Inline 250 with Holley 390 cfm high idle problem

[jcramsey]

Just started to notice my old truck running a little fast at idle after warming up. Got a stock inline 250 with Offenhauser intake and a Holley 390 cfm and upgraded to HEI distributor. Ive never done anything to the carb, just bought it off a guy, slapped it on and it's been running like a champ. Don't notice any high idle on startup, but once its been running when I come to a stop, it is noticeably high. Was out last night and pulled into Sonic and it started to stumble a lot and could definitely smell the fuel. Got back out on the road and it seemed to be running fine again, but still had a higher idle when stopped.

Where should I start?

Thanks!

[TJ's Chevy]

What intake/exhaust manifold gasket did you use between the head and the manifold flanges? If you used a metal material gasket then you have an intake leak which will cause high idle, I so; I would change to a hard material gasket, one with rubber so your manifolds have something to seal on. My Edelbrock is dumping gas out of the front for some reason....but I don't have a stumbling issue, Your idle will be higher do to the amount of fuel. When I have come to a stop, my idle sometimes sits at 1,000 for a short time and then quickly drops down. It doesn't do it any more, because I am running a rubber material gasket in between the head and manifold flanges. Also check you carb mounting plate gasket, if those are to thin then you can smash them from over tightening the bolts. I would double stack them. Also re-adjust your idle mixture screw/screws. If you have two, one will control the left side of the carb and one the right. Begin by turning the screw in until the engine struggles...then screw out slowly until the engine frees up. Do the same the the other side. THAT'S if you have two idle mixture mixture screws. If you only have one, you can do the same procedure. Also check your vacuum lines....re-tighten those. Especially your advance line, and pvc vacuum line. Might be the carb. And are you running single or dual exhaust? If you are running single exhaust then you are stuffing lots of fuel in and the motor is probably having issues with a lot of back pressure. I would convert to dual exhaust....Remember what goes in must come out. Stuffing 390 cfm worth of fuel, and shoving all that out one small 2 1/4 exhaust pipe doesn't go to well.....hope this helps.

[NEWFISHER]

Mine started doing the same thing. I adjusted the choke pot and set the choke idle arm ( at the back ) 1/2 turn less and helped a little. It was less frequent and subtle , but still was doing it until I removed the air cleaner and emptied a can of fuel injection/carbon cleaner into it with the spray attachment. It has to be running and I recommend changing the oil afterwards as it most likely will wash the rings and end up diluting the oil. Worked well and now the throttle shaft and plate don't stick causing my high idle issue at stops.

[jcramsey]

Aweseome....great info guys. Thanks a ton!