I have an '86 F150 with the in-line 6 cylinder (4.9) and factory air.

It ran at high idle, backfired and ran-on after shut-down for almost a year until I finally figured out that the carburator throttle-body had separated and was allowing air in, so I disassembled the carb, tightened it back up and adjusted the idle to run at @ 750 RPM. It ran beautiful for about a week, and then all at once it started running very,rough. Low power, I have to floor the thing to get it to go and/or start. It smells of raw gas at the exhaust (very strong). I pulled the plugs and they were all completely (and evenly) carbon fouled. I replaced the plugs, wires, distributor cap, rotor and air filter. The running improved slightly for a while, but not by much. It's been 2 days and I think the new plugs are carbon fouled again already.

My Haynes Manual says that carbon fouled spark plugs are either caused by too rich fuel mixture or weak spark.

I found a valve on the left side of the carburator which I assumed adjusts the fuel/air and I closed it completely. This had absolutely no effect on the problem at all. Also the when I removed the distributor rotor, the end where it passes around was blacked and carboned, but it was at least 2 years old and so I don't know if this is normal.

I replaced the Voltage Control Module about 3 years ago, and I don't know if this could be the problem. Also, there is a small metal cylinder attached to the side of the engine and is very exposed and looks very old. I think this is the condensor.

So my suspects are:

Bad Voltage Control Module
Bad Condensor
Bad/Broken Fuel/Air mixture Valve (could this happen)
Something inside the carburator closing off the airway (I had been running without an air filter for a while....)
Timing Belt slipped (if the spark is firing too late, wouldn't that cause this ?)

Or something else.

Any help would be greatly appreciated. It's Sunday and I have a job tomorrow and would like to get this thing fixed sometime today.

Thanks in advance,

Johnny

 

I'm not the greatest mechanic but It sounds to me like you might need to give your pickup a good tune up, and a good carb cleaning. I would get some carb cleaner and start there and try to find any little holes for air flow that might be plugged since you ran it without an air cleaner for awhile. I like to use split fire plugs or even platnuim plugs since they have a hotter spark and won't foul as fast. It could also be your air flow sensor. With all the new tech sometimes it's a proccess of elimination. If you know someone that has a tester that hooks up to your computer it might be your best bet to start with, sometimes part stores have one they will hook up for you and try to diagnose your problem...Good luck.

 

I spent the day messing with it. Discovered the carb has something called a fuel/air feedback solenoid. Assume this controls the fuel/air mixture and at this point it is my prime suspect. I have voltage (12 VDC) on both blades of the connector going into it, and after moving it around this morning it started running right for a while, and then it went bad again, so I am thinking it is an intermittant problem that is becoming permanent.

The only other theory I have is that the solenoid is good, but whatever sensor that feeds it information (voltage) is bad, but I have no idea how it works.

What determines the voltage to the fuel/air solenoid ? Is there a sensor somewhere ? How does it know what & how to adjust it ?

 

I am not very familiar with this truck although I would think the oxyogen sensor would feed the ecm then the ecm would control the air fuel mixture.I would trace the wires and make sure there are no shorts and then i would wiggle the wires again and start the truck . if it is running rough tap on the selinoid and if it smoothes out replace the selinoid. you may wany to concider a rebuilt carb. As far as you carbon fouled distributor cap that is normal. The pickup on the distributors are known to go bad alot on fords replace it they are less than $20 you may want to also look into a weak coil. Although the selinoid sounds like it is bad..