Rebuilt carb - poor response in upper RPM

[bcarlson78248]

I rebuilt my 17080213 carb (80's vintage with APT) using the HP kit.   Carb starts and runs great at lower speeds, although after it warms up the AFR shows about 12.5 when idling in park.  I assume I need to lean this out.  At 50 mph cruise I have an AFR of 13.5 and vacuum is about 14-15".   At 70 mph the AFR is about 13.0 and vacuum is about 14-15". 
I also provided a lot of background info at the end of my posting.

When I floor the accelerator at about 45-50 (2200-2400 rpm)  I do not get any obvious bog.  It downshifts and starts to accelerate fine up to about 3000, then begins to lose steam and finally flattens out (very slow rise in speed) at about 3500-4000.   It doesn't feel like I'm getting much out of the secondaries.  I am using the dark blue power piston spring and stock setting on the accelerator pump. APT is 3.5 turns out.  At 4000 RPM with the pedal floored the AFR is showing about 11.8-12.0.

Adjustments I am considering:
1.  Open up the hole in the choke pull-off to reduce delay.  Currently its about 2.5 seconds.
2. Adjust tension on secondary  flaps.  Its now at about 3/4 - Which way should I go?
3.  Try richer CP secondary rods.  Stock for this carb is the G hangar and DP rods.  I have an R hanger and CP rods from another carburetor.  I could get AY rods from Edelbrock.

Thanks,

Bruce

Background Info
 
Other stats: 75 GMC pickup, Edelbrock performer manifold, replacement GM 350 crate engine, stock 8.5 compression heads, 194/204 @.050 cam, iron manifolds and dual exhaust, TH350 and 4.11 gears.  Initial timing at 12 degrees. HEI distributor with 22 degrees mechanical and 22 degrees vacuum.  Idle vacuum is 20".

I used all stock parts, except for the the idle tubes, float and other parts that came in Cliff's kit.  APT is set to the original setting of 3.5 turns out.  Power piston spring is the dark blue one from the rebuild kit.  Carb has a new a secondary pull-off  connected to both the choke and the secondary air flaps (Airtex 1C1148).  Primary pull-off (Airtex 1C1044) is there, but doesn't seem to do much.  Spring on secondary flaps at about 3/4 turn.

[Cliff Ruggles]

Verify FULL opening of the secondary throttle plates, and that the airflaps open unrestricted to their full open position.

The A/F being around 12 to one isnt' going to make an engine stop making power or pulling to peak power.  At the dragstrip I've jetting from so rich black smoke was bellowing out, to so lean the engine was surging some, and the difference in ET and MPH was negligible.

That cam isn't going to make any kind of strong top end charge, DONE by about 4500 if not sooner.  It still should pull pretty hard from 2500 to 4500rpm's or so.  If it takes off fine then "flattens" out, it's either fuel delivery related, or a restriction in the exhaust, intake, carb, cylinder heads, camshaft, or all or most of the above.....Cliff

[bcarlson78248]

I verified the throttle plates went to full 90 degrees when I rebuilt the carb, and the secondary airflaps do move freely.  However, I will also check that the secondary plates are actually moving to max when the accelerator pedal is floored.

I tried disconnecting the secondary pulloff, just in case it was keeping the secondaries locked out - no change.
I moved the tension on the secondary spring back from about 3/4 turn to 1/8 turn - no change.
I also tried a set of richer DP secondary rods - no change.

It might just be running out of air flow in the heads and cam, but I thought it should at least pull strongly to about 4500 rpm.   However, my TH350 shifts out of second at about 3600 RPM, so I'm only getting to  4500 RPM after I hit 3rd gear.    It doesn't seem like I'm running out of fuel, since I would expect it to go lean if it was fuel-starved.  I may be just reaching the best upper end I can get with this conservative cam, heads and stock mufflers (brand unknown).

Since I'm getting a cruise AFR of about 13.5 would it be a good idea to trying moving from a 73 jet to a 72 jet?  Or should I use a slightly leaner/larger primary rod to get cruise AFR closer to 14.7?

Thanks,

Bruce

[Cliff Ruggles]

Use the APT system to fine tune part throttle A/F.  Don't change the jet size unless it requires more fuel at heavy part throttle.

The crate engine may be using poor flowing emission style heads, which never did make much power anyplace.....Cliff