Is there any way to correct a "U" shaped AFR curve?

[70GS455]

So dyno testing data is showing a "U" shaped AFR and BSFC curve vs RPM. Starts at 3500 rpm around 12.5 AFR, then dips slightly to 12.1 or 12.2 around 4300 then slowly climbs to 13.5 or .6 at 6000. BSFC shows a similar trend: starts off at .44 dips to .41 in the middle and climbs to .45 or so at the end. Is there a way to make this more straight? Or does it even matter? Manifold vacuum at the end is 2.1 to 2.3"

[77cruiser]

What size N&S, how much pressure?

[70GS455]

.135, 7 psi

[lightning boy]

Just a thought, have you tried tapping the rear air bleeds for screw in and trying some different sizes. .028"or so.
Or maybe a set of custom machined sec. rods.

[70GS455]

Just a thought, have you tried tapping the rear air bleeds for screw in and trying some different sizes. .028"or so.
Or maybe a set of custom machined sec. rods.

Good idea, would have to be a small screw as the tube OD is like 0.090". Thinking about soldering it and re drilling, then open up the bottom one

[Cliff Ruggles]

Larger fuel inlet seat is the first place to start and keep the pressure at least 7psi.  Even if the bowl doesn't go empty if it doesn't stay completely full for the entire pull it will show lean on the top side of the pull.  For the later carbs with the improved hinge pin location you can run at least 7.5-8 psi with a .145" seat w/o issues even if you use the smaller float from the later carbs.

Custom machined metering rods with long tips are also a good idea.

I've actually changed to smaller tipped secondary metering rods on the dyno and shown leaner in the next pull.  This happens because of the tip length, size and taper on the upper section, and included angle/radius leading to the tips.  The factory "CC" rods with .030" tips are HORRIBLE for this problem due to the big upper section and taper down to the small tips.

Most of my customers who drag race a lot, Class racing, etc, have me machine a set of rods for them from the same cores.  This provides consistent and predictable changes going from one to another.  That is next to impossible using factory metering rods even if you are "juggling" hanger height to compensate........Cliff

[70GS455]

Larger fuel inlet seat is the first place to start and keep the pressure at least 7psi.  Even if the bowl doesn't go empty if it doesn't stay completely full for the entire pull it will show lean on the top side of the pull.  For the later carbs with the improved hinge pin location you can run at least 7.5-8 psi with a .145" seat w/o issues even if you use the smaller float from the later carbs........Cliff

This is a 7043240 with a stock looking larger float. With a 0.145 n/s, do you think regulated 7 psi will be ok without flooding?

[Cliff Ruggles]

Yes, I use the smaller float in those at 9/32" and fine with 7.5-8 psi.....

[70GS455]

Yes, I use the smaller float in those at 9/32" and fine with 7.5-8 psi.....

I got an edelbrock .145 N/S, my .148 drill smooth end just barely fit thru there. Left fuel pressure at 7.5, no flooding. Made 6 more ft lb peak, peak hp stayed the same. It seems better, but left and right AFR diverge from about 5000 rpm up, about 1.5 afr difference. Midrange is fine. Strangely enough, on a pull later, the addition of an air cleaner base caused them to be much closer, like less than 0.5 afr at 6k

[Cliff Ruggles]

Q-jets are pretty sensitive when it comes to air cleaner bases and tops.  They will typically HATE drop based style air cleaners especially if/when they move the lid close to the carb.

Many years ago I installed a Performer RPM on my engine which is apprx 1" taller than my stock intake.  I had to use a 1" shorter Shaker assembly and a 1" shorter air filter moving the lid 1" closer to the carburetor. 

It didn't look like a big deal and still plenty of clearance between the carb and air cleaner lid.  However, it induced a stumble/hesitation/bog that I could NOT tune out.  It also KILLED some upper mid-range and top end power and the car slowed considerably at the track.

I removed the air cleaner lid and it was fine, put it back on and it immediately started acting up again.  So I made a custom filter for the Shaker opening and moved on.

Many years later I actually dyno'd my "modified" iron intake against an Edelbrock RPM and the iron intake outran it by 6hp!

That prompted me to remove the RPM and put my stock intake back in place and go back to the stock Shaker, stock air cleaner base, filter, lid etc. 

The car actually went QUICKER at the track in that configuration than the RPM and custom air cleaner in the Shaker opening.  I picked up .02-.03 seconds in 60' and about .05 seconds on the run at about the same MPH........