style top cover:
Steve Thomas
True Classic
Bear in mind that there is a correct geometric ratio between choke diameter and bellmouth height and maximum diameter. If you deviate too much, all you get minimal benefit - a proper entry radius is still a good thing tho. A 40DCNF needs a bellmouth that is 84mm diameter at entry and a height of 40mm. The entry must be clear all around and above. It is immediately obvious that "proper" bellmouths are very difficult on a DCNF as the choke spacing is so close.
petex19
True Classic
As always thank you, you've always been a wealth of knowledge. Yes the cut ones were custom cut by previous owner for the BOI airbox I purchased from him. He provided a second set of original tops for no air box. I'm very intrigued on best tops and bellmouth stacks available or anyone that has a 3D design for 3D printing. I have read that some members have went that route.
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fiatfactory
Steve Cecchele
Take the "shortened" DCNF top cover and shorten more, level with the top... then use the available aftermarket DCNF style bellmouths bolted to the flat face... Original DCNF bellmouths have a "lip" on the underside to fit into the corresponding recess on the "flat" topped DCNF's, which are 42 and 44 models... so original weber bellmouths are not really suitable for the purpose... the aftermarket onse from greece/croatia are made without the "lip" on the underside.
Whoever shortened these DCNF tops doesn't have much of a clue about how a carb functions... leaving the tall auxiliary venturis in there well above where the airs being drawn in is going to badly mess with the metering and atomisation of fuel ... no wonder you're having issues tuning it!
you can use short aux venturis from a 44IDF or some models of 36DCNVA, it gives a stronger signal to the aux venturi.
even though the aftermarket bellmouths are "far from technically correct" they still make an appreciable difference to the air entry into the carbs... with no other changes but going from 32 to 34 chokes and adding the bellmouths (to modified dcnf top covers) we picked up 8 rear wheel HP on a 1603 engine, so they must be working....despite not being perfectly technically correct.
SteveC
Whoever shortened these DCNF tops doesn't have much of a clue about how a carb functions... leaving the tall auxiliary venturis in there well above where the airs being drawn in is going to badly mess with the metering and atomisation of fuel ... no wonder you're having issues tuning it!
you can use short aux venturis from a 44IDF or some models of 36DCNVA, it gives a stronger signal to the aux venturi.
even though the aftermarket bellmouths are "far from technically correct" they still make an appreciable difference to the air entry into the carbs... with no other changes but going from 32 to 34 chokes and adding the bellmouths (to modified dcnf top covers) we picked up 8 rear wheel HP on a 1603 engine, so they must be working....despite not being perfectly technically correct.
SteveC
petex19
True Classic
Excellent advice! My bellmouth are aftermarket and flat. The short aux venturis from idf carbs. Do I source 40idf aux venturis or do the 44idf aux venturis fit 40dcnf's? I found these flat tops online. Not inexpensive but an option.
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fiatfactory
Steve Cecchele
The regular Fiat 40IDF 13/15 carb auxiliary venturi is completely different, so not suitable for a 40DCNF.
Most 44IDF, the 42DCNF and some 36DCNVA carbs use the short or medium length aux venturi in the correct size, 4.5.
this metered size corresponds with the cross section in mm squared of the fuel orifice... the smaller the number the earlier (lower air speed) the carb will pull in the main circuit, 4.5's seem to work best on the SOHC, but if you were having a bad transition AFR's (spiking lean towards the end of the progression circuit at 3000-3500 rpm or so) then it would be a tuning tool to correct that by using a smaller cross section and drawing from the main circuit earlier
weber part number is 31966.002 for the medium length 4.5 aux venturi (in the middle) ... usually the suffix (in this case 002) would correspond with the metered size, but not in this particular parts case ... just to be frustrating.
the long aux venturis that come standard on most 40DCNF are 70503.450 (to the left)
the really short aux venturi (on the right) comes out of a 36DCNVA and some 42 dcnf's and other weber models, I'm not 100% sure of the number but I think it's a 31956.002 for the 4.5 size
no need to buy new flat (or in that case stepped - the picture shows the type with the recess for the bellmouths with the underside lip) carb top sections, a hacksaw and a file will get the machined set you have flat.... the metal is very soft and it files easily
SteveC
Most 44IDF, the 42DCNF and some 36DCNVA carbs use the short or medium length aux venturi in the correct size, 4.5.
this metered size corresponds with the cross section in mm squared of the fuel orifice... the smaller the number the earlier (lower air speed) the carb will pull in the main circuit, 4.5's seem to work best on the SOHC, but if you were having a bad transition AFR's (spiking lean towards the end of the progression circuit at 3000-3500 rpm or so) then it would be a tuning tool to correct that by using a smaller cross section and drawing from the main circuit earlier
weber part number is 31966.002 for the medium length 4.5 aux venturi (in the middle) ... usually the suffix (in this case 002) would correspond with the metered size, but not in this particular parts case ... just to be frustrating.
the long aux venturis that come standard on most 40DCNF are 70503.450 (to the left)
the really short aux venturi (on the right) comes out of a 36DCNVA and some 42 dcnf's and other weber models, I'm not 100% sure of the number but I think it's a 31956.002 for the 4.5 size
no need to buy new flat (or in that case stepped - the picture shows the type with the recess for the bellmouths with the underside lip) carb top sections, a hacksaw and a file will get the machined set you have flat.... the metal is very soft and it files easily
SteveC
petex19
True Classic
I believe this is the 4.5 aux venturis you are referring to.
