40 DCNF Carb Tuning

[fiathead]

I've been going through a thread on the tuning of the IDF carbs and find myself wondering about the 40DNCF carb on my 1.4L twincam in my '69 Spider. This carb is not running very well at the moment. It works and runs well overall, but I don't think it's tuned well. First off, it's a bear to start, but since there is no startup circuit in this carb, that might just be the nature of the beast. When running though, it stumbles upon acceleration kind of like there is a bad accelerator pump diaphragm. It also does not like to idle well when cold and at the present, it's set to idle at around 1500 rpm warm just so it will idle at all when cold. I've been thinking it may be due for a rebuild, but other than running like crap, I really have no evidence that it does need a rebuild.

The thing in the IDF thread that makes me think a bit is regarding the fuel pressure regulator. I am currently running a Facets pump at whatever pressure it provides since there is no pressure regulator in the system. This car does not have a fuel return line, so I'm not sure if I should be running a fuel pressure regulator at all since my understanding of a FPR is that it needs to be between the carb and the tank, not the pump and the carb. I have had fuel pressure issues in the past though, such as the engine stumbling badly during a long WOT period while ascending a grade.

I'd love to replace the current pump with a more suitable pump and regulator to give the carb a constant 3 psi if I could figure out how to do that with a system that has no return line. Given a constant pressure to the carb, I am confident that I could figure out what that carb needs in terms of setup, but without the basic fuel pressure issue taken care of, I'm not so confident.

Also, on an different note, when this carb is set up correctly, what should I expect in terms of startup? There is no start circuit in this carb and the current start process consists of pumping the accelerator 6 times, then trying to start. Depending on the temp outside, it might start on the second or sixth attempt. Once started, I need to keep my foot on the accelerator to keep the revs around 2K for two to three minutes before it will idle. Is this normal for a carb with no choke or is this a symptom of the setup?

Thanks,
Tim

[sptcoupe]

Are you running a single DCNF or duals?

The idle issue is more likely related to 1) incorrect idle mixture and/or jetting, 2) retarded timing, 3) if you have duals, bad synching, 4) incorrect float level adjustment, or 4) an air leak under the carb or at the inlet port. There is no particualr cold start issues with DCNFs. I have run them on TCs in Wisconsin and Ohio winters with no starting issues, even without a manual choke.

Get a fuel pressure regulator and put it downstream of the pump in the engine bay, and upstream of the carb inlet. Even an inexpensive in-line regulator is a big improvement over no regulator. For DCNFs, run 4.5 lbs of pressure.

An engine stumbling badly under long WOT can also be cuased by an undersized needle valve. I would run 200s.

Can you tell us what jets you have?

[fiathead]

This is a single 40DCN14-1b carb on a 1.4L Twincam. The main jets are 150s, air correction 180s and the emulsion tubes are F38s. I'll check the float level tonight and will adjust the idle mixture too. It's good to hear that they do run well when set up correctly. I've had this car for 14 years and it never has run well when cold for me and I had always assumed that the lack of a choke was to blame. I'd love to get this to run as well as my '77 Spider did after swapping a 34DMS carb on to it. That carb completely changed that car's feel.

I will do the FPR installation between the carb and the pump, but am not which one would work in that configuration. The IDF thread mentions the Inglese NG1081. Would this be appropriate for a system without a return? The specs I've seen don't specify if it requires a return. I've also seen other FPRs that specifically say they are for a non-return system, but seem a bit high in the pressure range, running from 4 to 12 psi. I don't see any mention of adjustability on those, but I'll keep looking.

Thanks for the advice on this. I am really looking forward to the day that I don't have to spend 10 minutes warming the car up before going anywhere.

Tim

[engineerted]

The fuel pressure regulator does not need a return line to tank ,and yes it is installed between the pump and the carb! More internet miss information. Just run down to the local autoparts store and pick up one of the MrGasket universal regulators, most Advanced Auto parts. PEP boys and others carry these by the performance dress parts or at Summit racing.

http://www.summitracing.com/parts/MRG-9710/

Ted

[fiathead]

Correction to the above post due to idiocy while reacquainting myself with this carb. The mains are 150s with a 180 air correction jet and F38 emulsion tubes. The diaphragm looks to be working well and was replaced about 10 years ago. I have upgraded the fuel pump and installed an inline regulator and gauge and am getting 4.5psi now. Before, it was 3.5psi. I'll adjust the idle mixture screws after I have a chance to see what I have so far. Also, the float level was adjusted to 47/56 per a spec found online for the 40DCN14 carb. Once I get a chance to run it a bit, I'll let you all know if there are any differences.

Thanks,
Tim

[fiathead]

With the above mods, the car is running better than before off idle. It was running well before off idle, but it seems to pull a bit better now. The idle is still an issue though, but that could be the cams making it a bit lopey. It did start easier. Once warmed up, I adjusted the idle mixtures and reset the idle down to around 900-1000rpm.

The issue that I still have is that there is a huge flat spot just off idle upon heavy acceleration. If I apply throttle at a measured pace, it seems fine, but if I mash the throttle, I can actually get the car to stall. I've looked down the throats when adding throttle and the accelerator jets seem to shoot fuel nice and cleanly. I'm wondering if moving to a larger pump jet might help that. I'll have to pull the current one to see what's in there and might give that a shot.

Tim