Hey Guys, I'm new here. I purchased my first vehicle, a 72 F100 from the second owner (owned since 79). I purchased it on November 25,2009. The Truck has a 351W from a 72 Ford LTD that was said to have been rebuilt and a mild cam installed. Also converted to an AOD tranny (also said to have been rebuilt) The truck ran pretty well when i bought it aside from dying on idle sometimes. The truck would run pretty good until heading up a grade and then all of a sudden it started missing and jolting and barely would run. I rebuilt the carburetor and replaced the distributor only to find no help. The Engine has gradually gotten worse and now it does in fact miss all the time above 1000RPM. The spark plugs are very lean from the powder white color I see. And the truck idles smoothly but misses bad under acceleration. I have tried playing with the mixture screws to no advantage (Autolite 2100?). I have the engine timed and the firing order correct and the points and spark plugs gapped. New rotor and cap and wires. Fresh oil and filter. New air filter too. I have reasonable vacuum (18) and i have 125-135LBS of compression on each cylinder. I also installed a new fuel pump and fuel filters. Yet it still misses and is running very lean. Also it has been getting AWFUL gas mileage about 2-3MPG . Any help would be greatly appreciated. Thanks

 

Welcome aboard, Solomon.
This is what I would do: Put the crank at tdc on the damper marker. Pull the distributor cap and mark the outside case with white-out exactly where the rotor is pointing. Put the cap on. A lug should be exactly over the white mark. I suspect that the distributor might have been off a tooth and you put the new distributor in at the same orientation. Under acceleration, the vacuum advance might be making matters worse by dragging fire to an adjacent cylinder. Worth a try.
Altering the idle mixture screws won't help you with your problem. The idle circuit is out of the picture when you are accelerating.
Let us know what turns up.

Semper Fi

 

Also, Solomon, read Xstrange's post in the "Big Red is running rough" thread. He gave a good explanation of how and why a blown vacuum diaphram might be causing your problems and also might explain the lean indication on your plugs.
You can do a check on the diaphram by plucking the rubber line to the carb sucking on it. It should hold vacuum and the distributor plate should move.

Semper Fi

 

Your symptoms sure sound like the problem is with ignition timing. A blown vacuum advance unit diaphragm is a possibility.

But first, your timing may just be way off. On old Ford engines, you can't necessarily trust the timing marks. The reason is that the outer ring of the vibration damper will creep over time, so you'll get a false reading of the notch against the bracket with the marks. On the 352 in my own '67 F100, the mark had drifted off about 15 degrees! When I first got it, I used a timing light and set it right to spec......and it would barely keep running! By trial and error, I found where the timing wants to be. According to the marks, it's about 15 degrees advanced from where it should be. Someday I'll do the depth stop in No.1 cylinder routing and make an official new mark on the damper. For now, I just know where to set it to make it right.

That's the first thing that I would recommend....loosen the distributor clamp, ignore the timing marks, and try advancing and retarding the timing to find the point where the engine sounds happy. It should run smoothly at idle, and jump up smoothly when you blip the throttle. Then road test it, and experiment with moving the timing a few degrees back and forth to find where it will accelerate smoothly without pinging going up a hill. Once you've found that, use the timing light to see where that point on the damper is in relation to the original mark.

 

thanks for the replies guys! The truck is sitting at a storage so I will try and get to it tomorrow. The vacuum advance is brand new but I will check to be sure it holds vacuum. I lined up the rotor and made a mark when I reinstalled the new distributor to be sure I dropped it in the same spot. I had played with the distributor for at least an hour. I have given it all the way up to around 60* of advance and eventually all the way to 60* of retard. And played with it all in between. It just doesn't run correct. I was told it could be stretched rocker studs or a bad cam. Is this true? Thanks for all the help guys. Why is it running so lean? Thanks everyone!

 

Hmmm......Okay, it sounds like you've already been through the basic stuff.

I wonder if you're having a valve lifter problem? You said that the cam has been replaced with a mild street cam. What about the lifters? Are they the stock hydraulic lifters, or aftermarket replacements? I'm not that familiar with the 351W, is there an adjustment at the rocker arms? I'm thinking that you may have a situation where, as the engine revs up, some of the valves start floating (that is, not closing completely). This could be caused by rocker arms being adjusted down too tight (if they're adjustable), or some mis-match of parts, causing the lifters to run out of travel.

I doubt the "bad cam" suggestion. When a cam goes bad, it usually just wipes out a lobe or two. Then you have a dead cylinder and a constant loud rapping in the valve train.

There's a possibility that the cam wasn't timed properly. But usually the symptoms of that are that the engine is hard to start and backfires through the carb.

Again, the symptoms of breaking up at higher rpm and the lean plugs really sounds like valve float. That's assuming it's not a basic ignition or carb problem.

 

Thanks for the reply. I am clueless as to weather or not they are adjustable, but i doubt it. I will pull the valve covers ant take a look. What will the engine do if they are stretched? I purchased the truck used so I'm clueless as to if it has the proper lifters.

 

It sounds like it would be a good idea to pull a rocker cover and have a look. Your timing chain might be off a link and you can tell by rotating the engine and watching valves. Also, check for lift, bent rods.
The hydraulic lifters are self-adjusting. You set them by getting the cylinder at tdc, firing, so that the lifters are on the heel of the cam, bottoming the adjustment and backing off to center the piston in the lifter bore.
You have a puzzling set of symptoms and the guys are scratching their heads.
Let us know what happens.

Semper Fi

 

Running lean hmm our you using the correct spark-plugs and heat range? Are there any vacuum line going to the tranny that maybe suck air to lean it out? what did the old spark plugs look like when removed? Check and make sure you have the correct coil for you ign. system. Some guy thing any coil will work just because it a coil. Like a dura-spark coil on a standard ign. system. Also check for a intake leak.. my 2cents
orich

 

Sorry took so long for a reply, im moving soon so i have been busy. The AOD transmission uses no vacuum. The old plugs are tan like they should be. The real problem must have come around once i played with everything....
Im going to check out the truck in the next few weeks, since I can have the truck at my new house 24/7 instead of at a storage unit that is far away. Clueless about the coil. Keep throwing Ideas at me guys! Thanks EVERYONE! Will write back asap.

 

So I found the problem. The new distributor I bought had a faulty vacuum advance unit. The diaphragm held air, but the little arm moved too easy and played with the timing way to much. So I installed an old vacuum advance unit and adjusted the points and it runs great now. Thanks for all the help!