My 1986 F150 has recently come into a new problem and its been a problem for around 2-3 months now. Now I know that idling problems can be caused by lots and lots of things but I'm trying to pinpoint it to try and find out what is wrong with it. I have the 4.9 6 cylinder engine and I just did a tune up on it yesterday and yet it still continues to idle weirdly. The problem only comes around when the engine gets to operating tempreture, and never when I start it up. Once the engine is in operating temp and it starts to idle it idles fine for around 20 seconds to a minute, then it shudders and drops from the normal 900ish RPM's to around 600 RPM's then continues to go from around 750 RPM's to 300 RPM's and continues to go up and down between those two rpm ranges before it finally dies. I'm thinking of rebuilding/replacing the carb if that does anything but I don't want to spend that money to find out that's not the problem. If you guys could help pinpoint this problem at all that would be much appreciated.

 

Check your choke, be sure its completely open when its warm.

Also look down into your carb while its running. Make sure the fuel is squirting and you dont see drops.

Then just for the heck of it pinch off the rubber fuel line with vice grips or something. Just before it dies out let it go. This blast of fuel may dislodge any dirt in your jets. Or if it starts running better it will tell you your float is sticking over flowing your bowl and flooding the carb some.