I have a 79 f250 with a 460 and Holley carb. About 2 months ago I turned the key and got nothing, lights wouldn't turn on etc. I tried the key a few more times and it started right up and I chalked it up to bad luck. A couple weeks later it started to miss a little bit. So I put in a new ignition switch.

About a week later it was still missing and started to backfire and then completely shut off on me. After I coasted to a stop it would crank but wouldn't start. So I went and bought a new ignition module and as soon as I put it in it fired right up. However, when I got home I tried plugging the old ignition module in and it also started up so I guess that wasn't the issue. It continued to miss, especially after being on the throttle. It seems worst when I'm giving it a lot of gas in 3rd and I shift to 4th. Within 10 secs it usually starts to miss.

I also changed the reluctor and the pickup in the distributor but it's still missing. It doesnt have to be super warm to do this, it can happen within a couple minutes of it starting up and driving. I really thought this was an electrical/ignition issue of some kind because of the initial problem of it not turning over or having power at all but maybe the issues were unrelated. Is there anything I'm missing here? Thanks for the help in advance

 

Is it "missing" or is it more of an undetermined stutter that could be fuel or ignition related? By "backfire", do you mean out the carb?

I know more about the 400 and 390 engines, but I think there are a few things that are somewhat universal. I would double check that your breather system is working properly through the engine. Make sure you are getting vacuum at the PCV valve and that the grommet is still sealed tight to both the valve cover and the PCV valve itself. Double check that the breather is letting air in as well. This is the primary culprit for backfire through the carb in my experience.

Once you know the PCV is working, I'd then be sure you don't have a vacuum leak somewhere. A weak vacuum signal at the PCV may still produce a backfire, such as when the brake booster is on the same "T" and is leaking and your normal manifold vacuum level is already on the low side, or something to that effect. It usually would be a leak near where the PCV pulls it's vacuum. I've never had one with the smog pump (my '79 with 400 had it deleted before I got it), so I'm not sure how that would affect things. If you have a smog pump, it could be a factor, but I really can't say either way (seems like it shouldn't contribute, but maybe it could).

I would then check the fuel to be sure it's not got water in it, or water collected in the fuel filter. I would also check the fuel filter itself in case it is starting to get clogged. I've had stuttering caused by bad fuel, but I can't recall having a backfire at the same time.

Since you seem to have looked the ignition over pretty well, I'll assume that you probably checked the timing. Beyond PCV and timing issues, I don't think there are many other ways to get a backfire (again assuming out the carb rather than the tail pipe).

Just to be sure you don't have a valve issue, I'd check the manifold vacuum and see if it stays steady.

As simple as it may be, I've also experienced a night and day difference once by cleaning the negative battery cable where it mounts to the engine. I replaced the cable and couldn't believe how much more alive the engine was afterwards. I did have some indication that it needed to be done though, such as the measured voltage to the battery not showing any higher than the battery level of just over 12 volts, then suddenly jumping to 14V and randomly fluctuating between. After replacing the negative cable, it is always steady and the ignition just seems to be more powerful.

Just a few thoughts on what you could check......

 

I'm not sure about he 460 but on my truck with a 300, I had times when the thing would start and when it did it would pop and fart while driving down the road and it turned out to be the choke sticking and doing weird things. I cleaned it up really well and made sure it moved freely set it properly and all my problems went away. I might be right out to lunch here but it's another thing to check.

 

Thanks for the replys. I think it's missing and not stuttering since usually its really quick and it goes right back to running great but I guess it could be stuttering a little too. As for the backfire, it sounds like its coming from the exhuast so that's what I'm assuming but is there a more definitive way to know other than by where the sound is coming from? Im starting to think it might be carb related since it has mechanical secondaries. I'm not sure if when the secondaries open up that's causing an issue but I don't know much about carbs.

I'll take a look at the stuff mentioned though, thanks again.

 

I'll defer to those guys above but in the very first post read that you replaced the ignition module but, then re-installed the old one. What's in there right now? The old one?

Heat will eventually cause coils, modules and other electric gee-gaws to fail or go intermittent, though they work fine at first or when cold.

 

Agree with rechecking the module....if your new one isn't a Motorcraft, with the blue wire grommet, it will fail sooner than not. Also, old brain boxes will "go intermittent," then die for good.

 

I did reinstall the new ignition module and that's what I'm running now, I just tried to old one out of curiosity. The new one isn't motorcraft, it's what I could pick up at the time to get me home but I'll look into a motorcraft unit. Are there any tests I can do to try to isolate the problem?

 

Sure. You could check the magnetic pick-up coil (ohm it out) as well as the wires connecting it (for brittleness and a good connection) in the distributor.....

Check coil wires and connections, ohm the coil out (for grins and chuckles).

Make sure all yer grounds are clean and snug same with the battery cables, solenoid and wires, etc.

 

Just wanted to update, even though its been a while. It ended up being the coil. After I replaced the coil it it ran fine for a while, but then the coil went bad again so I'm not sure why it keeps going bad.

 

You probably know this already, some coils require an external resistor, if there isn't enough voltage drop the high current through the ignition system may burn up the coil.

 

I knew there were different types of coils, but I replaced mine with the same model as was in the truck when I bought it. Is there a way I could check this? What would I check the voltage drop across?

Thanks

 

Usually the coil will have "Use with external resistor" printed right on it. Measure the voltage at the coil with the key in the run position.