I'm running out of ideas on this one. '82 bronco w/ '69 351 Windsor.

I've got this thing going on every so often where I'll be driving down the road and the engine will start missing (sputtering and light popping). When it does this it will start after a mile or so and sometimes clear up, other times not. I've pulled the plugs several times and #6 is always black. The rest of them look almost lean around the electrode and insulator but very black down inside.
The plugs are (new) Motorcrafts.

Not too long ago I pulled the heads because of two burned exhaust valve. I had them install hardened seats while they were in there. Before I pulled the head it was doing kind of the same thing but I'm not really sure because of the valves.

Since then I've done:

Compression- good
Replaced cap and rotor
Plug wires are fairly new and resistance check was right on
Switched #6 plug wire with another-no change
Different coil
Distributor-changed out for a points dist
Carb-Holley 650 DP-Pulled apart and cleaned, replaced acc. pump, power valve, and needles and seats

I don't know if this is carb related or electrical. About the only thing I didn't change out in the ignition is the wire that goes from the + side of the coil to the ballast resistor.

I take a fair amount of pride in fixing my stuff without bothering others and I've been pretty successful over the years but this one is really starting to make me question my worthiness. If anyone has any ideas I'd be very greatful.

Thanks, Travis

 

Sounds like it's starving for fuel. Check the fuel level in the bowl and change fuel filters. Assuming those are ok it's probably the fuel pump. The popping can ruin the power valve in the carb so install a power valve saver too:
http://store.summitracing.com/partdetail.asp?autofilter=1&part=SPE%2D2408&N=700+ 4294925239+4294839063+4294847241+115&autoview=sku

 

sounds more complicated than a fuel pump...???


Some ideas off the top of my head:

Blow the line back using compressed air to be sure it's free of gunk- whats the filter look like even if it flows?
fuel pressure guage?
whats the vacuum doing?
did you do a leak-down compression test? results? (sounds like exhaust valves to me kinda) but had them ground etc)
quality of fuel- drain tank- clean and use a different gas station to check

switch carbs if someone has a working carb you can borrow just to check. I had 2 holley 600's rebuilt and both had my freshly built wheezer running badly...put on a edelbrock 1405 borrowed of my running truck and it ran perfectly.

Also imo 650 too much for that engine...but shouldn't cause this....

all the above is off the top of my head before going to bed....take no offense and may not even be close with any of it.

 

When this first started happening my first thought was a blocked fuel line. I tapped into the fuel line right at the carb with a 'T' fitting and ran a hose into the truck hooked to my fuel pressure gauge. It held about 5 to 6 pounds under normal driving but would drop maybe 1 pound under heavy acceleration. When the truck started acting up there was no change in the gauge readings.

When I tore apart the carb I checked the power valve (by sucking on it) and it was OK but I put a new one in it anyway.

I have the vacuum gauge in there now and it seems to be steady through out the driving range (idles at 20-21)

Last weekend after I had the carb apart it ran like a top for almost a week. Then I let it sit for a couple of days and it didn't last a mile before it started acting up.

Something I don't think I mentioned was when I had the points distributor in I didn't unplug the ignition box plug with the 2 wires (red and black) Don't know if that matters.

Thanks for the responses guys. I'll check back a little later if anyone has anything to add. I appreciate all you guys do.

 

Well I might be on to something. This evening I run a wire from the +side of the coil into the cab of the truck to my volt meter. With the key in the run position (engine off) it has 6.8 volts. When I start the truck it jumps to 10.6 or so and will rise to the mid 11s with engine RPM.

I would have to believe that bypass in the ignition switch is bad and sending too much voltage to the coil. Sound reasonable?

 

Can't send too much voltage to the coil. Its designed to run upwards of 14 VDC. What your readings DO say is that there isn't enough voltage there and if its fluctuating with engine speed then your spark quality is suffering in direct proportion with the change in voltage to the coil. Check connections @ the coil and take readings all the way back to the battery to find out where the problem is. The coil shoud be getting full battery voltage as soon as you turn the key on and it should get fairly constant voltage supply during operation.

 

OK I'm a little confused now. I've talked to a couple of Ford guys I know and they said the same thing you did about having 12 volts at the coil. One said that it doesn't have a ballast resistor. I know thats true on the older Duraspark and I'm not sure about later 80's but according to both books I have the Duraspark II has one (1.5 ohm resistance wire). I checked the resistance between the ignition switch and the coil and it showed about 2.5 ohms. (I also see that I failed to mention that it is a Duraspark II system I'm working with).

I guess what has me puzzled the most is I have 6.8 volts at the coil when the key is in the run position which the book say is correct, and jumps to 10 to 11 when its running. Where is that extra 3 or 4 volts coming from? As far as I can tell, anything that comes into the coil has to go through the ballast resistor.
If it is supposed to have 12 volts, can someone explain how? I've got it stuck in my head how this should work but if I'm wrong, I need to know why so I'm not wrong anymore.

 

When it's running the coil receives a pulsed dc signal. Most meters are looking for an average level so the pulsed 12v may look like 10-11v. It could be a bad ignition box. When bad they tend to die as they heat up, then come back after they cool down. Often they just cut out, but it could cause sputtering too. Try swaping it out. They are relatively cheap and its good to carry a spare anyway. After going through 3 ignition boxes over several years I switched my 85's duraspark II to a MSD-6A. Much better spark and far more reliable, but expensive.

 

D7AZ-12250-A .. Resistor Wire-Ignition Switch to Coil (Motorcraft DY-213) 49"-long, #16 wire. Color coded red with green stripes.

As original, the 1960's/70's Ford fuel pumps have a cartridge fuel filter inside. On the pump is a tomato juice sized can with serrated edges. Inside that can which unscrews, is a fuel filter. Most ppl don't know it's there, so the filter rarely gets changed.

C4AZ-9365-A .. Fuel Filter (Motorcraft FG-1)

 

Did you say it happens under load?

Have you checked the vacuum advance check valve and or the weights and vacuum for the distibutor?

Have you replaced the distributor resistor and or have you tried to run without it? (No resistor will shorten points life).

How old is the Fuel tank and is it possible some debris is clogging up the outlet.

Do you have catalytic mufflers on this unit they could be plugged.

One other major thing folks don't do is to check their power and control wires and cables and connections.

Always remember if you are checking an electrical circuit you need to do it under load.

It could be something as simple as alternator is not putting out enough juice because of a loose fan belt or bad alternator or a bad battery cable or end or a bad regulator, there by starving the ignition of voltage on speed or load condition.


I don't know if any of this helps but maybe it will get you thinking in the right direction.


And remember you only need air fuel and fire to run just check each part of that equation seperately.


LG Rick.

 

Well I stumbled across something this afternoon that might be a factor. I had the positive cable off of the battery last night, and when I put it back on today I started tightening it down and the clamp broke. I thought it was kind of odd because the cable was less than 6 months old. As I was taking it off I noticed the clamp was almost black from heat. I could see a bad ground being the culprit, hopefully. So tomorrow I go to NAPA and get cables to go from engine to frame to body. And if this is the cause I guess it serves me right for not checking the basics first. I'll check back in tomorrow with an update. Thanks for all the replies, I've taken them all into consideration.

 

that just happened to me today...even replaced a starter when the cable was corroded in a hidden area.. different problem but it was just a cable.

 

Todays adventure began with adding ground cables to frame/body/engine. No change. Just for the hell of it I unhooked the coil wire and run a wire straight from the battery through a 1.75 ohm Chrysler ballast resistor. Still no change. Its frustrating because it fires right up and runs good for a mile or so. I have been noticing its starts happening after I leave a stop sign and get in second gear. After I get up to speed it seems to clear up until I come to a hill where I have to step on the gas a little. I wish I had another carb to throw on just to make sure it's not that. I've got an old Carter AVS but have no idea if its any good. Well there's always tomorrow....