hey guys I just snagged my grandpas old 73 ford F100 with a 390 in it. it hasn't run in about 7 years, but was rebuilt about 5 years ago. so I've done all the stuff I thought I should to get good fluids in it again, and crank her up.

got her runnin, but not pretty. she idles great, but at about half throttle, and starts to stumble real hard. and will die pretty suddenly. I've gone through timing, replaced the carb (found a crack in the venturi cluster on the old one), replaced all the fuel filters, replaced the condenser in the dist. replaced the coil. the few things I know to check to find out what it is. unplugging all the hoses coming of the carb and plugging, no change.

what are some things I'm missing to check, and how do I check them? thanks guys, but I'm no super mech like a lot of you, just kind of a tinkerer, so all your advise will definately help.


Ryan

 

You mentioned the condensor inside the dist, but what about the points? If those aren't in good condition and properly adjusted, the engine will not run very good, no matter what else you try.

 

yeah i had to readjust them, and they do look pretty decent, they're not all crudded up or anything, and I slid a penny back and forth between em to clean em (old codger trick), and regapped em, and after I did all that was when I finally got it cranked for the first time. sorry I did forget to mention that.

and wouldn't that keep it from running good at idle to though? cause she purrs like a kitten at idle.

 

They're cheap. Replace them, the dizzy and the cap too.

 

Sounds like you have a bad diaphram in your vacuum advance. It will idle real nice because you haven't pulled any vacuum at the distributor advance port on the carb, but when you give it some throttle your distributor advance port starts to pull vacuum for your advance and you develop a very nice vacuum leak. See if that's the problem by plugging the advance port on the carb.
Also check to see if you have a worn bushing in your dist. that may cause the dist. shaft to wobble affecting the gap on the points.
Bud in AZ

 

Except for the fact that the engine dies, it does sound like weak igntion. I've found sparkplug wires to be the number 1 culprit for ignition problems.

The more load you put on the engine, the harder it is for the spark to jump the gap, and the spark starts finding alternate routes to ground, and leaky sparkplug wires are often the problem.

I had a weak spark situation a while back. Engine would idle perfectly, but would sumble and misfire very badly under some load. The engine would almost die, but if I backed off the throttle, it would clear itself and return to a decent idle.

I suppose it could stall, but mine didn't.

To check things out, open the hood at night and look for any arcing, sparking or corona around the wires. Don't put your fingers in the fan!

On the other hand, fuel starvation can be an issue. With an older vehicle that's been sitting, you may have a lot of rust in the tank. This rust can break loose and plug up lines, and even a brand new filter can plug up quickly. Fliters are cheap, so you may want to cut your new one apart and see what's in there.

Alternately, you could do a cranking test and make sure the fuel pump is pumping out sufficient fuel.

Ford says it's fuel pump for a 390 should pump 1 pint in 24 seconds (at a 500 rpm idle, which won't last long with the fuel line disconnected). That's a minimum flow requirement. I got much more fuel when I did my test, although I use an aftermarket Carter Pump. However, I was only cranking with all leads from the coil (-) disconnected (to prevent a fire).

Not that it means that much in this case, but Ford specs the fuel pressure to be between 4.5 and 6.5 psi. If you can test it, that's great, but it may not tell you a lot compared to the flow test. Depends on how you do the pressure test. I'd say, just do the flow test and see what happens. Have a fire extinguisher handy!

(Had a friend who caught his car on fire doing a fuel pump flow test...not pretty)

 

I have seen the intake gaskets dry out and suck air after sitting for a few years. That can cause the problem you are having as well. Just a thought.

 

agree,
puttster

 

X2, but with other items to check.
Since you said this truck/engine sat for a few years, it's possible the mechanical advance is stuck. They do tend to rust in place when left sitting for a while.

The only other reasons I can think of for a great idle, but runs worse as the RPM goes up, is an improperly adjusted/stuck choke causing the mixture to get way too rich as the RPM goes up. But wouldn't restart easily after dying.
Restriction in fuel flow inside the carb could do the same thing, but in reverse. The mixture would lean out as the RPM went up. This would fit with the quick restart described.

If it was a low fuel flow issue not within the carb, it wouldn't restart easily after it died.

Then, there's the coil and/or coil supply voltage. A weak coil, or low coil voltage, could supply the current needed for idling, but as the RPM goes up, simply can't keep up with the demand. Bad plug wires can do the same thing.