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How I tuned my carb - step by step primer (long)

1904 Views 5 Replies 5 Participants Last post by  zeke67
Step by step, here is what I did to tune up my car for improved driveability and response. I hope this is helpful, sorry it is so long.

Motor: 327 with Edelbrock RPM top end, 1407 Performer Carb w/electric choke and Mallory Unilite. Starting Point: 16* initial timing, 34 degrees total timing, approximately 25* vac advance (more than Mallory's original setting) connected to port vacuum. Carb tune: Out of the box rods, jets, step up springs, unknown squirter, top hole on accel pump arm.

Once warmed up, it idled well but a little fast. Cruising was very smooth and clean. It made smooth power, but a definite and obvious off-idle stumble at mild and moderate acceleration. Passengers could feel the stumble. From a full stop going to WOT caused a severe stumble or even would kill the engine.

Please note: Edelbrock's tuning manual and calibration charts were followed as to steps to take to improve the tune up and the order in which to follow them. Each change was made on a FULLY warmed up engine after at least 6-8 miles of driving and getting a feel for the symptom several times before making a change.

Weekend number 1.
Step 1: Cruise mode (rods down) was very smooth and crisp. No hesitation during steady state cruise or very slight acceleration while at speeds from 25-45 MPH. Go to next step.

Step 2. Power mode (rods up) was very smooth when transitioning from cruise to mild acceleration. Very hard to feel for any problems, lots of driving during this step. Read and re-read the manual. Off idle and hard acceleration problems persist. On to step 3.

Step 3. Step up springs for power pistons (think PV on a Holley, only "backwards"). Went from stock spring two stages slower to slowest spring. Hesitation improves off idle. We are on to something. Weekend is over. I'm happy because the car is more driveable. Passengers cannot feel the stumble. WOT problem persists.

Week number 2:
I re-read the manual and think a lot. I assume my unknown squirter is a .028 and decide to verify that. Discover it is a .041. I put in a .035 squirter in, since that is the out-of-box calibration

Step 0: I come to my senses and realize that correct idle mix influences off-idle performance. And take a lesson not to skip steps in tuning. I take quarter turn out of curb idle to slow down idle speed. Took 1/8 turn out of idle mixture. Idle speed in drive now at 900 RPM. I drive the car a lot feeling out the symptom.

Step 1 (same as above): Problem is back. I realize that the change to the original squirter has caused a lean condition. I realize that my big squirter was a problem and I compensated it with slower step up springs which masked the symptom but didn't fix it. Now we are really on to something. Before continuing with step 1, I go back to the original step up springs the the problem diminishes. I continue with one step faster springs and then another step faster and the problem really starts to go away. I really have to pay attention to feel it, but the problem is still there.

Step 1, continued: I try one step richer for metering rods on the cruise mode, using the calibration chart. Right away, the car is a little louder and a little rougher. It is loading up on fuel ever so slightly. Honestly, I could have missed it but at this point I am not going to second guess myself. I decide to go one step leaner for cruise mode than the out-of-the-box setting. This restores my previous driveability and has no ill effects. If figure leaner is better for steady state cruising and leave it. Next step.

Step 2 (same as above): Power mode. According to the manual, I have no problems with power mode calibration. My gentle but steady acceleration is smooth and strong. Going from gentle to hard acceleration has no stumbles. I do not have metering rods in my tuning kit that let me experiment with going only one step richer given either my current tune for cruise mode or the original cruise calibration. So I can't play around with it. Frankly, I am confounded that Eddy wouldn't provide these parts in the kit designed for this carb.

Step 3. No sense playing with step up springs according to the book. I either have to order new rods or try something else. I start to ponder that Edelbrock may have deliberately left out the metering rods I crave because they know something that I don't. The stumble is so slight you can't feel it unless you are the person driving and it doesn't happen under all conditions.

Back at the garage, I figure I keep the vac gauge on and set the idle. I can't do any better than what I already have (score one for me). I back down the idle to 850 RPM in drive. I take 3* out of the vac advance can, down to 22.

One more test drive and the problem seems to have gone away. Score one for Eddy, there is a reason they left out the metering rods I thought I needed. And score one for the Mallory, I never should have tweaked that vac advance can as much as I did. Weekend was over. I need to live with this a while before doing anything else. I also need to check off-idle to WOT. And add two degrees of timing, maybe four.
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Thanks for the detailed writeup. I appreciate it very much since I also run a Performer carb and have been tweaking. I was a bit surprised that you're running a 750 cfm on your 327. I've gotten mixed reactions about trying it on my 355 (currently a 1405, 600 cfm). Is your motor mild or pretty built that caused you to choose a 1407? Did you buy the carb new or rebuild a used unit?
I have Edelbrock's Performer RPM package on the motor - heads, Air Gap intake, camshaft (.488/.510 lift), even the timing chain. The 750 CFM is what they call for for this combo. I was more interested in it for the calibration than the CFM. I bought in as a "remanufactured" unit, which I assume was a refurbished warranty return.
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