Quick help. My bronco has 351m and today started running like crap. I want to ohm the spark plug wires but what should the readings be.?

Also, what should the gap be for the pickup coil inside the distributor?

Thanks

 

I'd pick up a Haynes manual if you haven't already. I know the pick up cool gap is in there and just found the spark plug ohm limit.

 

 

Testing plug wires for ohms is pointless, you would need the manufacturer's specs per foot and would need to test without the crimped ends. What gap are you referring to? The only gap is the air gap on the magnetic pick up and that's not adjustable.

 

Maybe they meant 500?

Quick google search says most modern vehicles are 10-12,000 per foot

And 500 per inch comes out to 6,000 per foot.
Not sure how older vehicles relate to newer tho.

When in doubt just pick up some new wires. I wouldn't bother ohming those wires.

 

I think he's referring to the breaker points since he mentioned inside the dizzy. That's for sure adjustable (see pic I posted) but I also just adjusted mine a few months ago.

Unless his 351m doesn't have a points dizzy?

 

The 351m debuted in 77, all ford trucks had duraspark ignition starting in 1975. He doesn't have points unless some terrible swap was done.

 

Thanks for the info. Didn't know that

OP my breaker points info is for a points-type ignitions only. (Also mentions that in the pic)

 

[QUOTE=ford390gashog;18509759) The only gap is the air gap on the magnetic pick up and that's not adjustable.[/QUOTE]
I know the gap on the magnetic pickup CAN get out of spec. Years ago I had one out of spec by .002 and the engine would not fire.
From what I can tell, I have 2 wires not getting any or sporadic spark.

Thank you to everyone for the input. I would like to track down and not just throw money at it. Back in June/July I had to replace the dizzy with a new one. I was out of town so I was over a barrel. I also changed the wires.

Thanks again. Any other input or suggestions will be appreciated.

 

That 5000 ohms per inch seems high, but I've seen it in a Ford shop manual. Remember that's MAX. When it goes over, that's a replace indicator. Less is O.K. Many aftermarket wires are normally far lower.

As far as the stator to armature gap in the distributor goes, another FTE member said it should be 0.018 inch. I have not, so far, been able to confirm that from a Ford spec. source.

 

Checking the resistance of the plug wires only assures they are continuous and not open.
You cannot check AC high voltage resistance with a low voltage DC ohmmeter.
A plug wire with spiral core will have low resistance. It's funny that they tout "our resistance is lower than yours" when lower resistance in a spiral core wire means the windings are farther apart because the total length of the wrap wire is shorter. Less wire, less cost.

If you have isolated the misfiring cylinders, change out the plugs on those two. It is not unheard-of for plugs to be bad right out of the box.

If you have eliminated plugs do the same for wires. When I replaced plugs in my wife's 4-cylinder car I also changed the wires. A few months later it started bucking in traffic. Turns out my Beck-Arnley plug wires had carbon tracking in the boots.

If not that, get out your trusty clamp-on timing light and clip the plug wires on the offending cylinders. It doesn't matter where the spark is jumping, if it does jump you'll get a flash.

Also, check the reluctor air gap for all eight teeth.

The final thing is distributor phasing. That's where the rotor is pointing when the spark happens. If the rotor Is pointing between two cylinders it's possible for the spark to jump to the wrong terminal. This usually happens when the vacuum advance is highest.

I'd disconnect the vacuum advance hose and plug both sides. Then take it for a spin. If the problem still exists I'm all out of ideas.

R.