This is my first time posting. I have a 1978 F-100 stepside, 300 inline 6 automatic, good shape I bought it new. I have been having trouble keeping it running, but not all the time, It will run for about ten minutes then it will start running rough wanting to stall I have to put it in neutral and feather it and get it to a safe place in a hurry before it dies. When it dies I can hardly get it started sometimes I have to keep the gas pedel to the floor to start, I have to wait about ten minutes, and feather the gas to start. Most of the time its better but not all the time. Put a new stock sngl barrel carb. on last year,plugs, wires,fuel filter,dis. cap an tune -up. I also put a new gas tank on this year, replaced vacuume lines under the hood. Also this year I replaced the ignition module.If anyone can point me in the right direction it would be great. Is there any advantage to putting on a electric fuel pump? Whew. Thanks

 

I don't think I'd stick an electric fuel pump into it, at least not until I found out what's causing the problem. That's just complicate the analysis problem by adding another possiblity. Even then, I don't see any advantage if your current pump is keeping the carb bowl full of fuel.

One thing you didn't mention that you've replaced is the ignition coil itself. When the engine dies and won't start, do you have a GOOD spark? Pull of a plug wire, stick a Phillips head screwdriver into the cable end (one that has a good plastic handle on it!), then hold the shank of the screwdriver about 1/2" to 3/4" away from the top of the plug or a ground point like a bolt head. Have a friend crank the engine over, and watch for a blueish/white spark to jump to the plug top or ground, along with a nice "snap" sound. Hold he screwdriver by the very tip of the plastic handle, away from the shank, unless you want you're eyes to light up. If there's no spark, then the coil is shorting out when it warms up. A new coil should be less than $20.00 at any auto store.

You say you did a tune-up. Did that include changing the condenser that's in (usually) or very near the distributor (sometimes located outside the housing) and connected to the moveable side of the points? I've seen them short out when they get hot after running a while, and of course that kills your spark. When they're cold, they'll work just fine ... just like a bad coil.

Please post back and tell us what you find out.

 

Ok, makes sence. I replased the coil about 3 years ago, but that does't mean it isn't gone bad. I can't remeber if the condenser got changed.Is there a condenser on a pointless system? ( Electronic Eg.) I will post back. Thanks alot for your intel and your time. Don

 

Typically there is NOT condenser on a pointless system. At least, not one that you can change. When I read you were asking about a 1978, I made a bad assumption that that vintage would have points. I tend to associate EFI and fuel injection with anything 1987 or later, and anything previous to 1987 with carbs and a points-type ignition. I tend to forget about carbed electronic ignition units. I apologize for this error on my part.

Your local auto store may be equipped to test it your coil for you. However, if it is a temperature-dependent type of failure then the testing probably wouldn't catch it. The coil really needs to be tested at or above normal operating temperature and under full load. Many stores just run a continuity test on the windings, and then check for a short between the primary and the secondary windings. If nothing shows up, the coil is allegedly good. However, thermal expansion or insulation breakdown can cause an intermittent-type problem to rear its ugly head. These can be terribly frustrating to try to analyze.

I hope you find the cause of the problem. Please post back with the results.

 

I had an puralater electric pump on chevy it caused more problems becuase it put to much pressure on carb.

 

Make sure the ignition module on the firewall is grounded correctly. I had this problem on my 78 years ago.

 

Well i had the same problem with my 1986 F150 I know exactly what is wrong with your truck and would bet on it.

Change the battary cables and it will be fixed!!!!!!

It's hard to beleave but if you have old *** cables when they heat up they get more restince and then you get the problem you discriped.

Hope it works it did for me and it's cheep. I did it for like 10 bucks at autozone.

 

I am with phrog - I would bet it's the negative cable or at least the connection.

Negative cables corrode on the inside and don't always show a problem.

I would change them both - also good ground on the duraspark ignition box is also a must. Make sure the mounting bolts holding the module are clean - it won't hurt to clean the bolts & threads good and use a little dielectric grease.

Possibly the coil - you should be able to have it checked - when it's hot check the primary resistance it should be between 0.5 and 1.5 Ohms.