Tuning Update: She was starving, apparently!

[R4K 10]

Finally! A happy resolution to a near month-long search into my problem.

 

The problem, if I haven't updated you all lately on my first (finally successful!) rebuild, was this:

 

Once the engine broke in a little, about 300 miles or so, I started to get a real high Park Idle (nearing 1100), which was causing a dieseling/run-on when the car was shut down. Problem was, I was getting a stumble while in Idle in-gear, and had to set the idle higher there (about 800 or so) to keep from stalling while at red lights. Even then, if it sad idling too long (like the duration of a really nasty red-light at an intersection), there would be a horrible stumble directly off-idle. WOT performed fine, as did part-throttle at Cruise.

 

No amount of tweaking the A/F mixtures could get it idling smoothly. It seems to like more initial timing, but nothing that would keep the stumbling problem away.

 

Now, initially, I had suspected either ignition or fuel delivery. I knew that my tank was rusted, but I really didn't realize how much. One day a couple weeks ago, it just died on me on the way back from the performance shop (where lots of the locals were darned sure it was all a timing issue). Finally, the rather new fuel filter stayed clogged long enough for me to see the accelerator pump wasn't delivering ANY fuel.

 

AHA! Problem isolated! Luckily, the fuel filter unclogged after a bit and allowed me to limp home.

 

Fuel tank and all soft-lines were replaced (yes, I know I should've replaced the hard lines, but I'm still cheaping it and didn't, so shush!).

 

Problem still there. Definitely better, but still there. Okay, there was a fuel delivery problem before, probably low pressure. Need to tweak the carb again. No amount of tweaking would make it go away.

 

So on a hunch, I decided to check the spark on all my wires using my timing gun. I found that RIGHT DURING THE STUMBLE, the timing light would go dark if on plug wire #8.

 

AHA! Problem found! I'd planned on replacing the wires anyway, so I took the opportunity to get some new 8mm Mallory wires. Also put the original (hotter) plugs back in, so if the problem was still there, it would rule both plugs and wires out.

 

Problem still there. Now I suspect the Cap or Rotor (the module going wouldn't be restricted to just one wire, it would be random). So, I rotated the orientation of all the plug wires one to the left, set the rotor correctly, and reinstalled. Now the miss showed up on the #5 wire...which didn't make any sense, because the #5 and #8 aren't adjacent in firing order.

 

Replaced Cap and Rotor. Luckily, then the dark-spot miss started jumping to other wires.

 

AHA! Problem found. The module is going after all!

 

Replaced module. Problem still there.

 

At this point, somebody convinced me that I had too much lifter preload on my non-adjustable rockers. So I spent a day measuring that crap (which after a false alarm because of sloppy scribing), and hunting down SBC valve springs to shim my pedistals. Then realized I didn't need to, now that I measured more carefully.:pinch:

 

 

So, after replacing:

 

Spark Plugs (was okay, wanted to experiment with cooler plugs anyway)

Spark Plug Wires (already planned on getting)

Distributor Cap

Distributor Rotor

Distributor Module

Distributor Power Filter

Coil (already had)

Gas Tank

Flex fuel lines

Fuel filters (1 before and 1 after pump)

 

as well as having the Carb Rebuilt (wanted to do this myself, but was trying to diagnose a problem and didn't want to experiment for my first time and possibly cause more problems)...

 

...not to mention trying to search down SBC valve spring shims of the right ID and thickness and smoothness...

 

What did it turn out to be?

 

As I happened to be out looking for those shims, I picked up an Edelbrock Calibration Kit for my carb. I was beginning to wonder if the random lack of spark wasn't the CAUSE of the stumble, but a SYMPTOM of the stumble, and I suspected was caused by a lean condition.

 

Now, before you say "You should have looked at the carb early on", let me explain.

 

I can't count how many times I've heard from people when they hear I have a 390 (402 really) with an Edelbrock 1411 750cfm carb, "That's too much carb" "You're overcarbed." It's like a Tourette's tick or something.

 

But people had me convinced. So all throughout this process, I've got all those know-it-alls' voices in the back of my head "too much carb too much carb" and, being a novice at carburetion myself, took it to heart. If anything, I HAD to be running rich! Everybody says so!

 

There was one or two lonely voices out there in FE land that 1) didn't have a Tourette's tick of "Replace that Edelbrock with a Holley" and 2) said that, for an even slightly warmed-up 390, the 750 was not too much carb, and even at that, still too lean at stock configuration!

 

Now the Eddy 1407 is the same as the 1411 that I have, but it's manual choke, and they leaned the 1411 electrical choke out (too much). I used the Calibration Kit I had to go 2 stages richer at idle, and 1 stage richer at power mode, achieved by changing to a narrower rod.

 

Unfortunately, I don't have the jet/rod combination the stock 1407 uses. I have the jets (presently have a .110, will be changing to a .113), but won't be able to get my hands on the rods until Monday.

 

When I'm done with this, I'll be running even a little richer at cruise, which will be fine, and a little leaner at power mode. I think the existing jet/rod combo I presently have is running a bit rich in power mode, so this step down should be just what I'm looking for.

 

I'm pumped!

 

This thing, before, was stumbling to stay running at 800-850 RPMs for weeks. Today, I had her down to 600 and she was just loping and sounding rank-ass mean. I had to just chock the wheels, leave it in drive, and stand back and take a smoke as I just listened to her.

 

Sad thing is, thinking back on it, this thing was probably running lean even BEFORE I rebuilt it. I put a longer duration cam on it, put better-flowing and more port-matched heads on, as well as increased the compression ratio by 1.5 or so.

 

Once I get this carb the way I want it, I'm off on my experimentation into recurving my distributor. There's guys with near the same setup that swear theirs likes 18* - 20* initial!

[bswor 77]

Glad to hear that you finally tracked down the problem. It sucks that the last thing you check always seems to be the problem, but i guess thats the way it goes. So does it feel like another car or was your problem just on the low end?

[R4K 10]

Yeah, the problem was always previously on the low-end. Now it idles solid, jumps right up at off-idle and responds well to acceleration (although I may stiffen the springs up on that to get a little better response time).

 

However, it sure seems like it's weaker than it was at WOT. The rods I replaced the stock ones with put my Cruise Mode 2 stages richer, and my Power Mode 1 stage richer. I'm thinking that the old rods had an adequate Power Mode, but the Cruise Mode was too lean.

 

The combination of the new rods I have ordered with the .113 jet will go just a bit richer even on the Cruise, and actually lean up the Power Mode a bit, so I think it'll put me very near where I want to be.

 

Once I've got the new jet/rod combo in, if my Power Mode is still weak, I can tweak plugs and gapping and play with timing (wanting to tweak my curve to run more initial and still keep my total advance to 36*, all-in by 2600 or so).

 

But it all makes sense, now. The high-drive, low-park idle problem, the dieseling on shut-off (lean runs hot, creating a glow-plug effect in the combustion chambers), the stumble at idle. Even never before being able to start the car without accelerator help...now, just pump it once and turn the key. Amazing.

 

I just never woulda thought the stock jet-rod combo of a 750cfm would be so lean.

[Pakrat 1,043]

Yeah, I could have told you all that.:001_tongue:

 

Seriously though, congrats on finally chasing down the gremlin. Too bad you had to replace all the uneeded stuff but hey, now you have extras at least.

 

BTW, the Holley site says you can go as large as 770.

[bnickel 10,004]

i just changed out my headlight harness on my car a couple weeks ago and it cured a starting problem i had. what i think happened was that when the last harness got installed somehow the start-up ignition resistance wire bypass didn't get hooked up so i wasn't getting a ful 12v to the coil on startup, just the 8-9 volts the coil gets once the engine is running. the problem was that the old harness was cobbled together with parts of the original harness (which got fried from a broken strap allowing the harness to fall over on the exhaust), most of the harness from a 6 banger parts car (not correct for a v8 car) and part of the charging system harness from my old 70 cougar. so somewhere in all that mess i had an extra hot wire that no one could figure out what it was for, i'm guessing the resistance bypass wire for startup. anyway, i swapped the harness out and the car hasn't started this good since before i quit driving it teh first time.

[Max Power 74]

It sucks that the last thing you check always seems to be the problem, but i guess thats the way it goes.

 

LOL.. well yeah, cuz after you have found the problem, you quit checking! :thumbup:

 

750 is just fine for a warmed over big block in the 400 cube range. David Vizard has taught us the you could go MUCH bigger if you wanted to with proper tuning.

 

I ran a 750 on a near stock 351 for years and it ran great.

 

Those charts are VERY conservative.

[bswor 77]

LOL.. well yeah, cuz after you have found the problem, you quit checking! :thumbup:

 

LOL i actually thought of that as i was typing it... zing!

[R4K 10]

LOL, I was going to say the same thing, but your drift was right on. I sure with there weren't so many "Nope, that wasn't it!" moments during the troubleshooting phase, it would have been better (and cheaper!), hehe.

 

Part of my problem is that this was my first muscle car as well as my first rebuild. The only previous experience I had was with this car, for only about 5 months, before the rebuild...and since it's apparently always been starving, I didn't have anything else to compare it to.

 

Another part of my problem is that all the mustang folks I know are online, not close enough to swing by and borrow some parts to swap out. I'd have swapped the carb much earlier if I had a buddy's to borrow, hehe.