1977 302 no idle running rough.. puzzled

[Cotyadams]

I got a new wiring harness installed in my 1977 302 and it idled great in garage. Took it down the street for a test drive and noticed it wasn't shifting. hose going to modulator valve was broken. Got it replaced and tried to drive bronco to work today and it wouldn't idle without giving it a little throttle. Wound up driving it about 5 miles before it started stuttering and cutting out. I could get it to crank again after letting it sit a minute and it would run while giving it throttle for about a mile then would spit sputter and go dead. It's running like it's not getting fuel. I'm going to by pass the electric fuel tank selector switch and run straight from the auxillary to the carb when i get off and see if that helps anything. Looking for suggestions...

 

[69red]

first thing id check would be all of the vacuum hoses and make sure they are all hooked up and not cracked.

also check that any of the vacuum ports on the carb that are not used are plugged off.

 

[Aptosgrown]

Hmm, maybe it was idling great in the garage off the fuel in the lines, but could have sucked up some junk if you had it up and down on stands which finally made it to your fuel filters?

Other item that comes to mind that usually causes running rough and what not is a vacuum leak. maybe something was loose and once you started going down the road a vacuum hose/plug popped off?

I do agree though, sounds like its starving for a drink. Good luck

 

[JAFO]

If the engine ran fine before the new harness, and now does not, you kind of need to backtrack as to what changed or was affected. You sure you have good consistent spark? I'd verify spark first. Then fuel pressure. Then vacuum leaks.

 

[Cotyadams]

Everything ran fine before new harness. Everything ran fine in garage after new harness. The only thing i changed was i hooked up the electric tank selector switch which the previous owner had mounted to frame rail. They had it bypassed and were running straight from the auxillary tank. I will change fuel filter and switch back to running straight from auxillary bypassing switch and see if anything changes. Ill also check vaccum tree for leaks. Maybe i pulled a cap off when I put new line on for transmission.

 

[jckkys]

If you turn the idle mixture screws out IE give it a richer mixture, does it idle better? This would indicate a vacuum leak. If you remove fuel filter and simply blow through it easily, the filter is OK. Replacing parts is a poor diagnostic method. Test what you have first. Is the manual fuel switch gone or just circumvented? The slight inconvenience of reaching down to switch the tanks, is better than not knowing if the carb is getting fuel. It's easy to run a hose to a jar while cranking to check fuel delivery to the carb. But since the idle is the only problem, as I understand it, then fuel getting to the carb isn't a problem. The idle mixture test should tell you what direction to go next. Another consideration is the ignition module. The first Duraspark modules in '77 would die killing the engine when they got hot. after cooling down they worked again. Ford fixed the problem in a few years but many EB owners bought cheap Chicom replacements that were never dependable. The 5 miles may have been enough to over heat a bad module.

 

[suthernboy]

Everything ran fine before new harness. Everything ran fine in garage after new harness. The only thing i changed was i hooked up the electric tank selector switch which the previous owner had mounted to frame rail. They had it bypassed and were running straight from the auxillary tank. I will change fuel filter and switch back to running straight from auxillary bypassing switch and see if anything changes. Ill also check vaccum tree for leaks. Maybe i pulled a cap off when I put new line on for transmission.

Are you now running off the main tank which had been sitting dormant for some time? Or, still running off the aux, just through the valve now?

 

[Cotyadams]

Drove bronco for twenty minutes today's ms she drove fine then started sputtering and sputtering and went dead. Will fire back up but when I put if in drive it goes dead. Any ideas

 

[Fairlane514]

What ignition system do you have?

 

[Cotyadams]

Duraspark

 

[Cotyadams]

Just went back to check on it and it fired up and ran fine around parking lot. I'm clueless and need some help. I don't understand how it would idle fine in park or neutral but when I put it in drive it goes dead...

 

[1963hotrod]

Fuel screen on pick up

I had a problem like that with my 69 mach 1 , it ended up being the fuel screen on the pickup. Varnish was slowly coming free in the tank and plugging the screen that covers the pick up. After sitting for a while it would fall off and it would run fine again for a while. I also blew air back through the fuel line and that cleared it off. I had to take the tank out and have it cleaned, then it was perfect.

 

[Cotyadams]

The kicker here is that when it starts running like crap in drive I can put it in neutral and it will idle fine and rev up good. Put it back in drive and it bogs down and wants to go dead.

 

[jckkys]

The needs of an engine under load, ie. drive, are different from a no load, ie. park or neutral, engine. The spark has to be stronger to over come the increased resistance of a mixture compressed by a load on the engine. The original coils are oil filled and the oil can leak out. The coil then over heats after running a short time. A cool down restores the spark and things work again for a while. Your new harness may have put excessive voltage to the coil too. I'd try hooking the engine up to a scope when things go south. Any shop can do this and give you a diagnosis.

 

[Rustytruck]

Check the mag pick-up in the distributor.

http://classicbroncos.com/forums/showthread.php?t=120627