I have a 69 with the 350/350, original intake, and Holley 4165 spreadbore. One of these days I will take some photos so people can see my daily driver. Anyway over the past few years I've been working on one system at a time-- right now it's the carb/fuel.

I've always felt like the carb/intake system was holding back the engine. Well after driving home tonight, I removed the air cleaner, pushed the throttle pedal all the way down, and noticed that the carb secondaries are only open maybe 25%. I can't believe I never checked that before! I guess I always operated the carb throttle arm whenever I was playing with the carb, but never from the actual pedal.

Anyway, I'm trying to figure out how to adjust this. Obviously the accelarator pedal can't go down any farther-- it's on the floor. And I have no extra floor mats or anything else behind the pedal to limit its travel. The carb has mechanical secondaries-- so I assume that's pretty set. So I figure either the secondaries aren't opening soon enough, or I need to adjust my throttle cable.

So I have two questions-- when should the secondaries start to open-- as soon as the primaries are wide open? Second-- if I need to adjust the throttle cable, how do I get more travel out of it (if that's even the adjustment I need).

Any help or advice is greatly appreciated. I'll keep plugging away for another hour before calling it a night. I can't wait to get this fixed and hopefully take it for a test drive tomorrow. I'm hoping with the secondaries opening all the way that the car will have a little more pep.

Andy

 

The secondaries only open as much as the engine can handle, and never open full up.

Found the below in the Lars carb tuning paper...

"Most Q-Jets are 750 cfm carbs. This is more airflow than most small block engines can ever handle. Yet, GM used Q-Jets on everything from Overhead Cam 6-cylinder Pontiacs and Buick V-6’s, to 500 cube Caddys. How?

The secondary airvalve on the Q-Jet effectively makes the Q-Jet a variable-cfm carb. The spring windup of the airvalve combined with the bleed-off of the choke pulloff diaphragm allow the secondaries to open only as much as the engine can handle. Thus, if the engine can’t handle all of the cfm, the secondaries simply don’t open all the way.

The primary side, however, is used throughout the rpm range. It is always in use, and provides the metering for the majority of the power produced by the engine. Let’s look at the scenario:

You’re at the stoplight. You bring the rpm up slightly against the torque converter – 1500 rpm. You’re on the primary side of the carb only, and this is what is producing all of your torque right now. The light changes, and you put the pedal to the metal. All of your torque at launch is being produced by the primaries only, as the secondaries don’t see enough airflow to open. The rpm comes up quickly: 2000, 2500, and now the secondaries might be starting to crack. Almost all of the air is still passing through the primaries, and the secondaries are now starting to compliment it just a tad. 3000, 4000 rpm, and the secondaries might be half-way open. The primaries are still providing most of the airflow and metering. 5000, 5500 and you hit redline just as the secondaries hit about ¾ open. Second gear, your rpm drops, partially closing the secondaries back up, and you’re back to sucking the majority of the air through the primaries once again.

So we see, the secondaries provide only a compliment to the primaries. The primaries provide the vast majority of the fuel metering, and primary jetting is absolutely the most critical to proper performance. You cannot compensate for poor primary jetting by re-jetting the secondaries. So we are going to concentrate on jetting the primary side for peak performance, and then we will set up the secondary side to provide a proper compliment to the correct primary jetting."

I realize that Lars talks primarily about the Rochester Q-Jet, but these same things apply for Holley and Edelbrock carbs as well. Lars has said so about a gazillion times here on the forums.

 

Thanks for that info. This 650 cfm carb has mechanical secondaries, so their opening is directly based on throttle position. I think the carb linkage is ok-- the secondaries are starting to open when the primaries are about half open. And then they both come to full open at about the same time.

So in reality, neither my primaries or secondaries are fully open when I floor it. So I'm thinking I should adjust to get them fully open when the pedal is fully depressed.

 

It appeared that my throttle cable wasn't fully opening my secondaries either, I only needed about a few more degrees of throw, what I did was put a small clamp around the throttle cable as a spacer to hold it aft a bit and this did the trick. Have a look.

 

That's a good idea. I ended drilling another hole in the carb throttle arm, a little farther to the front of the car. Worked like a champ. I just got back from a test drive-- man what a difference now that the secondaries are opening at WOT! Unfortunately the car stumbled at WOT-- so that will be another post....

 

My 76 had a stretched throttle cable like yours. I didn't drill a new hole, I just put a piece of wood under the throttle pedal between it and the carpet and pulled up on the arm really hard to bend the pedal enough to take out the slack. It works find now and opens the carb all the way. I guess it's had the pedal to the metal too many times.

 

I did a forum search and found that the acc pedal rod bending is a known problem. When I looked at mine I saw that someone had actually replaced my rod and with a two piece assembly. I thought about trying to bend it or adjust it, but the throttle hole solution seemed easier. I'm a fairly big guy and it is a PITA for me to get anywhere under that dash!!!

 

CRP makes a nice chrome "adjustable" throttle cable braket that bolts right on to the carb studs. Fixed mine easily.