Okay, I'm using the stock Motorcraft 4350 on my 460. When the truck is warmed up and I'm going down the road, she accelerates nice, I can ease into the gas and she's just as smooth as she can be. Secondaries are a little iffy. This carb has vacuum secondaries. It engages for like a second or so, which seems a little odd for me. But that's not a big deal in the grand scheme of things, since I don't drag race the truck. However, when the choke opens after warmup, she's temperamental until the truck has been driven for a few minutes. I don't know if the choke is opening too soon, before it is truly warmed up, but it just cuts off when I first try to pull off. From the research I've done on this carb, the only adjustment at idle is the idle mixture screws, which, using a vacuum gauge, I've got dialed in as best as I can. I took a ride to my dad's house after work today (it's actually his truck) and it did great on the interstate. I had it up to 80 for about a mile and she just purred down the road. When I went to leave, I started it for him and we tinkered with the idle screw again because it was idling low. It was burning so rich, there couldn't have been a bug alive for 100 feet. He said he recognized it too. However, there is no smoke coming from the exhaust, even when you rev it up. The long term solution is to replace the carburetor with a new one, I just need to put some money aside over the next couple of months.

I also did a short MPG test today. I say short because it was only 60 miles. But with a wonky fuel gauge and having ran out of gas last week, I just decided to calculate it today. It ended up being 6.7 MPG. I've always said it was about the smiles per gallon, but good Lord, that's pretty rough. Now, I did flog a few times, don't get me wrong. It's hard to drive this 460 with a light foot. I'm sure a lot of it is because the carburetor, but I don't know what else I can adjust on it. I feel like if I push my luck, I'll just make the carb worse and at least it's drive-able now. I won't even ask about the Holley vs. Edelbrock debate, I've read enough threads on that for a lifetime.

 

You should definitely get more like 8mpg..............................

You are working with both the idle speed screw and the idle mixture screws to max out the vacuum... yes? What vacuum did you achieve?

 

Depending on how high the idle is, I get anywhere from 15 to 17 on the vacuum. It's been a real battle to find the sweet spot. If I set the idle screw too high, the truck wants to pull when I'm at a stop. Too low and she will stumble and cut off once it's put into drive. The only thing I told my dad I was going to do is try another spacer. The original phenolic spacer broke on him years ago and he had installed a Mr. Gasket spacer, but it was one of those types that was just a giant square opening. I have the spreadbore carb, so when I was getting this back on the road, I decided to get a Ford spacer with the EGR valve attached. It came off a 77 Lincoln and has the same spread bore pattern. This truck didn't originally come with an EGR valve. I have the valve capped off, but I'm wondering if I'm losing vacuum with this spacer? If I can locate the gaskets for this Mr. Gasket spacer, I may try it and see if there's a difference in how rich it is running. Would a vacuum leak cause the try to run rich?

 

Heck yes vacuum leaks will make it not want to run well at all. You'd have to turn it super rich just to stay running. Probably a good thing to look at first. And if is an EGR problem no amount of fuel will help since the air coming in has no oxygen left in it

 

Idle mixture is just that, though it needs to be set right and misadjustment can cause flat spots on acceleration.

There are other carb circuits to include transition, power, and cruise. The latter is where fuel efficiency (at least on the highway) can be improved through correct jetting.

Jetting is often poorly understood and used to compensate for problems elsewhere in adjustment. That's probably what's going on here. Get in the shop manual and get things back to baseline.

 

X2 on vac leaks. Does your state smog test? You can increase that mileage considerably with an aftermarket non-egr type intake manifold and carburetor. Just getting rid of the smog garbage increased mine from 10 mpg to 15 mpg with a 351M. The 400 I built is getting 18 mpg. A friend has a 460 in his '79 and says he gets 14 mpg after we installed Edelbrock Performer intake and 650 cfm carburetor. Getting back to vacuum leaks, if you've gotta pull your intake manifold to regasket and seal it, why not get a better intake and carb?

 

After looking at the documentation for this carb, the only other thing that I can think to adjust besides the actual jets are the metering rods, that go into the jets. Setting these to a leaner adjustment may help with my issue. I pulled the carb off of the truck tonight and I'll see how I can adjust it tomorrow. I wanted to look at the documentation one more time to make sure I knew what I was doing. I have found a site that sells replacement jets, but I don't have a clue what jets I currently have or what I would need to move to, as I'm completely new to carburetor adjustments like this. I will see if there is a number stamped on the jets when I remove the metering rod assembly.

 

No, no smog test in Virginia. I definitely plan on replacing the manifold and carb at some point, but not in the near future. I wasn't going to pull the intake just yet, but I would like to take this spacer with the EGR attached and just replace it with an aftermarket spacer. My gut feeling is that the EGR plate is causing an issue. Looking through the Summit catalog the other night, they make spacers similar to my bore (Quadrajet) that I might be able to try.

 

You can test the EGR valve to make sure it's closed fully.

 

By just a visual or do you have to pull a vacuum on it or something?

 

im having a maybe similar problem with my motocraft on a 78' 351W. itll start and idle great, throttle in neutral is responsive. but under load once hot itll take a dive and evenshut off. starts right back up, then as soon as I stomp it, it dies.

any tips on checking the vacuum system? ive already done the usual, cleaning, new lines, fuel filter, etc.....

if i spray the body of the carb with brake kleen it bogs down, so i could just have a intake leak killing it once warm. idk.

 

Definitely sounds like it's leaking. As long as you're sure you're not accidentally getting some of the spray into the throat of the carb, then it's getting sucked in somewhere. And don't worry too much about whether the idle increases, or decreases. Just the fact that it's changing means that some of the spray is getting sucked into the engine.
You have to pinpoint exactly where it's getting in.

Paul

 

yea its a air leak. not coming through the intake. ill replace the base gasket and see what happens. any body every use a carb spacer on these?

 

Just put a 1/2" wood Edelbrock spacer on mine to alleviate some heat issues i get with my FE... More importantly, when you have your carb off, take a good look at the thread holes in your intake that your carb bolts down to.... If this is an Aluminum intake you may have a hole that is junk and needs to be repaired. If this is true you may have a vacuum leak under load from the base plate of the carb as I did... I recently Heli-Coiled all the bolt down/stud holes on my intake...

 

Well, not sure about the 460 w/4bbl, but pretty much all of us with other engines have spacers from the factory. All the 2bbl carbs I've ever worked on had spacers, and many of the 4bbl carbs too.
They were factory versions either with or without EGR, but usually were for vacuum ports and heated air transfer to the base of the carb.

Do you mean that type of spacer, or one of the aftermarket performance spacers? If the latter, I've only installed those on later model trucks and cars. I have a square-bore version here somewhere, but that's about the time I decided to go EFI, so it's going to be going on under a TBI on a 302 instead.

If your carb was originally set up with a spacer of some kind and it's not there now, this may be a source of leakage. In some cases massive leakage.

Paul