1984 fullsize bronco, 351W, recently rebuilt (2years). Less than 1000 miles on it since. Still stock, although this engine came out of something else a long while ago when we first had the truck, so I do not know its history. Had problems with it running, shop recommended removing 2B carb and sswitching to earlier one that was not controlled by EEC-IV. Put on earlier ford carb. I also changed the ignition to an MSD6A, MSD coil, and swithced out the distributor ro a Duraspark from the TFI. Engine runs fine at speed. At idle, seems to run rough. Have to run a pretty high idle in neutral so when it is put in gear, it idles around 750. The transmission was also rebuilt w/new torque converter at the same time. I figure that the carb might need some adjustment for the idel, but the real problem is that it continues to "diesel" when it is shut off. Tried toying with the timing and that does not help. The only vacumn items running are the brake booster, transmission, heater control, and the port for the vacumn advance. All others capped off. EGR not connected either. MSD has a blurb about run-on casued by back feeding of voltage through the charge light. Mine has the gauge. Anyone out there had any similar problems?.

 

The MSD's are real sensitive to any transient voltage...but you should only get a couple of fires at the most and not a whole diesel effect. Usually deiselling is caused by excess fuel in a HOT cylinder. How hot does your engine get and how rich is your carb running?

Make sure your MSD is hooked to a power source that completely shuts off with the ignition. Some components take several seconds to completely power down and have a continuous flow of current or hold a charge for a few seconds. This could fire the MSD. When the magnetic pickup in my distributor went bad, the truck would quit running but you could hear the MSD box still firing (as long as the rotor was lined up to a plug wire post).

 

The run on is caused by your high idle. Something is wrong, and you have the butterflies open too far, trying to get it to idle. Look for the famous vacuum leak all the books tell you to check for first(well, I guess it's true). I wonder, your egr is not hooked up, but could it still be leaking. Make a block off gasket to see if it helps. When you get it all straightened out you should hook the egr back up. You might be better off to go ahead and get an aftermarket 4bbl and manifold if you can't get it straightened out.

 

[updated:LAST EDITED ON 31-Jan-02 AT 08:54 AM (EST)]I have my idle set at around 650-750 in gear. It just seems to run good at that idle speed. I had it dropped to 500, but then it sounded like it was goind to die at the stop signs...I don't have any trouble with dieselling...A rich condition will contribute to run on though...and your idle should be nice and smooth...does your idle smooth out after the engine is warm?

 

He said it's idling around 750 but, since the engine is not running right, I bet he's got the idle screw turned way in more than it should to be to get 750 rpm.

 

I'll have to agree with everyone else in that the diesaling is probably due to the idle adjustment being way off. My money is that you have a vacuum leak somewhere that is causing your faulty idle, which you tried to fix by tweaking the idle screw a bit too much.
Try to find the source of your rough idle, and I'll bet that solves all of your problems.
Ben