I bought a 83 F250 with the 351 automatic. The engine runs great, but idles really low. I'd like some help troubleshooting it. The guy who sold it to me said he's 100% sure it's a vacuum leak.

Here's what he said. It was idling fine. Then one day he ran out of gas. He said he kept trying to start the truck but both fuel tanks were empty. After a while he put in a couple gallons of gas and got it started. Ever since then it's been idling really low.

The idle needs to be fixed because it shakes the truck pretty bad.

Any suggestions on how I should fix this?

Thanks!

80skeys

 

Vacuum leaks, unless really bad, will typically raise the idle. Listen for any hissing around the carb and check vacuum lines for cracks, particularly larger vacuum lines such as the PCV and brake booster. If all looks good, just raise the curb idle screw on the throttle lever and adjust the idle mixture screws on either side for highest RPM (out-rich in-lean). Generally go out until it smooths out at highest RPM then go in until it just stumbles or roughens up, then out 1/4 turn. Or if you have a vacuum gauge adjust for highest vacuum. Also would be a good idea to set the initial timing while you're at it, generally around 10* is fine.

 

I raised the curb idle screw and adjusted the idle mixture jets. No difference.

The truck idles fine when in "Park" and "Neutral."

It idles too low when put in "Drive" or "Reverse."

 

Slush boxes should have idle adjusted in gear (parking brake set, wheels chocked). Generally 550-650 RPM should be fine for a stock engine.

Also, get a can of carb cleaner and spray out the air bleeds and venturis real well.

 

On a carbed engine a vac leak WILL cause a low idle unless your idle mixture was rich to begin with.

 

Incorrect, small vacuum leak will raise RPM, large will result in rough idle and lower RPM.

 

This is what I was taught; a vacuum leak on a properly-tuned carb will
lean out the mixture and lower the idle.

 

Me too.

-Enjoy
fh : )_~