7044211- Lean Idle?

[tmny277]

Hi all, I just discovered this forum though I've been using Cliff's book for a few years.
I have a 7044211 installed on a 74 L82 Vette.  The motor is nearly stock except for Dart Iron Eagle heads.

The motor has a slightly rough idle and intermittent surge under light throttle (about 2000 rpm).
It pulls 15" of vacuum at about 900 RPM, which is the stock idle.  The idle is not steady though, it floats about 20 rpm up or down but can spike up to 50.  When I screw the mixture screws in the idle will get really rough and nearly kill the engine.  When I adjust out I get to about 4.5 turns before they stop responding.  When I partially block the air horn I can get up to 16" of vacuum before killing the engine.
I'm thinking that it might need more fuel at idle, possibly due to free-er flowing heads??
What do you guys think?

Caveats-  The carb had an incorrect air horn when I bought it so I got a replacement from another 74 model year carb (7044213 I think). This new  air horn is slightly warped. The gasket takes up the warpage easily though as far as I can tell.

Thanks!

[tmny277]

More Info- The carb was set up un-modified with stock jets and rods, 75 jets and 43 rods.  And rebuilt using Cliff's book as a reference.

Thanks again!

[429bbf]

i would take a very good look at the electrical system on this to start with before i would blame the carb. do you still have points? even if its converted id look at it . electrical problem give a lot of carbs a bad rap.i worked with a guy once that changed 3 carbs before he realized it had a stuck set of timing advance weights.fwiw

[tmny277]

Thanks 429,  The ignition was the first thing I checked.  It is also completely new, Distributor checked, new points, condenser, rotor, cap, coil, wires, plugs.   Dwell and timing checked and re-checked.

Do you think my supposition about needing more fuel is incorrect?  I'm looking for any feedback in this area.
According the Cliff's book my cam is somewhere around his second idle example with 222/222/114 specs.  So, I'm thinking it needs more fuel at idle....

[429bbf]

what size are your are your idle tubes, dcr, and idle air bleeds now . that would give a guy a good idea of what your dealing with . did you totally strip the carb and measure everything and put shaft bushing in . a small vacumn leak through the throttle shaft will cause the symtoms you have .fwiw

[tmny277]

I have it apart right now so  I measured all the relevant ports.
Upper Idle bleed- .070"
Lower Idle bleed- .063"
Idle Tubes-new- .035"
Down Restrictions- .046"
Idle mixture screw ports- .067"

The shaft has bushings installed and is tight.

Thanks!

[77cruiser]

I wouldn't hesitate to open the idle tubes to .037 & maybe the idle ports too.

[429bbf]

if it was mine i would increase the dcr to at least mid .055 and make the lib .070 and id also make the idle mixture holes to .085 . thats what i would do.you can always go bigger easy but it takes time to smallen things up.hth

[tmny277]

Thanks Guys, I'll open up some of those ports and see what difference it makes.

[Cliff Ruggles]

Leave the IAB's at the stock sizes and go up .001-.002" on the idle tubes.  If that isn't enough idle fuel open up the DCR's to the next size.

Keep in mind that idle fuel down add to the main system, so baby steps there.  Those carbs are also very lean on the primary side and have the huge MAB's, so 75 jets is actually pretty small and it will be lean at light part throttle with the 43 rods in it depending on where the APT has the PP positioned....Cliff

[tmny277]

I wanted to let you all know the results of my mods.
I opened up the Mixture ports to about .08 and the lower air bleeds just a hair.
I had started to open up the DCRs, before Cliff's post, but I must have mis-measured those because the drill bit just slipped right through with virtually no resistance.
I also installed 77 jets while I had it apart.

The car runs great, best its run since this engine has been in.  I got it up to 16-16.5" vacuum and it is so much smoother.
The idle rpm is much steadier too.  There is still a slight variation maybe +/- 15-20 rpm, but no more big spikes either way.
The car really likes the extra fuel all around.
I think it might even want a bit more at idle but I'm pretty happy with it for now.

Thanks to all who responded with advice!

[Cliff Ruggles]

Good news!....Cliff