My freind has a 00 250 explorer.Just out of the blue it won't rev,it starts & idles fine.Comp is 90psi cold engine.Have taken the carb apart & blown out all passage ways,all clear.Have disconneceted the rev limeters(single black wire under hood) carb boot no tears.This is about to drive us crazy!Any suggestions would be great.Thanks ahead of time. Oh the only way we can get it to rev is by partly putting hand over carb inlet.

 

Anything under 100psi is not good.But is sounds like your pilot jet is stopped up or the air mix is off.But yet again it could be low comp.

 

I have had the needle clip break and not let the needle raise. They wont rev when that happens. I suspect you would have caught that when you took the carb apart,though. Like ftwflh said, your pilot could be plugged up. I would say check the clutches, but if you can hold your hand over the end and it goes, it has to be the carb. 90lbs is low, but it will go with that. Gotta have a minimum of 80 to start.