I have a 1974 F700 with a 361 5 speed and two speed rear that I just recently purchased. I seams to start and run fine when cold and when not under a load, but after about 15 miles of driving it looses all power, loaded or empty. It had a new cap, rotor, points, condensor, and wires when I bought it. I have since replaced the plugs, set the timing ( it was off ), checked the fuel filter, checked the fuel screen at the carb, checked for vaccum and mechanical advance on the distributor, and every thing seams to work fine. The carb seams to be set propperly also, I do not even have to use the choke when its cold, it starts right up, runs a little rough for the the first minute or so, but smooths out nice. This truck is no power house to begin with, but it goes from running 60mph on the flat road, to having to drop a gear and maybe getting 45-50 mph on the same flat road, no matter how much throttle you give it. Once I loose the power I obviosly push a little more on the throttle and if I let off it back fires. It idles fine even after it looses power. Does anyone have any suggestions as to what I should check next?
Thanks, Ned

 

You've done the most likely things, fuel, spark, vacuum check and seem to be used to diddling with our old engines

You have a tough problem, it may have an easy answer, but in the meantime these are suggestions to try. One is a recheck of the vacuum lines and manifold seams. Spray frosty freon on the vacuum lines to display any crack that is otherwise invisible. Use any freon except the mildly flammable R406. Our old trucks have the good rubber vacuum lines, but everything wears out and leaks are sometimes hardhard to spot.

Intake manifold vacuum leak: Without going to gauges, there was *something* to spray or dab around the manifold to detect leaks, maybe someone can chime in with what it was.

Plug wires, volt reg and coil: A little bit of money now. My '79 lost power after warming up, pulling a load, because the Good carbon core wires I'd put on wore out after 7-8 years. Carbon core wires gradually disintegrate inside as current passes through, and need to be replaced every 40?50? Kmiles. I replaced with spiral-wound wire plug and coil wires, major improvement. New less expensive carbon core wires will work too just not as long.

And twenty years ago my '79 was losing power, I replaced the volt regulator and coil after checking everything else and it ran fine then. Can't say which part it was.

 

Thanks for the suggestions, I have a new coil so I will try that next. I also had someone tell me that I could have something floating in the gas tank (rust, plastic etc.) that would be to big to be pulled to the filter but only partially block the fuel line. This would explain why it runs good after is is shut off and sits for a while, the vacuum from the fuel pump would be lost and allow the foreign object to fall back to the bottom of the fuel tank again. Anyway, I will keep working on it. I am sure that I will figure it out sooner or later.

 

While you're diggin', I'd check the governor and two speed if it's vacuum operated. I don't ever remember seeing a bad one, but you never know. The other thing to check is crank case blow by. If you've got a lot, that's bad news.

 

That's a good tip, that big 'ol F700 woulda prolly been at construction sites etc working and could have got "anything" into that tank when filling w/ gas...

It won't help if the item is too rigid and big for the fuel line, but I just blew crap outta my '79 gas tanks by unhooking the fuel lines and sticking a airhose in the neck, wrapped in a rag to sortof seal it. 110psi blew out a lot of rust flakes and unidentifiable stuff, it seems to have worked pretty good 300 miles later. Rinsed out the gas filter on the carb again after driving a few miles...

Cheap and easy solutions first

 

Check the vent on your tank , it sounds like your pulling a vacuum in your tank.
I've hade trouble with our rollback , same symtoms - Ended up being a mud dauber nest in the fuel tank vent. Next time it does this loosen the fuel cap and check for a vacuum in the tank.

Just Some Ideas-------------Hotwrench

 

Since you've checked the ignition and fuel systems, it's time to do a hot and cold compression test.