Need help getting my car to Idle properly

I have been dealing with this too-fast idle on my 1978 308 GT4. Yesterday I had just finished re-building my ignition system (set exactly to 7 degrees BTDC static) and I have a wild thought about the USA carbs: DCNF 72,73,74,75

In its USA configuration for meeting emissions, these carbs were leaned out quite a bit at idle and my crazy notion is that the idle circut is designed with this in mind. Therefore it was designed so the idle speed of 900-1000 can be achieved with the thing running way lean and timed at 3 degrees AFTER top dead center. Then, years later, when the cats are removed, the timing is set to 7 degrees BEFORE top dead center, and the idle is made more rich (I have #60 idle jets), it will idle faster and there is not enough adjustment range with the idle SPEED screws to slow it down.

My car currently idles around 1800 with the idle SPEED screw backed out almost all the way (but just in far enough so the levers don't loose contact and allow the plates to bind in the throttle body).

I know that leaning out my carb's idle MIXTURE screws certainly will slow the idle down. Currently they are backed out just beyond where they start to slow the idle (which is just one of a hundred ways to set these jets).

My plan tomorrow is to crank in all the MIXTURE screws in a tad at at time to lean it out and bring the idle speed down. My impression is that this will bring the idle down, give me better CITY fuel economy and still not be so lean that it pops and coughs.

I have searched f-chat on this subject and I think that the people that do not have a fast idle problem have the idle MIXTURE screws in farther than you or I.

Again my thought is that there is something in the design of these USA carbs (DCNF 72,73,74,75) that won't allow enough adjustment of the idle speed when the idle is richened up from the USA smog settings and the idle timing is advanced from 3 deg. ATDC.

 

Okay, first off I wont say I am any carb "expert". The things have fooled me so many times they facinate me.

However. It is my understanding that at low speed idle, all fuel is coming through the progression holes in the idle circuit, controlled by the idle screw.

If the throttles are actually closed all the way shut, and the mix screws are screwed down fully, I dont understand how the engine can get enough fuel to even fire. Unless there are internal leaks within the carb. I dont even think leaky throttle shaft bearings would alter anything other than let in more air. And a vacuum leak below the carbs would only tend to lean the mixture further, reducing idle speed.

Anyone out there know enough about these carbs to offer na accurate opinion?

 

Beta I am starting to think maybe there is a greater difference between the 76-77 cars vs. the 78-80 cars. I have a stock 77 that was built without cats and had the twin air pumps. I understand when they went to cats and a less agressive set of cams it must have changed the carb set up. I have no problem getting the idle down to 700 rpm . I set it up at 1000 rpm at 7*BTDC as recommended by Don King who did my dizzy. As mentioned earlier I bump it to 1100rpm to compensate for the AC in the summer. As far as I know (I have not verified) the carbs are at factory settings and jets. I did notice that I need to turn the A/F mixture screws out to 5 1/2 - 6 1/2 where it runs sweet. I also run a hotter plug to prevent fouling and this works great. I wonder what all of the other 76-77 guys are running at with their set ups. So my net is these motors should be able to run at the factory 1000 rpm settings . Unfortunately this problem is difficult to find the problem but you gota keep trying. Enjoy the ride

 

I once used water to find a vacuum leak in the condenser of a nuclear power plant. This took a fire hose though, a weld had cracked in an inaccessable area. It worked great. I would be reluctant to use water on my Ferrari engine. I still will use propane.

 

Hi,

Just a few points based on info in John Passini's book on Weber Carburettors (picture scan (c)John Passini)

1) At idle the fuel/air mixture comes through the idle feed orifice, not through the progression holes (the progression holes are for when the throttle valve opens more)
2) Even when closed all the way the throttle butterfly should probably not touch the throttle barrel to avoid damage or excessive wear. (Therefore some air will come around the butterfly when "closed")
3) The idle circut has its own air-fuel mixing (emulsification) and those idle mixture screws control the amount of MIXED air/fuel that enters the throttle.
4) You are right in that when the mixture screws are screwed in all the way and the throttle is closed the engine will not run because it is no longer getting fuel. In Passini's book he indicates that most Webers WON'T cut fuel/air all the way off with the idle mixture screws all the way in, but on these USA DCNFs, (if I can remember correctly from when I was cleaning mine), the idle screws WILL shut the flow off completely when screwed in all the way.
5) In terms of vacuum leaks: if you don't know it is there, then you unknowingly compensate for the lean mixture with whatever method you chose to set the idle mixture (colortune, RPM drop, etc.) and now more air and fuel is coming in, causing a FASTER idle.
6) Also, of note, there is a controlled air leak coming through the charcoal vapor chamber. At least on my car, there is a slow continuous feed of air through this chamber to the manifold under 3 of the 4 carbs.

The picture shows a progressive 2 barrel, but the right side barrel is similar in funciton to a DCNF.
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Thanks for the info on your car. Does your carbon cannister still empty fumes into the intake manifolds, or are these capped?

 

Just remember that the cars didnt leave the factory running that high. 30 years of who knows who working on it could have balled something up.

I need to pull the carbs off mine and see what I need to do to fix them. Someone put the late model replacements on the car, and I think the progression holes are not drilled correctly. It has a burble just off idle, or, at light throttle crusing speeds, a sputter. If there arent enough holes, or they are to small, you have a lean condition. Also, are you absolutely sure all of your butterflys are closing down and entirely blocking the carb bore? I have heard of twisted throttle shafts, and misaligned throttle plates, either one of which would cause idle speed trouble.

Once I opened up my mix screws a bit, I found I was able to idle the motor way down, as low as 500 rpm and smooth, barely a tickover.

 

Canister is removed and vac. lines are plugged. Not emission stuff left except the exhaust rail on the firewall side. I will remove these some day when I get adventurous .

 

Artvonne:
I was reading some of your older posts and you mentioned : "With the mix screws in so far, it starved for fuel. This nessesitated opening the throttle plates to a farther open position than normal, forcing the the engine to draw fuel off the progression holes." This is exactly what I was thinking when I imagined the USA carbs somehow speed the idle up without raising the plates too much (so that they DONT draw off the progression holes when they are leaned out for emisions testing). I wish I had the USA and non-USA DCNF to compare side by side. I know the progression holes are different but is there anything else that is different?

Also, at one point you mentioned: "When driving, if I take my foot off it, it just cruises along with virtually no engine braking, like the throttle is stuck open. And if I shove in the clutch it still hangs up there at 3000 and slowly comes down to idle." I also am having that symptom, did you ever get rid of that? Am I correct in that you do NOT have the USA carbs?

Thanks for sharing your information.