1973 F100 390 FE
Truck has been running like a top the last 8 years. Parked it in the garage the other day and two days later went to start it and it had a very rough idle, seemed like it wasn't firing on all cylinders, and died when put in gear. After a few more tries it wouldn't start at all (though turns over no problem).

A few days later I went to see if I could figure out the issue and it started right up. I let it warm up awhile and was getting ready to take it around the block. Before I could do that it started stumbling again. Gave it a little gas and it died. Would start for a few seconds, but very rough again, and then wouldn't start at all (again).

Truck does have a MSD box. I checked all the continuity of the wires and they were good. Then after waiting awhile I got it to start and idle again (though rough), I pulled some plug wires at the cap and could see that they were firing. Started being very rough again, then died. Pulled the plug wires again while jumping the solenoid to turn it over and no spark.

Maybe it's the MSD box I thought, but I switched it out for another that I know is good and it has the same issue.

Couldn't check with starter fluid for a vacuum leak as I couldn't get it to idle long enough (today, will see if I can get it to run awhile again tomorrow when it is cold). But, visually all the vacuum lines look okay and it seems unlikely that it would spring a big vacuum leak at the intake or under the carb in a day (but possible). When it was running a bit I did try putting my hand over the carb and it died. Giving it more gas makes it die.

The only other things that were a little strange was that I could turn the distributor even though the clamp/plate is bolted down and secure, but it looks to be pointed in the right area. Also, sometimes when it dies you get a lot of atomized fuel mixture shooting out the carb, I guess because it is not ignited and getting pushed out on compression.

Seem to be getting fuel just fine.

 

Are you seeing a good stream of gas when you crack the throttle as looking down the throat. I'm thinking maybe accelerator pump diaphragm leaking.

MSD, did that replace points?

 

Yes, I have two good streams from the accelerator pump.

Technically the points are still in there, but yes, the MSD box replaced the need for them. They still look good though.

 

Say what points are still there but the box replaced them??
So tell us what is telling the box when to send a signal to the coil to fire?
The box needs something to tell it when to fire be it points that open & close or a signal from a electronic dist.

No spark when you checked 1 time is not good. No spark no fire, no run!
I would take a good look at the points again. Take points file to clean them up then a clean lint free rag to get any dirt left from the filing.
Now check the point gap and set it just like you would with out the MSD box. I would set by dwell, match book cover gap is just to get it running.

From sitting points can get a crusty funk built up on them and cause issues.
If you still have the issues change the condenser. Gut tells me if you have a newer condenser installed that is the issue but ........
You dont need to change the points if they look good but if you want to go ahead.
Let us know how you make out
Dave ----

 

Sounds like the box is a CD system Had an aftermarket capacitive discharge ignition on a '68 Plymouth, but it kept it's points too ..... the CD just reduced the current being switched by the points so they lasted longer, but they still would eventually need some care. Rub blocks wear, points get dirty from arcs, etc. Window switches of my '07 often do the same thing.

 

Okay update...

The following day I rewired everything and then put a new set of points and condenser in. Fired right up and ran great. I let it run parked for 30 minutes to be sure the problem didn't reoccur, then took in for a test drive. I got about a mile down the road and it clicked off as if I'd taken a wire off the coil. Everything still looked good, but despite some roadside fiddling, I wasn't able to get it to fire again and had to have it towed.

I was baffled as to what it could be. I had ordered another set of points and condenser when I used the last one and when it got here I put them in. Truck fired right up and ran as if there had never been a problem. No test drive yet. I'll update you on that and let you know how it turns out.

While I am thinking of it though, does anyone have a source for quality points and condensers?