DCNF help.....

Donald Wetzel

Donald Wetzel

Daily Driver
Dual dcnf 40's, and a question on idle jets.....

Here's what I have.....
130 mains
200 air correctors
F24 tube
50 idles

Can get it to start, but takes well over 3 turns of the mixture screws to get down below 14 on the a/f meter at idle (800ish)....and cracking the throttle takes it immediately well into the 18-19 range.

My thought is going immediately bigger on the idle jets.....but to what? Or better off with a pin drill and enlarge as I need to?

Just looking for a best direction by those with more knowledge than me....
 
55 is a good idle jet size.
I would not start drilling out jets. Build up a collection instead.
 
What venture/choke size is in the barrel? Also what camshaft are you running.
If just opening the throttle plates a bit is causing the idle to go up abruptly it is telling me that you are too rich and more air is needed.
So turn in the idle adjuster. Do not expect the engine to be cranked and it will start, you need to prime the engine by pushing the gas pedal to the floor a couple of times then crank.
Also are you using the cold start enrichment of this carb when starting? ( will need a choke type cable for it to work or a servo)

TonyK.

Grimsby Ontario Canada.
 
TonyK....

Running a 32 choke tube and 4.5 venturi....cam, was told by the PO it was stock but did not wheel it to check...a stockcarb came off it, so I assumed stock.

The problem isn't that the idle is jumping, the a/f is going way lean by just cracking the throttle.....

It's acting that it needs a larger idle jet, just not sure how big of a step I should take with it
 
Are you testing the mixture by driving it or just working the throttle by hand? I would drive the car and watch the A/F and see what happens. Three turns out on the mixture screw suggests the idle jet is too lean. On the IDFs on my spider typically the mixture screw would be perfect about 1/4 turn or less but that seems to be an IDF thing. Buying four of every jet can get expensive and I have no problem drilling jets, used to do it all the time with mains. At least drilling the idles will show if larger helps and then you can feel a big better with buying the actual jets. Also, maybe you can borrow bigger idle jets from folks here.
 
hi. the setting is perfect for 1500cc. for 1300 put 30 venturi. with 55indle go little better but all dcnf 40 42 44 at low rpm have problem. only dcnf 36 dont have.
 
This configuration worked fine for me:

Dual Weber 40DCNF 47 7L, Sprint Manifold

Choke: 32mm
Auxiliary Venturi: 4.5mm long
Emulsion Tube: F36
Main Jet: 1.35mm
Air Corrector Jet: 2.20mm
Idle Jet: .55mm
Idle Air Orifice: 1.6129mm (measured)
Pump Jet: .45mm
Pump cam type: 11
Float Level: 50mm
Starter Emulsion Tube: 60F6
Needle Valve: 1.75mm
 
Donald Wetzel said:
TonyK....

Running a 32 choke tube and 4.5 venturi....cam, was told by the PO it was stock but did not wheel it to check...a stockcarb came off it, so I assumed stock.

The problem isn't that the idle is jumping, the a/f is going way lean by just cracking the throttle.....

It's acting that it needs a larger idle jet, just not sure how big of a step I should take with it
Click to expand...



Are you sure it is going lean. Most O2 sensors will read lean when they are actually very rich the condition will not resolve it's self by adding more fuel. This comes about when either the settings of injectors are way off or the same with jetting with a carb.

Having a stock cam will with bigger and better carbs gives a lot of bottom end, the cam will strangle the flow at higher RPM above 5,500 RPM.

I ran a single 36 DCNF and the idle jets were only 50 you have twice the barrels so I cannot see how adding 2 more barrels with bigger jets will case a lean situation with a stock camshaft. If the engine is running as lean as you say it is, then pull a spark plug to prove it. The plug will be grayish. If it is black you are too rich and the O2 sensor is tricking you.

TonyK.
 
The design of the O2 sensor is to work within a range, when the mixture is too rich and out of that range it will report as being lean and no matter how much fuel you add it will still report lean. Pull a spark plug and see if the engine has been truly running lean or as I suspect running very rich.

TonyK.

At Bob Martin's in Radcliff Kentucky.
 
Not ignoring anybody's input, just not much time to get in the garage during the week....I appreciate all the directions I have to go with this....
 
Well.....Had to go back to my good old military mantra...."attention to detail"......

Started from scratch.... checking timing, vacuum leaks, everything......still very lean, couldn't get a good idle under 3 turns with 50 idle jets........plugs, wires, coil, and dizzy are brand new....checked them anyway, all good.

Only thing it could be was the carbs....and started with what I could without yanking the carbs out.....making sure nothing was plugged, and checking jet sizes with pin drill bits......

Well.....mains were good, air corrector good.....but when the idle jets are marked as 50's, but you can't even test fit a #80 drill bit (equiv about .34mm).....that might explain things. ;)

Ended up drilling out to a #75 (.53), and may go up to a 74 .(.57)......but MUCH better....

 
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i have found that you want to be sure to purchase the jets from a well known supplier. I made the e bay mistake one time with idle jets. That was the last time.
 
artz1731 said:
i have found that you want to be sure to purchase the jets from a well known supplier. I made the e bay mistake one time with idle jets. That was the last time.
Click to expand...

No....no one to blame but myself on this one...

Had to purchase new mains and ACs, but the idles were the correct size (on the carb from the PO). I assumed that a good soaking followed by ultrasonic would be sufficient.....
 
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