Here's a problem that has baffled every person I've presented it to, including some well known engine builders.
82 F100
351 Windsor, 20k on rebuild.
Mileage/towing cam.
BBK headers with Lighting like exhaust system.
Edelbrock 4bbl intake and Edelbrock carb.

Here's the history.
When I switched to the Edelbrock intake, it ran much better over the old 2bbl but it always felt slightly lean right of idle in normal traffic driving.
When I installed the headers, the problem became more pronounced so I bought the complete jetting kit and set out to fatten that circuit but when I improved that area it killed my cruise mode mileage. No amount of jetting ever made the off idle right.
I bought an oxygen sensor gauge and it proved my gut instinct. A vacuum gauge and tach also tied everything together.
Convinced that it was the carb, one of my customers in the carb business sent me a Holley to try. Slight improvement but same symptoms.

Before I lose you to boredom, some other possible clues. Vacuum in gear is 17. The problem starts happening as I open the throttle slightly and it drops to 15. It goes away if it gets down to 12. Power circuit kicks in at 7.5.
Vacuum is always steady and about 17 in cruise mode, 21 at idle in neutral.
Everything in the ignition is brand new from plugs to Duraspark box. (I tried that out of frustration)

There has to someone out there that has a new idea and I'm open to anything. I just want the problem to go away.

 

Well, I can't say for sure if this may be your problem, but a co-worker of mine had a similar sounding problem using an Edelbrock intake, and it turned out that the casting had a very small hole in it causing a vacuum leak and lean condition at low RPM's. Not saying that this is it, but at least something to ponder.

 

You may have hit on something. Where was it and how did he find it?

 

I don't know about his co-worker, but I have had good luck using a propane torch. Don't light it, turn it on low and with the engine running pass it around the intake, gaskets and carb base. On the off-idle lean condition, on the Edelbrock (Carter AFB) carbs, Chrysler used to use a special raised jet and a two stage step up piston. This allowed a part throttle enrichment and still decent fuel economy. Some of these pieces are still around. What kind of advance curve both vacuum and centrifugal are you running? Ignition issues can cause part throttle lean conditions and headers will magnify the problem. Are you still running an EGR valve? if so, try restricting the EGR flow or temporarily disconnecting it. EGR leans out the mixture. On the Holley, you could try a two stage power valve, some of them enrich very slightly at around 15" and full rich around 5-7". Which Edelbrock intake are you using? Open plenum types can give a part throttle lean, and an open style, thick carb gasket can give some funny results also. Without actually seeing it and doing some testing myself, it's kind of hard to tell. Good luck with it.

 

Take a look at your cam as well. An aftermarket cam won't produce as much vacuum as well as a stock cam will. The tighter the valve/lobe seperation, it will cause a drop in intake vacuum and cylinder pressure.

 

I think 85lebaront2 is on the right track. Try the richer power valve, if no luck then insure the accelerator pump is working, maybe stiffen it up. If nothing else works go to a dual plane manifold.

 

The leak on the mainfold was actually on the underside, open to the block. He didn't find it until he removed the manifold, and we are talking a very small hole, really had to look for it.

 

I had the same problem on a 1978 F150 with 460 engine. I installed the complete Edelbrock Performer package-cam,lifters,timing set, manifold, and 750 CFM carb. After numerous rejetting attempts and triple checking everything for leaks, I called the Edelbrock tech. people and they immediately responded that their 750 CFM carb. needed different booster venturis to stop off-idle stumble. They sent them to me free of charge and that cured my problem

 

i agree check for vacum leaks, and let us know what the specs are on that cam. whats the lobe seperation as well. anything below 110 with a carb would not be good

 

85lebaront2
I'll try the propane trick.
The Edelbrock has a 2 stage step up system also but it doesn't kick in until the vacuum drops to 8".
The new distributer had the curve checked and was declared good by the technician. I don't run the vacuum advance anymore because it starts to ping when ever the lean condition pops up. Slight grades on the freeway start sounding like I'm trying to rattle it apart.
The EGR is gone since I'm trying to eliminate all non essential garbage. The only change since I blocked it off was that it goes rich when I back out of the throttle.
The carb is an Edelbrock 1406 and is what they recommended.

big hoss 29
The cam is a Compcams towing and mileage cam. Nothing exotic and has more vacuum than the original. It worked fine with the 2bbl carb.

jimpennycuff
I've spent hours on the phone with Edelbrocks tech guys and they insist that I belong in a home for the mentally disabled. I've even offered to bring the truck to them so they can drive it.
In the beginning, I found a tiny piece of flashing in one of the emulsion tubes and removing it helped a little. One of the techs suggested that I drill out the tubes but didn't know what size. I don't think that I should have to modify one of their products to make it work right.

What I'm learning here is that 2 different brands of carbs with different circuitry react the same on my motor. The Holley was road tested on a 350 Chevy before it went on my truck.
Can I eliminate the carb as the problem?

 

I tried the propane all over the manifold and carb area with nothing to show for it except that there was no giant explosion. Now if I can just fill the crankcase with propane maybe I'll find a leak on the underside. Hmmmmm....

 

Do you have an egr valve?

 

What is the curve on the distributor? If the advance comes in real early (like drag racers like them) it may have way too much advance for the RPM. I never had a problem with part throttle ping on any of my engines, but I have never run aggressive advance curves. Even on my Shelby, I didn't bring the total advance in until around 5000 RPM. If you are running a Ford distributor, under the pickup plate there are two different springs for the mechanical advance, a light one and a heavy one. The light one controls the early advance and the heavy one serves two purposes. The first is to limit the amount of low RPM advance, the second is to determine when and how fast the higher RPM advance comes in. A 351W on modern gas probably shouldn't have over 36 degrees maximum centrifugal advance. It can, however, tolerate as much as 50-60 degrees at light load. The vacuum advance makes up the difference between max load and light load. Can you get the technician who checked your distributor to tell you what the advance curves are? If you can, either post, email or PM me with them and maybe I can give you some more insite. What initial timing are you running?

 

EGR was replaced then removed and blocked off.

The curve is a stock Ford curve. I set the timing for max idle RPM and then check it for total which turns out to be 35 total.
I have moved the timing around while the lean spot is happening and the RPM only drops, never gets faster.
I have tried advancing and retarding in 2 degree increments, 10 degrees total each direction, with no change except that it runs poorly.

 

Ok, lets back up a second, what plug gap are you running? What spark plugs? Duraspark II likes a pretty large gap on the order of .045 to .047 inches. I have had a problem with Champion spark plugs in Ford engines, Champion revised their heat ranges a number of years ago and since then the plugs run a bit on the hot side. I usually use Motorcraft plugs only. Bosch platinum for some reason seem to like to act like a lean misfire. You might also want to recheck your float settings, a slghtly low float level on an AFB will cause all kinds of interesting problems. Some of the old AFBs as was indicated earlier in this thread, had a "skirt" on the booster venturiis to aid in what is called "tip in" or the point at which the main jets take over from the idle circuits. Unfortunately, you are all the way across the country from me.