Problem with SPF 1207

[marlowe]

Have 200 miles on the Superformance and cannot get rid of the stumble and dead spot at 4300 rpm. Have 351\392 FMS motor, 750 vacum Holley. Dyno guys dialed in the carb and says the problem is in the ignition. Wants to change to MSD dist, coil, and box.
Figured I could tackle it one item at a time and see what it is. Bought a MSD 6AL with coil. Removed the Duraspark box and trying to figure out how to wire in the MSD using the cars harness. Can this be done, or do I have to wire it just like MSD shows in their manual? Some of you SPF gyuys must have done this before.

[gsharapa]

Don't use the harness except for the tach lead, ground wire and hot ignition wire.

Wire it per the MSD instructions, which takes two wires to the coil and two wires to the distributor. Ground the box to the ground wire and take the hot big red positive lead to the solinoid. Tach to the plug in on the box and ignition wire to the one coming from your ingition switch.

I've done two SPF Cobras this way and the dealers do it also!

Note: Make sure you change the coil to the MSD Blaster as the MSD box will burn up the stock coil. Flat spot at 4,300 I might be looking at the vacume 750 carb. Is it leaning out? That 392 stroker want lots of fuel and I use to run a 750 double pumper on a 385 HP 351. Something to think about. How is it jetted?

[Great Asp]

marlowe,

I will e-mail you a picture of how the wiring is to go on a Superformance. I install mine on #992, and this worked!

Suggest changing dist, coil and ignition all at the same time, use all the same brand matched.

Also, my suggestion is to place the 6AL under the passengers foot box, not where the Duraspark is now. (Mount it upside down, inside the cockpit, above the passengers right knee estimated).

Any other questions, ask away!

Eric

[marlowe]

Gary, Thanks for the fast response. Looking at my wiring diagram from SPF, its almost impossible to figure the wiring colors and routing! Wires start one color, go thru plugs and end up different colors. They don't produce a wiring diagram that I am used to seeing.
If I wire the MSD direct as you indicated, must I remove any of the SPF wiring? Does the ballast resistor need to be taken out? Do I need to put in the diode MSD suggests so it won't run on after shutoff? Thanks.

[marlowe]

Eric. Did you have the same problem I have? Is that why you changed to MSD? And my Duraspark box is already mounted where you suggested. Sure would appreciate your wiring diagram.

[marlowe]

Gary, I used the Holley that SPF sent with their installation kit. 72 front jets and metering plate at rear. Don't know what plate is in there, but the dyno guys say it is ok. Air fuel mixture right on thru several dyno runs. They did have to change the spring controlling the vacum secondaries.

[Great Asp]

I pulled the duraspark out, and rewired as per the pic I will send you. I just left the ballast resistor where it was for looks.

Eric

e-mail me at ericbrown1@hotmail.com

then I can send the pic!

[gsharapa]

I know a lot of people are mounting the MSD 6AL box upside down under the dash. MSD recommends you not mount it upside down! If you are in an area of high humidity, rain or big swings in temp, the box can build up condensation which can not run out if it is upside down. Bottom line is a MSD box that will fail down the road! Your choice as many of the Superformance dealers are mounting them this way.

The 750 carb sounds ok based on your dyno results, but I would think the jetting is a bit weak for a 392 stroker. I was running a 750 double pumper with 74 in the primary and 76 in the secondaries, on a 351 crate 385 HP. I never liked vac secondaries and always wanted the over all performance and feel of a manual double pumper carb. Maybe some of the guys with this engine can post their success with what carb and setup of jetting and power valves.

I'm running a 514 with a 1050 double pumper with 86 primaries and 87 secondaries!

The MSD wiring is great. You can leave your SPF wiring as is and just use the following:

SFP Wire: MSD Wire:

Brown (Always Negative) attach to the Heavy Black wire on MSD Box

Black/Blue hook to the thin Red wire going into the MSD box,

Black/Green hook to the Tach plug on the MSD Box

The rest is hooked up as per MSD instructions;

. White wire not used,
. Heavy Red to + side of battery or + side (Hot) of solinoide,
. Orange to + side of coil,
. Thin Black to - side of coil,
. Green (-) & Violet (+) to the MSD Distributor

FIRE THAT BABY UP!!!!!!!!!!!!!!

[Great Asp]

Gary,

Thats interesting what you said about mounting the box upside down.

What if you drilled a small hole in the case for it to drain?

Eric

[gsharapa]

Just don't drill into any of the internals!!!!!!!!!!

[gsharapa]

I've had three Cobras with MSD boxes, last two were Superformance Cobras and the MSD box was mounted in the engine compartment where the Ford box is. My Paxton Supercharger SFP was there and the 514 is there and I'm not having any problems!

[Mike Madonna]

Marlowe,

Have the same motor as you. 750 Vac. with same jetting. However, no stumble nor flat spots anywhere in the RPM range. Two questions: Idle mixture screws, how many turns out? What spring in the secondary?

Thanks,

Mike

[Hal Copple]

In my 392 custom stroked Windsor, when i had it on a chassis dyno a year ago, the dyno operator showed me on the power curve where he could tell my secondary's were finally opening, i think about 4k rpm or so. He put in the lightest (white) spring in my vacuum attachement, and i picked up 40 !! horsepower, mostly in the mid range, and with the earlier opening of my secondary's, the "bump" in the power curve on the dyno graph disappeared.

I tweaked and chopped on that 750 vac Holley, cut off the choke tower, and recently put on a 750 Pro HP Holley, again with a vacuum secondary, and it gives noticably more power. But both carbs were seamless in their power deliveray.

I doubt your problem is ignition related, more likely fuel/air inadequacies. I ran and run now front 71 jets, and rear 80 (which is bit rich, but more protective against detonation). Power valve on my car is a 6.5. You may not be running enough air cleaner, these engines can be hindered by inadequate air flow from too small an air filter, especially one that is low, and makes it hard for the air to make such a sharp turn to go down into the primary side, what with the choke tower. As my speed shop guy reminds me, the primary has to flow as much as the seconary system!

My suggestion, is to check your timing, i ran best at 32 degrees total at 4000 + rpm, put on a bigger filter, i run a 14 inch diameter, 3 inch tall DROP BASE ! K&N, and put in the white secondary spring. My idle needle's seem to run best about 1 and 1/4 turn out in the front metering block.

Oh, on a 4160 series Holley, which is probably what you have, you have a rear metering plate, not rear jets, as you don't have a rear metering block, which i swapped out to on my carb, making it into a 4160 series Holley, so i could swap out rear jets easier. But when you do this, your hard chromed fuel line won't fit, as the rear metering block is 3/4 inches thick, and makes the carb a bit too long for the fuel line you have, i put on braided stainless steel to my car.

I don't think you need to swap out your rear metering plate, it will cost about $100 to change to a rear metering block with new fuel lines.

As for the MSD, i have mine in my engine compartment, on the passenger side footwell, although i do have it partially shielded from radiant heat with some aluminum-faced heat shielding. No problems, now at about 42K miles.

My car runs with the smoothness of a modern EFI engine, as do most of these cars, so i suggest you not embark on some long and complex road to a highly modified carb. Just do the secondary spring, and get the big drop base air filter. I have seen these cars gain as much as 75 horsepower with a switch to a big enough aircleaner.

keep us informed.

[webebob]

I'm still choking on the idea that the dyno guys said a mid-rpm stumble was ignition box. Did he say why? Like...was the timing mark flying all over the dampener ...or something?
Did it run fine on the dyno by not on the street?
If that is the case, pull the bowls off the carbs and clean out the crap that washed out of the new fuel lines.
You also didn't mention if you could push through the flat spot.
Either way, I'd make a quick run without the breather and see if the problem changes at all. If it does, it's carb.
I think the swap to MSD is a good move regardless of your problem. But a stock duraspark has powered millions of vehicles to 4700 rpm.
Good luck.

[Mr.Fixit]

The stumble could very well be a mismatched powervalve which isn't opening at the right time. You don't need to buy the rear fuel metering block kit, you can just drill and tap the metering plate to accept jets and fill the holes that it used to use with loctite quick set epoxy. MSD boxes do "go out" rather frequently, that's why they sell a harness that allows you to run two MSD boxes parallel to one another and just flip a switch to go from box A to box B during a race. Mounting it upside down is "bad" lots of heat is "bad" vibrations are "bad" it's six-of-one , half-dozen of another. I like to put the box inside the pass compartment as I fell that heat is worse than the possibility of condensation build up. (personally, I run a Jacob's box, not MSD)