I have an '85 F-150 with the 300 I6. It ran beautifully until the last emissions inspection, which it failed. The garage sent the carb (Carter YFA) out for a rebuild. When I got the truck back it ran terribly. No power, hesitation, etc. I discovered that one of the wires to the feedback solenoid was broken and they hadn't re-attached the vacuum lines to the EGR or the valve under the air pump (don't know what it's called). I fixed all that. Now the truck idles and accelerates fine but when holding steady RPM for 3 or 4 seconds it starts stumbling. Almost like it's not firing on all cylinders or there's a governor kicking in. If I press a little bit on the throttle it will smooth out, accelerate and then start stumbling again.

For obvious reasons I suspect a carb problem. I discovered that if I unplug the connector that goes to the throttle position sensor the truck runs great. But I don't know what the ramifications are for leaving that unplugged. Any thoughts on where I should look next? None of the books that I have go into any detail on the carb or the wiring.

Don't suggest bringing it back to the garage. Their answer was to check the timing and then throw their hands up ("it's an old truck..."). I only went there in the first place because you have to have emissions problems "fixed" at certain garages. Grrr.

 

I think you just stumbled on the solution. When you unplug your Thottle position sensor, your problems go away. Grab a Ciltons manual, it will tell you how to test the TPS. Odds are it probably need replacement.

 

Good thought. Don't know why it didn't occur to me. I have a Chiltons and it doesn't mention, or even show, the sensor anywhere that I can find.

 

It's in mine. My book covers years 87-96, the EFI era of Ford trucks. Your book probably does all of the carburated trucks. 85 and 86 were the transition years from carb to EFI. Both carb and EFI were available these years. In my book it is listed on page 4-20. Testing it is easy. Put a volt meter across its leads. With the throttle fully closed it should measure .6 volts. Fully open should read 4.5 volts. There should be no dead spot inbetween. Basicly you are looking for a smooth voltage transition as you open and close the throttle. If there are any dead spots it is most likely (99% sure) its bad. They aren't expensive to replace. I just priced mine at Kragen's and they are less than 30 bucks.

 

Thanks. No sign of it in my book. Not even on the exploded carb diagram. I'll pull it off and see what I can make of it. They painted the carb during the overhaul and probably gummed it up.