I just rebuilt my engine and can't get it to fire, it isn't getting good spark. When I pull my lead wire from my coil to check for spark it is barely visible when physically touching a ground. It won't do anything at a 1/4 or 1/2 inch away from the ground. It has a new distributor (refurbished), new coil(i had it tested, and it's good), and a new ignition module that was replaced prior to the rebuild. So I would be very appreciative of any help.

Thanks,
Evan

P.S. This is a 1975 Ford F100 fe 390

 

You're not going to see a spark when you physically ground out the coil lead. The spark comes from the electrons literally jumping across from the coil wire tip to ground. There has to be air in between. 1/4 to 1/2 inch may also be too big to get the spark to jump. The secondary winding of the coil can only make so much voltage, and the distance the spark can jump is limited by this voltage. A typical spark plug gap is much narrower than what you're doing. This is why plug gap is so important. Plug gap is measured in thousandths of an inch, and usually gap is under 1/16th of an inch. I could be wrong about 1/4 inch being too much, but it's something to consider.

If you correct what you're doing to check for spark and still have a problem, then you're going to need a multimeter to diagnose your ignition system which we can walk you through. You may also need a test light (nothing a multimeter can't do, but they make some things easier). If you're going to diagnose electrical issues on an old pickup truck, then there is no way to avoid getting a multimeter. People on this website seem to do anything possible to avoid getting a $10 multimeter. I'm not saying that's you; I'm giving you a heads up. If you don't already have one, now is the time to get one.

 

First off, thanks for responding. What you are talking about with the air in between is what I was trying to say when I was referring to "physically touching a ground". I had someone turn over the engine while I would hold the wire up to a ground with a small gap and I got nothing. So then I actually touched it to the grounded surface and you could see a tiny bit of spark. I just got a multimeter the other day so I have been trying to use it but I don't really know what I'm looking for or where to start. So needless to say the "walking through" would be greatly appreciated.

Thanks again
Evan

 

Check out these threads, they may give you some idea of where to start:
https://www.ford-trucks.com/forums/8...-problems.html
https://www.ford-trucks.com/forums/8...oblem-out.html

 

Here's the first thing to check. Set your meter to read DC volts. Pop the hood, and put one meter probe on the TACH TEST terminal of the back side of the coil connector. There should be some metal exposed for you to get your probe tip in there. Put the other probe on engine ground - unpainted metal on the engine, just like what you'd use to check for spark. Have your friend crank the engine over from the cab, and the meter reading should jump between 12 and zero back and forth as it's turning over. Let me know what you find out.

Normally, if you had spark, I wouldn't have you run the above test since having spark eliminates the need to check whether or not the box is firing the coil. But I'm not convinced - the fact that you get spark when you have the coil wire physically touching ground is perplexing to me. The spark you see comes from the electricity jumping from one conductor to the other. If the two conductors are touching, the current just flows through. I'm thinking the spark you saw is just the zap you get when you bring a powered conductor toward ground. I'm guessing the spark only happend once, not repeatedly? Maybe I'm missing something?

 

Yeah I think you're right about the "zap" because yeah it was just a little flash that would occur when initially touched to the grounded surface.

So I checked the TACH TEST side and it would only bump the meter to about 2.2 volts. Do you think that is saying the ignition module is bad?

 

Did the engine start before it was rebuilt using the new module? or is this your first attempt at starting it with the new module?

 

I don't have enough information to determine if the module is bad. Did it flash off and on, or just stay at 2.2 volts? 2.2 volts sounds like you didn't have a good connection down to ground. Measure the battery voltage using that same ground and make sure you still get a full 12.

 

Thanks for helping here's where I'm at now. So I found a better ground and it was still going to about 2.5 volts while turning over. I just went and bought a new ignition module on Saturday and it started right up. It ran really good on Sat. I drove about 25 miles. I went to go out on a drive today and while at lower speeds (up to about 40) it's running good, when I get up to 50-55 it will sputter and lose power especially when I push on the gas. My first thought is a vacuum problem because I've only got one vac running from my carb to my distributor and it is a dual vac. I don't know where the other vac is supposed to come from. I also put a new carb on when I did the rebuild and have not tuned so I thought maybe it could be a fuel issue.

Thanks
Evan

 

Glad to hear you are making progress. As far as the high speed issue, it's unlikely that it's vacuum related because at high speeds, vacuum is very low due to the throttle plates being open quite a ways. Vacuum issues usually show up at idle, when vacuum is supposed to be at its highest. High speed driveability problems are often linked to fuel starvation. The fact that you recently replaced the carburetor supports this notion, because your float height could be off, causing an insufficient fuel level in the bowl.

 

Your vacuum advance will not be part of the equation at 50-55mph, check your float level, fuel pump, fuel filter, etc.

 

Thanks. Will do.

 

Ok so I've reset my timing and it looks to be about 10 or 11 degrees BTDC. I've changed my fuel filter on my carb. So I still have my old fuel pump and I haven't attempted the float level adjustment yet because I have never done it before and thought I would talk to you guys first. I got my new exhaust done today and on the way home it was still cutting out. It would do fine until about 40 45 then it would cut out, if I really creeped up on 50 it would do all right but then eventually cut out. I would pull over for a min or two and it would idle fine and take off again no prob. I could really get on it and it would be fine until I got to the 40 45 range. If it would die when it cut out(which usually only happened when I was trying to push the gas harder) it would start right up and idle great. I guess this is a lot of info but I'm trying to paint a good picture for you guys. Montana Highboy suggested checking a few fuel related parts including the fuel pump, so I was just wondering since it is like a $40 part, should I start there? Then move on to the float level.

Thanks
Evan
P.S. I keep meaning to mention that the fuel was like two years old. I did not add a stabilizer until right before I fired it up. I know it isn't intended to work like that but I thought maybe it would help a little. Let me know if my line of thought was way off.

 

It still sounds like a fuel starvation issue once you reach a certain rpm level, have you checked you float height yet? set too low and the bowl won't completely fill potentially starving the engine for fuel at higher rpm's, if that checks out ok then i'd move on to checking the fuel pump.

 

Okay so it has been a while but here is where I'm at. After I posted the question about the fuel issue I drove the truck to work. This was the day after changing the fuel filter. So on the way to work it was better but still would cut out, although at a higher speed and then by the time I drove home it was accelerating with no problems. Smash cut to today which is about a week later. I went out to my truck after work to warm it up and it started like usual and was idling for about 5 min and quit. I assumed it was flooded so I let it sit and pulled the plugs to dry them off in case that was the problem. That did not help. My thought is something to do with that ignition module. I read a thread that someone else was asking about those frequently quitting and the person giving the advice said that if you are having issues with it quitting then you need to trouble shoot to figure out why it is burning out. So I'm not sure that is the issue but I do know the electrical on this truck does need to be gone through.So needless to say I would be appreciative to hear your guys' opinion.
Thanks
Evan
1975 F100 390 4x4