Hello, all. Well, this might be a bit drawn out, as I've been combating this for ages. So, the truck is a 1985F150 with the 300-I6, feedback Carter YFA carb. It sat in storage since 1992. I got it out and went through pretty much everything. All new vac lines, fuel lines, some touch up to the wiring.. long story short it's all good. It ran perfectly from October until about mid-December, I live in Maine and it finally got 'very' cold. Even in the -25* days, she'd fire right up every time. I rarely take it over 55 or 60, but one day, just for some reason, i tried putting the throttle to the floor, and it petered right out and started to buck, let up and it kept running fine. It seems lean to me. I figured it was 20 below zero, of course it wont be happy. I chalked it off to a extreme cold issue. Well, for the last month it's been unusually hot, 40-60*. It's still doing the same exact thing now. Since, I've checked everything for vac leaks, cleaned the feedback solenoid, tested all the sensors, and when it finally came push to shove, I even tried simply turning the metering needle screw in more and more.. and no dice, really. Anything under 3/4 throttle, it's fine. Anything more, and it slowly starts to lean out, till WOT which.. sometimes it barely keeps going, others, it'll still die right out and just kind of cough and shudder. New tank, lines, fuel pump, coil, all typical tune up stuff.. timing is bang on. On that note, when I start to mess with it, if it's still cold and the choke is on, it won't do it.. anyone have ideas? It was running perfectly before winter came. And it still does.. so long as you stay out of it. Anyone have ideas?

 

probley something with the accelerator pump or something with gas flow in the carb. it can get too much and that will cause hesitation and temporary flooding and too little gas will cause some hesitation. i would run a vacuum gauge to the cab and drive it then watching the gauge that's what i did last time i had problems. nice looking truck though, like the color

 

Beautiful truck, by the way! The fact that this bucking and eratic running does not occur with the choke still on, it does sound like a lean condition. Is the idle steady? How did you check for vacuum leaks?

 

THIS!


I also second checking the accelerator pump. If it's sat since 92, all of the diaphragms and gaskets inside are going on 25 years old. Plus, ethanol wasn't as big of an issue back then, and new ethanol gas can eat that stuff up.

A good carb rebuild certainly couldn't hurt.


Otherwise, I'm surprised that the vacuum leak check came back positive. Time is brutal on rubber and gaskets, and the gaskets and vacuum lines getting old and brittle can cause lots of issues.

 

First of all, thanks for the compliments! As for checking for vac leaks, the old ether/propane trick. And on that note, I replaced all the vacuum lines with rubber, because as expected they were shot. Only thing inop in the vac system is the hot air damper on the snorkel, it's busted and nobody seems to have one. As for the accelerator pump, looking down the bore it's a steady stream of fuel all the way to the end of travel. I've had the carb apart to check the screw setting, as I mentioned.
The idle is dead steady. Like I say, anything under 3/4 throttle and it runs like a clock. I suppose I'll give a shot to rebuilding the carb. It all seems to be working well.. in theory.. I'll give it a shot though. I have it in my mind it's one of the sensors honestly.

 

IIRC you can pull codes using a test light. If you do a search in the 80-86 truck area there is information on it.
Also over there are guys that know the feed back system pretty good and could be of help.

First thing I would try and I am sure they would say is see if you can wire open that hot air valve to pull warm air into the filter & carb.
With that cold temp you could be icing the carb. With ice build up inside the carb it could also run lean.

Have you tried a different brand of gas for a few tank fulls? Gas changes with seasons and could also be a cause.
Dave - - - -

 

Funny you would say that. Yes, I do have the hot air plate wired in different positions. I've tried all different kinds of gas in it with no real differences in the last 3k miles I've run it. I had thought it was continual ice issues.. but now it's suddenly 65* (leave it to Maine.. snow on Sunday and 65 on Wednesday..) and it still did it then. However your point is true.. didn't think about winter grade gas. Maybe it'll remedy itself, that'd be nice.. I'll give it a shot over on that forum!

 

Maybe the distributor cap is faulty.

 

Could it be a faulty EGR valve.

 

When you pull the carb, check the inlet needle valve and seat for trash.
I assume you have changed the fuel filters and that didn't help.
After it sat for so long the fuel tank may have some rust in it.
The carb filter will catch some of it but not all.
If you find any rust, change any filter you have and install a see through line filter back at the tank.
That way you can check to see how much rust and dirt is coming out of the tank.