I am unable to get the 400 engine running in my 79 F250. It turns over just fine, but the best I get is a few backfires. I pulled a few plugs and I am getting spark.

It was running fine, and quit on my while driving down the highway at a steady speed, ~50mph.

The carb was rebuilt about 6 years ago. The rotor turns with the engine.

Anyway, based on what I read here, I thought the timing chain might have been worn/stretched, so I pulled the cover off, and it looks nice and tight. Both crank and cam index marks line up correctly, so it does not appear that the chain slipped a tooth.

I am getting ready to put it all back together. I could use some suggestions on diagnosing what may be wrong. Much appreciated.

 

Fuel, air, spark.

Are you sure it is getting fuel? Maybe something got sucked up and has clogged up a line or the carb? Maybe the float has gone bad?

Have you put a vacuum gauge on it?

You said you have spark, but is it hot enough? The D2 could be failing or the coil. Is the distributor rotor pointing to the correct cylinder?

 

possibly the roll pin sheared on the distributor drive gear which changes the ignition timing...but as caravaggio mentioned, your rotor wouldn't be pointing at the number tower on the distributor with the engine at TDC...

 

Please forgive my ignorance, what is the D2?

I have tried swapping the electronic ignition module, since I have a spare. It didn’t have any positive effect.

 

Chain doesn't look bad.

Try giving it a little shot of gas dumped in the carb and see if it fires for a second. always rule out fuel and spark before you dig deeper.

 

I am guessing he meant DSII = Dura Spark 2
So you did 1 of the first things of swapping out the ICM (box).
Now that will only send the signal to the coil to spark when the PIP (dist. pick up coil) sends the signal to the box.

They say to use a ohm meter to check the pick up coil but I don't know just what the readings should be.
I am sure someone on here can tell you.

I don't think its a fuel issue, not if it just shut down going down the road (50 MPH).
If out of gas it don't just shut down.
But you can try a little gas down the carb and see if that will get it to start.

I say its a spark issue
Dave ----

 

Sorry you had to go to the trouble to dig all the way into the front of these things! All those pulleys and belts are not fun to deal with.

The ohm reading between the purple and orange striped wires coming out of the distributor should read between 400 and 700 ohms. If it's outside of that range, or frankly if it's getting near the limits, I'd replace it.
The roll pin can shear and the rotor still turn with the starter, but not accurately or consistently. Usually it does not turn, because the force it takes to turn the oil pump is enough to make the gear move on the shaft so the shaft does not turn any longer. But there have been plenty of instances where the rotor at first appeared to be turning, but was no longer in synch with the engine.
I don't know, but can "assume" that grabbing the rotor by hand and trying to turn the shaft would allow the sheared pin to show. Maybe someone that's had the problem can shed some light.

Definitely try the gas trick, just to rule it out. Obviously because you are getting at least some backfiring you at least have some fuel, at least some of the time. So you're not wrong to suspect a timing slip. But a controlled amount of fuel into the carb will prove that out one way or the other.
Quality of the spark for sure. But timing wise it's easy to verify with a timing light. You can use one even while the engine is cranking on the starter. Helps to have two people, but one can do it with a remote starter switch connected to the starter relay.
Crank the engine and if the timing light shows that the marks are still lining up properly, move on to fuel.
Or if it shows that while the light flashes, but shows that you have erratic spark, keep digging into things like the coil (you can measure ohms on that too, but I forget what it is) or the problematic area surrounding your horseshoe connector on the coil where the Red w/green wire, OR the Green w/yellow wire might be compromised.

Also, do you have a tachometer? If so, temporarily disconnect it to see if things improve. A failing tach can kill the spark.
If you also still have the original radio noise suppressor (condenser/capacitor looking thingy) connected to the coil, temporarily disconnect it too. I failing one can kill spark as well.

Paul

 

Thank you. All of these suggestions are great. I am confident you all have provided me a path to diagnose and fix this issue.

while I had everything clear of the front of the engine, I decided to also replace the negative battery cable that was bolted to the block. Of course, that 3/8” bolt was frozen and the bolt head snapped off. Now my biggest problem is dealing with that. I tried drilling, but just snapped an easy-out bit flush with what remains of the bolt. Good times.

once I can resolve this bolt issue and get everything back together, I’ll report back on my progress resolving this timing issue.

 

It's always something!
The good news is that if you're buying a new cable anyway (and that was a great "while I'm here" addition by the way) you could simply get a longer one and run it to one of the starter motor bolts. That's what I did on my 400, but on my 302's they all have an unused threaded hole just ahead of the starter.
For those I keep some short studs that I thread into the block and then run the cable to with a nut holding it in place.

Did you also up-size the cable while you were at it? My go-to cable size any more is 2ga. While 6ga was factory, and 4ga is great, 2ga is even better. Because, well, it's bigger!
My mantra is "overkill is just enough" when it comes to battery cables.

Keeping the cable connections (or any connections for that matter) away from the messy gunky and rust-prone area in front of the engine is probably a good thing anyway. Lots of stuck bolts to deal with in that area. Not to mention the common frozen-in-place distributors too.

Good luck.

Paul

 

So I finally got everything back together and spent a day or two trying to free up the distributor, as it had frozen in place. It moves freely now. The truck turns over well, but the timing issue remains. I can rotate the dissy enough to maintain an extremely rough idle, but fuel is still spitting back up through the carb. I removed a valve cover to check for a loose rocker bolt and to check the rods. Upon startup, it is obvious that the cam is broken. The rockers for #1 cylinder are moving as they should, but no movement on the valves of the remaining 3 cylinders on that side. I have not removed the valve cover on the driver’s side.

This all makes sense now.

I have a pretty good understanding of what is involved with replacing a cam and lifters. My question is how to remove the broken part of the camshaft? Is that even possible? I might save this for a winter project, or possibly next spring. It is unlikely that I would swap the engine, but I will try to get the cam parts out if that seems possible. Opinions?

 

Well hell's bells and bummers in the summers!
Yes, very possible to get it out. But you might still get lucky (as if!) and it's just some wiped out lobes and/or lifters causing the problem. If that's the case then the cam will still come out in one piece. But that's assuming you can get the lifters out first.

Even if it is in 2 or more pieces, you can still get to it all. Just a matter of where it broke and why, and how much else was damaged, dictating how far you have to tear the engine down to get to it.
Easy enough to find out, by getting back to where you were plus removing the intake to get the lifters out of their bores. Then removing the cam gear and whatever part of the cam comes out with it.
Then, whatever is left might still be able to be finessed forward with screwdrivers and whatever you have to work with, pushing and pulling it forward a little bit at a time until you can grab it through the front hole.

And even a strong magnet will pull it forward, as long as the part you're pulling on is long enough to be supported by at least two bearings at a time and not fall down out of the bore.
But only digging into it will tell the tale.

Sorry to hear it was such a dire problem. Not unheard of in any engine, but not a very common thing to break in any engine either.
Good luck.

Paul

 

Great advice here. I will probably give it a shot. I’ll start by removing the other valve cover and see what I have there.

Any advice on replacement camshaft selection? I’ll be looking for something stock vs performance.

I’ll stop back in for an update when I know what I have, but it may be a while.

this’ll be an adventure for sure, and an opportunity to learn a lot.

 

Cams very very rarely just break. often if they do it's because a broken rod came up and whacked it. which is also rare.

You have a mess no matter what caused it. make it easy on yourself and pull the engine. tear it down, see what you have and go from there.

 

This will make getting the lifters easier to get out.
k-d 2079 | eBay