This is a long post, but I want to try cover everything I've done.

I just rebuilt my 400 in my 79 F250. It's been bored .030 over with Badger flattops, Comp Cams DEH265, Edelbrock performer 400 manifold and a 1400 carb. I used the 1400 because i wanted to maintain the EGR, AIR, and vapor canister systems. For reference, I'm running about 9.1-9.2:1 at 2500ft elevation. Referencing the original diagram I have constructed a fairly simple vacuum configuration that lets me run all of those properly. I'm convinced my vacuum configuration works and has no leaks. I'm presently having some issues with detonation.

In the beginning, it did not ping at a steady state cruise, but light accelleration it would have significant pinging. I started by enriching the fuel mixture and that seemed to help. Well, when i reached 40% over the baseline 1400 jetting I decided that was not the problem. However, at this point, I only had part-throttle detonation only with a load - climbing a grade. The plugs are still white which indicate either lean or too high of combustion temps. The exhaust smells like I'm running rich, and I'm getting right at 9mpg (I was getting 11 minimum with the old engine), so all signs point to fuel mixture not being the problem.

My next thought was timing. All along I was running the stock spec of 8* BTDC. The mechanical advance was stock and the vacuum advance was stock. I have two vacuum advances off of late 70s 351/400s. The one I was running was litterally the original. I do not know the exact intended emmissions config (i.e. year, CA/non-CA, EGR, etc.) of the other one. The original advance did not seem to be adjustable but my "spare" was.

After searching through the forums here, I saw that most people were running within 1-2 stages of baseline jetting on their carbs, so I put mine back to the baseline factory config.

Step 1. I unplugged the vacuum advance and drove around the block. Significant hesitation, chugging. It wasn't running well enough to see if I had detonation.

Step 2. Take my "spare" advance, dial in about 50% of the advance (seemed to be about 12-14 turns on the 1/8" hex). I tried to drive around the block and made it about three houses before going back. The truck just was not running well enough to drive. Giving much more than idle on the throttle and it would try to stall. This struck me as odd, since even 50% on the advance should run better than no advance - yet it did not.

Step 3. Take my "spare" advance, run it up to 100% advance. This time i was able to make it around block. It was drivable, but it still had notible hesistation as the throttle plates open until about 2000 rpm. The tells me that i might still have insufficient advance until mechanical comes in.

Right now I'm at a loss as to what I need to try next. I've read through the forums and I'm getting a better idea of how this all works together, but I still don't have a full understanding of it. Does what I described above stir any thoughts as to what I should try next?

 

Sounds like you are heading in the right direction, I would suspicion the timing. The other thing is, what spark plugs are you running?, they might be too hot of a plug.

 

this may be a stupid question, but how old is you fuel?? i see that you seem to be pretty knowlagable and i'd think you'd only run new fuel.

 

Spark plugs are the basic Autolite, stock replacement. And as for fuel, I'm running less than one month old 87 octane. All of my jetting and tweaking has occurred over the last 1-2 months. All of my fiddling with the timing was done yesterday.

And as for the stupid question, those are the ones I needed to hear. Yesterday when I went back to the baseline jetting, I wrote down what to put in, headed out to the garage, and made the swap. Well, I looked at that again, and I wrote down .095s in the primaries instead of 0.098s. With the base rods, that is setting me up about 15% lean from baseline! Talk about a stupid mistake. I corrected the jets and that made a noticable difference, to say the least.

Also, I came up with a third vacuum advance, non-adjustable. Comparing the three, I notice that the adjustable one is incredibly soft - like one finger to depress it. Recall from above the adjustable even at 50% advance was worse than without advance. I'm not sure if something is wrong with it, or if it was intended to have a different advance rate??? The one I took off, and the other one were both equally stiff and differed slightly in travel. The original one had more travel than this third one.

I put it on, and with the real stock jetting, things are looking pretty good. I have a flat spot from about 10" to 5" of vacuum which to me indicates slightly lean. Also, WOT at high RPM with the secondaries open pings slightly, though more than I'm comfortable with. WOT at low rpm is fine. This sounds like i need to go a size up on the secondaries. Most importantly, ALL of my part-throttle pinging is gone!

So, it all comes down to timing. And back to stock jetting (40% less fuel than i was running at its peak) I should be able to once again break the 10mpg barrier. haha

Sorry this is so long, but I typed it all out in hopes that if anyone ever has a similar problem in the future, searches and comes across this post, it will of use to them. Please comment or make suggestions. Would it be worthwhile getting an aftermarket adjustable advance, or just stick with this one that seems to be working?

 

Check your EGR for proper operation. The EGR operates at part throttle to reduce NOx which requires more advance during operation. That is why eliminating the EGR will produce part throttle ping.

Sounds like you have the right solution on WOT pinging although backing off on the mech advance may help also. More than one way to skin that cat. Checking A/F ratio with a wide band O2 sensor is a good idea.

 

EGR is working. The first day I put the 4bbl on this engine I forgot to plug in the EGR and had lots of pinging at anything except coasting. As soon as I plugged in the EGR, that problem went away and I was left with what I had.

I think I need to get the timing light out and see what my total advance is with this vacuum advance and compare it to the original one. That may fill in some of the details I'm missing. Knowing that total advance is the problem, where's the best place to take some out? Initial, mechanical, or vacuum?

 

All three advance settings must be correct.