carb tuning DNCVA/DCNF

If I knew how to search on here, I would find my thread a few years ago from the same issues. We did get mine all sorted out and it lists what parts we used. I'll see if I have it written down in any of my notes. More soon, I hope....
 
Here's my post from 2015 when we sorted it out.

Here is the combination that worked.

Idle Jets: 47
Main Jets: 150
Air Correctors: 175
Emulsion Tubes: F36

Hope this helps.
 
To check the main jet size, find a lonely road, get the car in 4th or 5th gear and floor it...and hold the gas pedal down for a few seconds. The A/F meter should settle down reasonably and give you a useful reading . My gauge rarely sits at a single number but you do get a feel for the range you are in. Don't forget, as you first tip into the throttle the accel pump will squirt in gas giving a richer false reading for a second or so. Most literature gives a range for proper jetting, not a single hard number. Obviously if you are in the 10s you are too rich and in the upper teens too lean.
 
A new update and new problem with the carb tune.

Lately the carb started experiencing high idle. I found out the throttle arm on the side was impacting my phenolic spacer by the slightest bit resulting in it not returning to zero. I fixed that but not i have a odd leaning out issue. A note, it was never quite right before.

When driving at steady state, at pretty much any speed or power, the engine is leaning out to the point of dying. My wideband sensor shows it just climbing to 20. I can easily resolve it on the road by blipping the throttle or changing the speed. It corrects it for a short bit and then back towards lean. Idle is fine and pretty trustworthy, so I am thinking it's somewhere in the air, emulsion, main jet combination or the float. I have not checked float on this carb as it was hard to remove the choke linkage. It's and S shaped rod #11 to be exact, that just wouldn't finagle it's way out of it's connections. I am sure I can get it but the last time I tried it took me long enough without success that I gave up, reassembled and drove. Wasn't worth it at that point.

My available jets are:
Air Correction: 175, 180
Emulsion: F36
Main: 130 150
idle: 140 147

Note I broke a tab on the F36 trying to open the top a tad to make it snug with the air correction jet. It was completely loose. Should they be loose or a snug 3 piece combination? I need to order at least another F36 from Pierce.

Are there any recommendations on what else to order to correct the leanness or would it all be in the float level?
 
If you have not checked the float level, I would do that first. Also worth checking/cleaning the float valve while you are at it. They can get sticky if there is buildup there. For the whole main/air corrector/emulsion tube setup to work as designed, the float level needs to be dialed in.

DCNF Float Level.jpg
 
GregS said:
As it has come on suddenly, check for blocked jets. Also check the fuel flow from the pump.
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Whats the safe way to check fuel pump functionality? I used to pump it into a cup but it's not the best to aim and ensure it hits the cup. Same method just as careful as you can be? Flying fuel is what burnt my friend's garage down.
 
dllubin said:
If you have not checked the float level, I would do that first. Also worth checking/cleaning the float valve while you are at it. They can get sticky if there is buildup there. For the whole main/air corrector/emulsion tube setup to work as designed, the float level needs to be dialed in.

View attachment 35474
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I will get to this this week.

In the mean time I need to find an F36 or two. According to Pierce's website they had none available. MWB has F24's. How do those compare? I see the F24 is a bit narrower at the bottom with different whole patterns.
 
I have heard that F24 tubes work well in X dual DCNF setups but have never tried them. If the tubes are narrower, they may run a little richer as narrow tubes leave more room for fuel surrounding the tubes. My carbs originally came with F25s by mistake. They do not work for this application and fortunately the supplier replaced them with F36 tubes.

Are you sure that you can't load an air corrector on the tube with the broken tab? Once it is screwed down, it may not matter.
 
Hey Andrew I won't be around til tomorrow afternoon but I could have a look to see if I have an F36 tube if you haven't ordered one yet? I could probably slip it in a simple envelope so it would cost you next to nothing?? Might be able to help with some mains too if you were thinking of a switch up of some sort?
 
CnC79X19 said:
Hey Andrew I won't be around til tomorrow afternoon but I could have a look to see if I have an F36 tube if you haven't ordered one yet? I could probably slip it in a simple envelope so it would cost you next to nothing?? Might be able to help with some mains too if you were thinking of a switch up of some sort?
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that would be great CnC! I am sure you car is dialed in but if you are offering parts dont offer ones you might want in the future. i would feel bad if you needed some in the future as well.

dllubin said:
Are you sure that you can't load an air corrector on the tube with the broken tab? Once it is screwed down, it may not matter.
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I am not certain i can't use the tube as is but given the fact it didn't run right before i thought it a time to dig in further to see what the issue is.

Perhaps at some i might dig into the F24s if the 36 doesnt help or I am unable to locate one.
Fortunately this is a single DCNVA where i only need to buy two of them. carb tuning alone is why I wouldnt be able to afford a vintage ferrari (not that i can afford a new one :rolleyes: ) Pierce had these at 24 a piece. Can you image how much that would be to "trial" a new tube for 3 DCNFs or however much some of them had?

dllubin said:
If you have not checked the float level, I would do that first. Also worth checking/cleaning the float valve while you are at it. They can get sticky if there is buildup there. For the whole main/air corrector/emulsion tube setup to work as designed, the float level needs to be dialed in.
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Top cover is off although i had to pull the cotter pin on the throttle linkage side that will probably require me to remove the carb to reassemble. Tiny, blind very cramped, and spring loaded... Thanks for the float directions snip.
 
AKimball92 said:
that would be great CnC! I am sure you car is dialed in but if you are offering parts dont offer ones you might want in the future. i would feel bad if you needed some in the future as well.


I am not certain i can't use the tube as is but given the fact it didn't run right before i thought it a time to dig in further to see what the issue is.

Perhaps at some i might dig into the F24s if the 36 doesnt help or I am unable to locate one.
Fortunately this is a single DCNVA where i only need to buy two of them. carb tuning alone is why I wouldnt be able to afford a vintage ferrari (not that i can afford a new one :rolleyes: ) Pierce had these at 24 a piece. Can you image how much that would be to "trial" a new tube for 3 DCNFs or however much some of them had?


Top cover is off although i had to pull the cotter pin on the throttle linkage side that will probably require me to remove the carb to reassemble. Tiny, blind very cramped, and spring loaded... Thanks for the float directions snip.
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Shoot Andrew, I have a ton of F30's and a couple F27's but no F36's ☹️. Sorry buddy. Just took them out of these carbs too just in case?
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20200820_204710.jpg
 
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Update: Step 42 in the pic read 52mm on my carb where it should be 50. tab bent to show a 50mm reading. the 58.5 was spot on. I now need to reassembly and install another F36.

The full position was large which would result in the bowl reading full at too low of volume. this would make things a bit more lean but do not think its 2mm would be the issue would it?

fiatfactory said:
I fixed the picture links in this thread. Perhaps refer to this to make sure your looking at the correct parts.

https://xwebforums.com/forum/index.php?threads/carb-help.29013/#post-242653

SteveC
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How do you remove the pump jet? If I have something clogging one side this could very well make it lean. When I blip the throttle all is good for a short bit. I didnt want to start twisting or prying without knowing. Your pic in the link doesnt show a thread or anything so is it just seated in there?
 
The pump jet assembly unscrews similar to the emulsion tubes. There is a brass piece that screws in and holds on the casting with the two jets. I think there is a gasket or a washer in there as well.

When I was trying to get F25 tubes to work, 2 mm did make a difference, but not enough to fix the problem. What is going through the tube is a function of fuel level, and where the holes in the tube are, tube diameter, and of course the jets.
 
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