The nice thing about tuning Edelbrock carbs on the primary side is that minor changes area easily accomplished. Just loosen the small cover screw, move it aside, and the primary metering rod and spring are easily accessed for metering changes. (Image/Jeff Smith)

I have a 1972 Jeep CJ-5 with a 3.73:1 gear and 33-inch M/T tires. The engine is an AMC 360 rebuilt with 8.75:1 compression, stock heads, and a COMP Thumper cam with 226/241 at 0.050-inch tappet lift with 0.491/0.476-inch lift and a 107 degree lobe separation angle (LSA). The intake is an Edelbrock Performer with an Edelbrock 1406 600 cfm carb. The ignition is MSD set at 14 degrees initial with 36 degrees total. It also has Edelbrock headers and a wideband AFR gauge with dual exhaust and Flowmaster mufflers.

My problem is my AFR gauge goes fat (and the Jeep feels fat as soon as I dip my toe into the main jets, and wide-open throttle (WOT) never goes leaner than 10.5:1 to 11.5:1. It only improves slightly as air conditions improve, but not much. Using my Edelbrock Tuning manual, I’ve gone to the #10 setting on the model 1406 Calibration reference chart which is 2 stages leaner. I’ve even leaned the secondary jets to a .089, and it still goes fat. I am thinking it has something to do with engine vacuum because Thumpers are known to have lower vacuum and therefore I’m thinking the signal to the carb is making the secondaries open too fast.

I want to fix this and have thrown multiple set-ups, springs, carb cleanouts and my thinking is there’s no way I should be running this rich. My plugs are hard to read, timing looks good, heat range seems fine, porcelain looks okay. I have it idle at 800 rpm, the accelerator pump is fine, I run ported vacuum advance. At idle my gauge is showing 9 “Hg. I know my cam is a little big even though I got the smallest Thumper. Is the cam too big? Is it dumping fuel because the vacuum signal disappears too soon? My cruise circuit is great around 14:1. It’s as soon as I get on it or go up a hill, it starts getting into the power and instantly runs fat.

Lastly, thank you. It is mostly because of you and a few other writers in the industry, and me reading your magazines for many years that I know as much as I do now. — M.A. 

Jeff Smith: I had to work on this one for a while before coming to a final conclusion. The difficulty with diagnosing these kinds of problem is that we have to rely on written descriptions as opposed to personally witnessing the issues. On the plus side, this reader has given us plenty of data which is very helpful. There are some situations here that deserve addressing.

First, it seems odd you can generate a 14:1 AFR at cruise and yet go extremely fat at WOT. I don’t think it has anything to do with the rate of secondary opening. I just ran the numbers and your secondary jet change is 12-percent leaner than stock. That is a major change in jetting. On a Holley, that would be roughly equivalent to four jet sizes, which indicates that something is off.

As a first suggestion, I would look at the fuel pressure. Edelbrock carburetors are sensitive to high fuel pressure — perhaps at WOT the pressure is excessive and making the engine run rich.

Upon further thought, the above scenario is unlikely. Instead, it could be that you are suffering from an errant oxygen sensor. I’ve experienced an O2 sensor that would do the same thing — report the engine was 10.5:1 AFR at WOT, yet it was close to correct the rest of the time at idle and part-throttle. Believing the O2 sensor, at the drag strip I leaned the jetting and then the engine lost power and the car slowed dramatically.

I went richer than where I started and the car responded quicker — even though the O2 sensor was telling me the engine was running at 10.5:1 AFR. It’s possible your O2 sensor is incorrect. On my car I had two different sensors (one in each bank) and both read incorrect at WOT. I don’t have an explanation for why. This occurred even after I re-calibrated one of the sensors.

I suggest this: Put the Jeep in a gear and record the time it takes to accelerate at WOT from 2,500 to 5,000 rpm (or any rpm range you are comfortable with).

Do this in a safe place and have a friend run the stopwatch. Perform this test at least three times and average the numbers.

Once that is completed, richen the carb back to the stock secondary jetting and perform the acceleration test again. That is your ultimate evaluation — my guess is that at WOT the engine is too lean even though the O2 sensor reports the opposite.

The best way to perform the timed run would be to keep the transmission in second or third gear. If it’s an automatic, run the test by manually holding the trans in gear where it won’t downshift. Try a gear that will allow acceleration from 3,000 to 5,000, for example.

Many years ago, I was trying to tune the secondary jetting on a Q-jet on a small-block and it appeared by reading the plugs that the engine was too lean. So I richened the jetting several times and the car slowed down each time based on my mph readings. My knowledgeable Q-jet buddy insisted I was going the wrong way.

After wasting much time and being thoroughly frustrated, it turned out he was right because he knew approximately where the jetting should be and knew my jetting was too rich. The lesson here was to watch the engine’s performance and use that to dictate the jetting. In your case, there is an instruments error.

All of this sounds simple and incredibly basic — but you would be amazed at how often people are led astray by false information.

While I don’t have the time to trade emails with most of this column’s readers, in this case, he eventually performed this test and then richened the jetting at first back to stock on the secondary side and then slightly richer. In both cases the Jeep responded positively to the changes.

He reported the Jeep has never run as strong as it does now, so apparently this cured his problem. He didn’t bother to re-run the acceleration test but it’s clear the results were positive.

We’ve listed the stock Edelbrock main jet and metering rod combinations for the primary and secondary followed by this tuner’s first changes, followed by the final changes that improved the performance. As you can see, there are some significant changes in metering. For most mild street engines on pump gas, making changes beyond three to four jet sizes (roughly 10 percent) on either side of the stock jetting may indicate something may be amiss. This is not true in all cases, but is an indication something is requiring a major change in fuel either lean or rich.

 StockFirst TuneFinal Tune
Primary Jet0.0980.0920.095
Primary Rod0.075 x 0.0470.070 x 0.0370.073 x 0.047
Secondary Jet0.0950.0890.095

In the Edelbrock tuning chart, the primary metering rod has two dimensions. The first is the large main part of the rod that sits in the jet during light throttle applications. The second number is the power tip. This is always smaller and when at WOT the spring pushes the metering rod out of the jet, leaving the smaller rod diameter to allow more fuel. The total fuel is determined by power tip area subtracted from the flow area of the primary jet.

Since the primary jet is larger than the metering rod it has a much greater effect on jetting. In the first tune, the primary jet is much leaner even though the rods are smaller. Overall, the primary fuel is reduced.

Combined with a much leaner secondary jet, the engine was starved for fuel. As you can see, his final tune is only slightly leaner on the primary side and the engine runs much better.

Author: Jeff Smith

Jeff Smith has had a passion for cars since he began working at his grandfather's gas station at the age 10. After graduating from Iowa State University with a journalism degree in 1978, he combined his two passions: cars and writing. Smith began writing for Car Craft magazine in 1979 and became editor in 1984. In 1987, he assumed the role of editor for Hot Rod magazine before returning to his first love of writing technical stories. Since 2003, Jeff has held various positions at Car Craft (including editor), has written books on small block Chevy performance, and even cultivated an impressive collection of 1965 and 1966 Chevelles. Now he serves as a regular contributor to OnAllCylinders.