Misses at hiway speed

FordGuy · Post by **FordGuy** » October 1, 2009, 12:04 pm

Hey all. Just got done rebuilding the 390 in my '65 F100 and it idles great. Around town is fine too, but when I get up to hiway speed it seems to miss. EVERYthing under the hood is new, so I checked the power to the coil. It's 8-9 volts at idle which seems about right, but I figured it could be some kind of short in that (original) wire so I made a detatchable jumper from the solenoid. It runs much better now, if not perfect. I have tried heavy-duty NAPA points and then Pertronix electronics just in case but there was no improvement. Has anyone run into a similar situation? If the wire is bad and I replace it how do I get the correct resistance to drop the voltage to 9? Is that even necessary with electronic points?
Thanks in advance for any help.
Tim

jwh f-100 · Post by **jwh f-100** » October 1, 2009, 12:12 pm

Jim,
Did you also use a Pertronix/flamethrower coil?????

I think (but please verify) the resister should be removed if using the Flamethrower coil..

FordGuy · Post by **FordGuy** » October 1, 2009, 12:20 pm

No, I just have a new standard coil.

Brian Taylor · Post by **Brian Taylor** » October 1, 2009, 3:47 pm

I have had similar problems and there were 2 fixes one was a fuel filter was dirty just enough to choke it at speed the other was the condenser on the distributor fine at idle but at speed it sputtered. I used my original rather than the new one I had just put on.
Hope this helps.

FordGuy · Post by **FordGuy** » October 1, 2009, 3:55 pm

Thanks for the input. The fuel filter is new, and I have eliminated the condensor with electronic points so I'll have to look further.

65f100_Thug · Post by **65f100_Thug** » October 2, 2009, 1:52 am

How clean is your tank? Ive made that mistake I hooked a new motor to a dirty tank and got the same results. I had a 79 bronco with a 351M and rebuilt it bored 60 with a 400 crank(yes IT sucked gas) thats why it was so evident i think. It ran fine putt putting around and would almost choke out when I punched it.The NEW fuel filter didn't show it at first but trash was choking it out.

FordGuy · Post by **FordGuy** » October 2, 2009, 8:27 am

I appreciate the input but I have checked the tank and it is spotless. The fuel pump and fuel filter have about 10 hrs. total on them. I'm fairly sure it is an electrical issue since performance improves when I jump 12V directly to the coil. I know points won't last long that way and I'll have to find a switched 12V source, but I have electronic points in it now. I was hoping someone had tackled this problem before me.

Tech · Post by **Tech** » October 2, 2009, 10:57 am

The harder that you work an engine the more voltage that it takes to fire the plugs. Regardless if it is your whole problem 8 volts is on the low side and will be costing you HP and economy if not missing. A classic case is the old truck that will barely accelerate without missing but runs ok if you don't give it too much gas. The resistance wires fail over time, connections get weak and the key can fail. The resistance wire should run about 1.1 ohms and you can run up to 11 volts with a standard coil. If your electronic module says that it can drive a 12 volt coil (no resistor) and you install a 12 volt HEI type coil it should give you a significant bump in spark energy. I have been using the Pertronix 12 volt coil .This will allow you to regap the plugs wider .040-.055" depending on the rest of your system. This includes the big cap and 8mm wires and should improve performance and economy. Bypassing the resistor was a good troubleshooting step. You can also take the plug gap down to .020" to see if it stops missing. A floating valve will still float and fuel problems won't be helped either. Adding more spark energy can however cover a few other symptoms.
If your this deep into the distributor I would put a spring kit in and get a little more total advance out of that conservative Ford distributor. You probably aren't pulling heavy loads up steep hills every day.
Tech

FordGuy · Post by **FordGuy** » October 2, 2009, 11:11 am

Thanks for the excellent information. I plan on implementing many of your suggestions as soon as I can.
Is resistance wire readily available? As an alternative would it be feasible to use an external resistor, and if so do you have any suggestions? Will a full 12V charge damage the coil? I have a family car cruise this weekend and may need a short-term fix.
Again, your input is very much appreciated.

Truckrat · Post by **Truckrat** » October 2, 2009, 1:15 pm

Fordguy, if you are running a Pertronix setup, you need to run a full 12 volts to the coil. You can eliminate the ballast resistor too. If you have a resistor wire coming from the firewall to the coil, replace it with a new regular wire so you are getting 12 volts. The fact that you used a jumper wire around the resistor wire and it ran better is the reason. I have run one of those for years, and they are great. Get one of those Blaster coils for it, too. TR

FordGuy · Post by **FordGuy** » October 2, 2009, 1:30 pm

That's good to know! I was worried I might damage the coil. So since the resistance is in the wire, I should be able to pick up 12V on the ignition pole of the switch, right? That would be the simplest and cleanest. Any comments?

Tech · Post by **Tech** » October 2, 2009, 2:25 pm

The resistance wire is in the pink wire that comes from the key. Most modern electronic modules don't need the resistor. Some coils do still need the resistor. They will overheat and puke oil out without it and eventually die. some electronic modules turn the dwell time way down while the motor is at low speeds, saving power and keeping everything cool.
big ford box = probably not
GM 4 pin HEI = yes
Pertronix???
Run it for a while and see if the coil gets too hot to touch. If it is hotter than a baked potato it will die.
The easiest resistor to find at a parts store is a ballast resistor from a late 70's Chrysler car or truck. It is probably easier to find a 12 volt coil these days. It should say 12 volts on the coil. They generally run about 3.4 ohms internal resistance.
Tech

FordGuy · Post by **FordGuy** » October 2, 2009, 2:34 pm

Excellent. That's the info I needed. I'll keep an eye on the coil and let you know how it goes. Much appreciated!