Ignition Woes

[farmer]

So I've come in search of which direction to take with my troubleshooting.

Backstory: Ranger with carb'd 351w swapped in using Duraspark ignition from the 84 donor F250. Using the original TFI coil and spliced into the wiring on that to Duraspark harness for my (+) and (TACH) wires. Also add in AutoMeter 5" Tach

Originally truck ran fine. Ran into an issue where it would lose spark and not run unless I unhooked the tach. Figured the problem to be the connection where I spliced the (TACH) wire to the Duraspark, ditched the scotch lock and cut and spliced in. Problem solved.

Current problem seems to be that it seems to "cut out" as I've called it around 4,000 RPM, or sputter and not want to rev past that. Isn't constant, but more common than not. The needle on the tach drops 3-500 RPM with each sputter.

Truck does sit a lot without being run, someone told me "jus' needs a good cleanin' out" but not sure I buy that logic.

Please, before I drop the coin on an HEI replacement, what should i look for next??

 

[Shoddy Cologne]

I would also suggest the carb maybe to blame especially if youre running pump ethanol fuel and the truck sits. Try dumping a can of Seafoam or fuel injector cleaner in your tank and run it good. If that doesnt help maybe take your carb off and take a good look at it

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[Shoddy Cologne]

Another thing could be your fuel pump. Theres alot to look at before spending the time and money on a new ignition system.

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[85_Ranger4x4]

Check the fuel for sure.

I have been at odds with my DS since I went V8 though. Either the coil or DS box will quit on me and it will just die like I shut the switch off. Last time whatever it was killed both the coil and box.

 

[RonD]

Tach signal is the Ground being cut at the coil, that is how coil and the spark works.

Coil is given 12volts, via "+" connection when key is turned on, "-" connection goes to TFI module(or HEI).
TFI cuts the Ground"-" based on distributor position sensor(replace points), with no Ground the primary coil's 12volts "jumps" to secondary coil with lower amps but much higher voltage, and that higher voltage goes to distributor, rotor, cap and to the spark plug where it jumps the gap to Ground.

V8 engine sparks 8 times every 2 RPMs, 4-stroke engine needs 2 RPMs for 1 full cycle.
So at 800rpm, 400 cycles x 8 sparks = 3200 sparks per minute or 53 sparks per second.

The tachometer signal reads this "open ground" as a pulse, it divides by 1/2 the number of cylinders and you get RPMs from that, 1/2 because there are only 4 sparks per crank revolution on a V8

Since the tach wire is connected to the "-" on the coil if there should be any kind of short to ground on that wire or in the tach itself then spark would stop instantly, lifting the ground is how the coil makes spark.

The RPM dropping suddenly on the tach and not matching actual RPM could be a short, in wire or tach, but also in the TFI module, because you are losing pulses.

Easiest thing would be to unhook both tachs and see if problem is still there.

Could also be the coil wearing out, it effects higher RPM first.

HEI swap would be what I would do regardless.

Newer coils can be run on 12volts directly, older ones should not be, it shortens their lives.
Older systems used a ballast resistor or resistor wire to power the coil when engine was running, coil would get about 8volts when electrical system was at 13.5volts(alternator power).
8 volts puts out plenty of spark for a running engine and coil stays cooler so lasts longer.
These systems would have direct power from battery to coil "+" when starter motor was active, this is because power thru the resistor when starter was cranking would drop to about 4 volts, battery voltage is only about 9.5volts when starter is on.
So this added wire gave coil the higher voltage but only when cranking engine over.
This was the "I" connection on the old 4 post starter relays

 

[farmer]

I'll have to double check fuel pressure. Electric in tank pump still with a bypass regulator on the firewall. I may have turned it down trouble shooting another issue, therefore running out of fuel at higher rpm. Otherwise the carb and fuel system function flawlessly, 2 pumps and on a 10 degree morning fires right up.

RonD you've got me thinking. I believe the old (TACH) side of the coil wire never got disconnected from the main harness. Being that there's no TFI module anymore (2.9 TFI module is in the distributor correct?) It shouldn't affect anything, unless there was a short in the wire somewhere. In other words cutting that wire at the coil (after my splices) shouldn't affect anything besides the stock tach right? Might try that.

Proform HEI dizzy is $160 thru Summit and I read that it doesn't clear the coolant ports on the intake. Summit HEI dizzy is just over $200

 

[85_Ranger4x4]

My current scheme is use a MSD box with the DS dizzy.

A plain 6A box is about $200.

And then I don't have a huge ugly dizzy that will mess up my air filter and Explorer belt system.

 

[Ford78]

I got the proform hei on my 351 they are super easy to hook up, one hot wire and one wire for tach. but you will have to ether get a smaller air cleaner or space it higher.

 

[farmer]

Ford78 (or anyone who has wired in an HEI dizzy) where did you run the 12v power to? I see the existing coil ho wire isn't recommended