Secondary tuning

[Too Fast]

Installed Kauffman 74cc D-ports, as cast.  Carburetor ran great with the previous 4X heads, about  8.1 compression with the .030 400 Pontiac, before and after the cam change below.

Last spring I changed the cam to an 041 and it ran great with the 4X heads and the carb I rebuilt 8 years ago with Cliffs (signed!) book, advice and kit.  The old cam was milder, dont have exact specs but I could get almost 16" vacuum at 750 steady RPM idle in gear.

Old system was:
4X heads with 2.11 intake, triple valve springs unknown pressures but held to 6000 rpm (too high!) with no float and really liked to rev with the 041.  These heads were rebuilt about 20 years old and about 25k miles with the engine, pretty sure they only had a competent 3 angle valve job and no porting.
1.5 stock rockers
.030 400, not sure about deck height but looked almost flush piston with deck at TDC.
3-tube Cyclone headers with poorly bent 2.5" into Flowmaster delta flow 40s and 2.25 tailpipes
Edekbrock Performer intake.
16" idle vacuum
Open element air filter
12 degrees intial, 36 total at 3500 RPM

New parts are:
Kauffman D-ports 74cc
1.65 roller tip rockers
Same .030 400 short block
Ceramic 4-tube from Summit
Pypes 2.5" mandrel bent X-pipe and dual electric cutout, Pypes Street Pro and 2.5" tailpipes (this exhaust stops people on the street, they turn around an look to see what is coming.  The old system rarely did that)
Offenhauser 360 intake
12" idle vacuum
New dual snorkel CAI from Spectre, one inlet in the cowl the other near the left headlight
17057253 carb, CV secondary rods with .073 main jets.
10 degrees initial, 34 total at 3500

HEI distributor, with another 10 with the vacuum advance connected.

The 1968 LeMans droptop with 3.55 gears, 2004R, 2300 stall, 3950 lbs with me in it ran 14.07 at 99.5 MPH last fall.  Several other runs were also 99.x MPH.

My new issue is with the changes I made, when the secondaries open it sure seems waaaayyyy lean.  Backfiring, never comes on strong like before.  So, I actually removed the CV secondary rods, and I could tell it was way rich with the secondaries, just to be sure I didn't somehow plug the carb.  Then I put one CV rod back in, and after it clears it's throat, really pulls hard over 4000 RPM but I can tell still too much fuel.  I also tightened the air flaps until they just start opening when I rev it with the air filter off.  Before, they would open quickly, ran great.  It has been horrible with the new setup on the secondary side but no issues before the heads/exhaust/rocker arm/intake change.

On the primary side the response is awesome, no hesitation and crisp response. It will easily pull hard to over 5000 RPM, I adjusted the APT about 1.5 turns richer to stop a hesitation it had with the new setup.

I am looking for advice on what to modify/purchase.  Today I ordered a dual wideband O2 sensor kit that I will install soon, to see exactly what is happening on both banks.  I am thinking with the changes I made I am getting about 50 CFM more airflow, and the secondary side is not keeping up.

I am hoping for 5 or more MPH at the drag strip with the new stuff, after the carb gets dialed in.  Any other advice on what to check would be great.
 

[Cliff Ruggles]

It looks like you made a lot of changes.

Most likely the problem has nothing to do with the carburetor or how it's set up.

Not familiar with that intake but I've had intakes cause the same issues you are seeing.  Same thing with spacers, air cleaners, air cleaner lids, etc.

Try removing the lid on the air cleaner and see if it helps?

Is that intake set up for a spread bore carb w/o an adapter?.....

[Too Fast]

The Offenhauser came off my 428 that blew up in 1997.  I've saved it all these years.  It is a square bore with a 1 inch spread bore 4 hole adapter, it had a Holley on the 428 and I also used an Edelbrock spread bore with an adapter back then.  Seems like nobody knows about those intakes.  I recently found a PDF file, and they sure talk up there products like everyone does, it is a split intake divided down the center with an air gap on the secondaries, then acts like a single plane.  It does not look like the Performer dual plane with 8 runners or a Torker II.

Actually I ran the Performer on with the Kauffman heads before I put on the Offy, and the LeMans ran the same, secondary operation acting like no fuel but ok on the primary.  I wanted to see if there was any difference but nope.  The Performer had a 1" phenolic open spacer.

I had that Spectre assembly on it before the head/rocker/header/exhaust change, it ran great so I don't think it is that.  I'll try taking the lid off....

You are thinking not a carb issue.  I'll verify I'm not doing anything stupid like somehow stretching the B+ to my distributor, or possibly messed up a fuel line or maybe the fuel pump is going bad.  I have not verified fuel pressure yet.  I could see the fuel dumping in the secondary side when I was free revving when I removed the CV rods.

Thanks for helping!

[Cliff Ruggles]

I've ran into similar issues as you are seeing and it was NEVER the carb.

Spacers, intakes, air cleaner assemblies and air cleaner lids have always ended up being the issue. 

To date my 17057274 carb has powered 5 different engines in my car and I've never touched it.  Every single time I ran into an issue with hesitation/stumble/bog going quickly to full throttle is has been when testing a different intake, or adding a spacer, or drop base air cleaner, goofy flow thru air cleaner lid, etc.

I'm not familiar with the Offenhauser intake but if it's a square flange unit and requires a spacer/adapter I wouldn't use it. 

My personal preference is the stock factory intakes for Pontiac engines.  I remember being on the dyno about 15 years ago testing a 428 we built here.  It had KRE heads, custom HR cam and 10.6 to 1 compression.  We made quite a few pulls with my iron intake and between pulls I was asked to bolt on an Edelbrock RPM intake to "see how much power it will really make".

The last pull was 497hp/540tq.  I bolted on the taller and larger runner RPM and the next pull was 491hp/535tq!

Good lesson learned there with intakes, bigger is NOT always the way to go and you can LOOSE power if the engine just doesn't need larger runners and more plenum volume.  To this day it makes me wonder how many folks buy Edebrock Performer and Performer RPM intakes for smaller engines making a LOT less than 500hp and think they are helping them some?.......FWIW......

[Kenth]

Standing beside the engine watching the air valve opens when revving the engine tells me the preload on airvalve spring is too loose letting too much air in too soon.

[Too Fast]

OK so I will put both CV secondary rods back in.  You are absolutely the guru, so I take your advice seriously. I don't have any stock 4 bbl intakes. I may put the Performer back on.  I haves read reviews about intake manifold testing over the past few decades and you are right about the OE intake, especially at my expected power level and RPM range.

 Something I found yesterday was at the distributor the B+ was about 2v lower than at the battery, so that might be part of it. 

It must be something I've done, since this issue started when I put the heads on.  I KNOW the new heads /exhaust/rockers is really letting the engine breath so much better, that's why I was thinking the carb was lean at WOT.

I should be getting the wide band O2 sensors this week, hope to install shortly. 

If anyone else has some ideas, please chime in.

I will report when I get it figured out.  Thanks!

[Too Fast]

Standing beside the engine watching the air valve opens when revving the engine tells me the preload on airvalve spring is too loose letting too much air in too soon.

Ok, I will adjust it a bit tighter.  It doesn't fully open, but starts opening enough I can see fuel flowing.

[Cliff Ruggles]

The carb doesn't go lean at WOT because you changed to better flowing heads, bigger cam, or anything else.

Matter of fact increasing compression, better flowing heads and better combustion chambers will like LESS fuel or lean A/F as BSFC is less with the more efficient set-up.

You have a transition issue from reading the first post.  If it were lean for A/F it never would pull very well anyplace. 

Transition issues more times than not are something else you've changed in the induction system, probably not the better flowing heads.

I remember decades ago removing my iron intake and swapping to an Edelbrock RPM.  It required a shorter air cleaner which moved the air cleaner lid 1" closer to the carburetor.  Other than the intake that was the only other change.

The engine developed a HUGE hesitation/stumble/bog going quickly to full throttle.  Once past it pulled OK.  I tried EVERYTHING to tune it out, nothing helped.  I removed the lid and ALL the issues went away.  I ended up making a custom air filter in the Shaker opening and eliminated the factory style filter and lid.

One wouldn't think that simply moving the air cleaner lid 1" closer to the carb would really make any difference as it still had a couple of inches of clearance, but my set-up HATED it.

Over the years I've experienced the exact same stumble/hesitation/bog with certain parts.  I found it IMPOSSIBLE to run an open spacer on any dual plane intake. Divided, 4 hole and semi-open were fine, but fully open produced a stumble that wouldn't tune out.  I also tried a dual plane intake once with the divider cut down a 1" clear across it.  Same thing, HUGE hesitation/stumble/bog that wouldn't tune out.  Put my intake back on and everything was fine......Cliff

[Too Fast]

Thank you Cliff.  I now have several things to check, I will do a single change at a time and test the result.

 I am also ordering a new fuse and relay box to power several things that need isolated 12v sources (feed to distributor, RPM limiter, cutouts, wide band O2 system that arrived yesterday) and will rewire all those circuits.  Who knows how stable the 53 year old wiring is like.

It must be something I've done during the big changes I made, because there was no secondary stumble before. 

[Too Fast]

So the issue seems to be too fast opening of the secondary flaps.  I checked the vacuum break/pulloff, it will hold vacuum with the line off when it is pulled shut with vacuum.  I release the vacuum tube, it fully releases in a bit under 2 seconds.  I think it is too fast for my combo.  I nearly plugged this vacuum line, and it is much, much better, taking about 4 -5 seconds to fully release.  Now I will loosen the flap spring.   It is wound about 1 turn.

The wide band O2 sensors were showing about 15.1 with the CV secondary rods when I am full throttle, accelerating in 3rd from about 3500 rpm on up.  I put CE rods in, now it is 13.5-14.0-1 in the same situation.  My first car with wide bands, they are gonna be a great for tuning.  Trying to get near 12.5 AF ratio for full throttle, then tune the other areas. 

Runs great low-part throttle, if a bit lean.  Instant 10.2-1 from the accelerator pump squirt, then moderate acceleration is about 13-15-1.  Steady cruise is about 13-1, so I have some tuning to do!  I will play with the APT now that I can see the effect in real time with the wide bands.  Sure was easy to adjust the carb perfectly looking at the AF ratio for idle!  The 041 cam will show a little lean at idle, then above ~1000 RPM steady it will be about 14.5ish.  Just about mirrors the old way of doing it with a vacuum gauge but now I know both sides are the same.  I can get full adjustment on the mixture screws, she idles pretty nicely in gear at about 750-800. About 850 in neutral.

BTW, the air filter lid on this new CAI setup is the same height from the top of the carburetor, and I am using the old RA fiberglass base that I have had on this car for 10 years.  The base that came with the Spectre CAI left about a 3/8" gap all around the bottom of the base, they told me this is designed into it but I wanted a total seal so no underhood hot air gets in.

[Cliff Ruggles]

I tune for best results first, then take a gander at the gauge to see where I'm at.  With a well thought out engine combo it will typically be fine in the 14-17 A/F range at light engine load, depending on speed, RPM and throttle position.

At WOT I tune for best ET and MPH at the track.  Efficient engines are fine in the low to mid-13's and also keep in mind that the readings are really arbitrary and vary some depending on how far from the exhaust valve they are mounted and how quickly the exhaust system gives up heat.  With headers, for example, the closer to the cylinder head the better for the sensor......

Decades of experience have taught me to use the A/F gauge for reference, and to monitor changes more than a target for best engine performance in all areas....FWIW....