possible carb tuning question

[68 Firebird]

Hello Cliff,
 
First off let me say I am not sure this is a carb issue in any way here.  The unit you set up
for me #7043266 has been excellent for the last year and a half.  But since I made some changes
to the car I have developed a issue where if I am at a stop and nail it she lays down abit and
sputters forward, and then sometimes may almost cutout and then return to idle.  I feel maybe something needs some tweaking since changes were made to car in last 3 months.
-new auburn posi unit running 3:08 gears(was open carrier)
-new tires BF Goodrich 225-60-R15
-new 13" continental converter I ordered from you a few weeks ago.
-TH400 that was rebuilt with a mild shift reprogram.old trans slipped  when hot.
Engine is a 68 pontiac 400 with 16 heads and melling spc-7 cam and 2" dual exhaust using original logs.
So far I have verified HEI timing 14 initial plus 20 mechanical in by 3200rpm with  another 10 from vac can on ported source.  Vacuum is 20" in park and 17 "in gear and steady.
Engine has under 2000 miles on rebuild. 
The carb is untouched other than setting idle , mixture screws and sec flap tension.  Mixture was set to 1.5 turns out and idles great in and out of gear(smooth).  In park I am at 800-825 rpms and 625ish in gear.
I also did adj secondary tension spring from 1/2 turn to 3/4 and did not resolve issue.
I don't mean to bother you with this I just was looking for some sound advice. 
If I pull a small vac hose off while idleing rpms go up about 70rpms and if i do the tipin test
the rpms go down a hair maybe 25 rpms. I know the posi was a big change but I feel there
should be enough bottom end to do a great burnout!!!!!!!!!
Please let me know what you think.
Thanks
Gerry Ciottariello

[Cliff Ruggles]

Check the accellerator pump shot and add some spring tension to about one full turn and see if it helps?....Cliff

[68 Firebird]

Verified accel shot and had 2 healthy streams.  I set flap to 1 full turn from
when flaps just come closed and flaps felt very stiff.  I backed it down to 7/8
and will give her another run. 

I may be asking a dumb question but this possibly be weather related as it
has been 90-100 degrees and relentless humidity.  This 400 would annihilate the
tires  fall and early spring!!!!!

Thanks

Gerry

[Cliff Ruggles]

When you work in this heat and get overheated and have trouble breathing, the same thing happens to your engine.  Power falls off considerbly as DA (Density Altitude) rises.  You can lean up the secondaries a bit to compensate, but the power still falls off, and woln't return till the good air does......Cliff

[68 Firebird]

I see your point on the heat/jetting relationship.  And agree the carb is jetted properly.
Took a much deeper look at the ignition and found the 3-wire harness that connects cap
to module&gnd has an intermittent break in the center black gnd wire.(no continuity)

Swapped out for another from a spare HEI(tested first) and will test out soon. 
Hoping the faulty gnd to primary side of coil was causing a spark issue under load.
Always the stupid stuff that's get me.

Thanks

Gerry

[68 Firebird]

Update

It was the faulty HEI harness causing the sputter under load.  I set the
tension on sec flaps back to 1/2 a turn and all is good again. 
Why is it the simplest crap that kicks my in the ***!!

Gerry

[Cliff Ruggles]

Good news is that you found the problem, and didn't have to pay someone else to do it!.....Cliff

[68 Firebird]

just wanted to give an update to this old thread.  I upgraded the duals setup that was on the car when I pruchased
it

[68 Firebird]

sorry I was typing and laptop farted and posted bedore i was finished typing.

What I was trying to say was that I upgraded to a pypes 2.5" exhaust with X-pipe and that old exhaust seems to be where the restriction was.  I wish I took a pic of the old downpipes, though the exhaust was 2.25 OD the bends were measuring down to 1 5/8 in several bends.  The bottom end has really woken up and pulls like a freight train.  No other changes were made and car even seems to idle smoother! If the throttle is mashed the motor jumps to 2700rpm's and she will pull thru all gears till you let off.  The 13 Continental feels bone stock everywhere else!

Gerry