I put my 1984 f150 with the 351w HO back together about a year ago now, i put a Edelbrock AVS2 650 cfm on it at the time. since then i moved from ohio to montana, and now I can't seem to get my idle quite right. I can't get my idle below 800 or so rpm and even then I can't get it to idle smooth, but it clears up when i press the throttle. it was also having hot start issues over the summer, but i believe that's a vapor lock issue I'm working on resolving. I've "reset" my idle mixture screws and started from scratch repeatedly. i have a fresh set of plugs and wires as well as a fresh distributor (they're copper plugs I've seen people's issues with the platinums). I've moved my timing advanced and retarded ended up just over 12° tdc since that had the highest idle vacuum. which I've only managed to get up too 19 InHg or so. i don't have any vacuum leaks. the only vacuum lines are for the pcv, ignition advance, and brake booster. the pcv is brand new and there's no notable change disconnecting and plugging the brake booster. i have my ignition timing connected to the "timed" vacuum port and I've confirmed no vacuum at idle. at this point I'm wondering if the carb is just too big for the air density here, since i could get a good idle at 650-700 rpm back in ohio. I'd be willing to follow other diagnostic paths before i shill out for a new carb though so if anyone has suggestions I'm willing to try. would a failing ignition module do something similar?

 

What happens when you try to go below 800? It runs rougher or the idle screw is not touching the throttle arm anymore?

 

i would just take what it gives you, you dont spend that much time at a hot idle anyway. And you dont drive a 351 for its mpg.

 

Going from Ohio to Montana could have changed your elevation. Higher elevation has less O2 per cubic foot and it is often needed to reduce your carb jet size so that fuel to O2 ratios are optimized. Check with the carb manufacturer about recommended jet size for your new locale.

 

the idle screw backs off of the throttle arm, disconnecting the throttle cable doesn't change it

 

true enough

 

hadn't thought of that, thanks

 

Have you check that the choke is not holding the idle up?
There has to be something holding the throttle open so it will not touch the adjusting screw.
I know you said no vacuum leaks how did you check for any? They are the only 2 things I can think of to keep the idle high and screw from touching.
Dave ----

 

I checked by spraying every connection and along the hoses with brake cleaner, i also disconnected the lines and capped them off at the barb fittings. i put a liberal amount of silicone around the intake gaskets since i had the rear coolant crossover blow out repeatedly. i made sure the choke was fully open and I didn't see anything holding the throttle open yesterday, but i can double check tonight

 

Sorry to say, but it does sound like you still have air coming in from somewhere else. That would explain the high uncontrolled idle and the rough idle.
1st off, lets double check what the other Dave was saying. When you are messing with it at a hot idle, the choke door on top of the carb should be fully vertical. If it is, we can dismiss the choke cam as being the problem.

2nd. It will not be the jets. They are not active at idle

Have you sprayed around the base of the carb? Do you still have the EGR spacer or some remnants of it under the carb? The EGR spacers tend to burn out the gaskets and deteriorate the aluminum. This needs to be checked. I will warn you, I would not use rtv on any gasket area where the fuel is. It's ok on the ends of the intake where the water runs through, but the fuel will turn rtv into a sticky goo.

 

the choke can be a little lazy to open. i think i need to adjust the cap. but with it fully open either from heat or by hand my idle is still off. i have sprayed the base of the carb, and where the intake meets the heads, there is no remnant of the egr system, the carb sits directly on top of the intake. i worried as much about the silicone but wasn't sure and needed to get the truck running for the move. i suppose if it has been eaten up by fuel then it's probably sucking air from the valley. sounds like manifold gaskets are in my future

 

That could explain why it was decent before, and now the idle is higher. It tells you right on the tube not to use it near gasoline, but of course I had to try it. I used the ultra blue silicone on a problem I had, I was trying to re-use one of those metal pan intake gaskets. I ended up taking it off a few months later, and the blue rtv had turned to a green slimy snot and did not set up at all.

I would also look at the intake closely. Most of the Ford engines when you use the Ford intake, you can't put the carb right on the intake without using a spacer of some kind. They usually have passages that let exhaust go up under the carb into the old EGR spacer. This exhaust hole needs to be plugged off with something that can stand the heat. I would also take your new carb base gasket and lay it on the intake and inspect it carefully that it is covering all it should be. And then put it on the bottom of the carb and see if it covers everything. Not all these gaskets are the same. You have the aftermarket universal type that will fit your carb, but then you have the factory Ford type that covers their spacer, but not necessarily your aftermarket carb. Just be diligent and look it over good.

 

i have an Edelbrock performer intake and if i remember right the gasket covered it nicely.

 

Good. It is the non-EGR version?

 

I'm not sure I'm pretty sure the carburetor mounting surface just that nothing more, it does have a heat riser on the one side though. i bought it second hand so I'm not 100 percent