I'm finally getting around to tinkering with my '78 F250. 400 engine. Finally got the radiator issue resolved, and now i'm working on another minor problem that I have noticed since I have had the truck.

On cold start and drive-away the truck performs flawlessly. Starts up with a pump of the accelerator and runs smoothly at a high idle. Tap accelerator, idles down, put in drive, accelerates great. Fast-forward to warm engine. As soon as the choke opens up the truck seems to get sluggish on the highway, hills, etc. Off idle acceleration isn't bad. Sometimes a slight stumble off idle.

Seems to me the engine is starving for fuel after the choke opens. I have rebuilt the carb. No change. New filter, etc. Float is set at factory specs-wet. The closest I can find on the setting WET is 13/32.

Truck has 22,000 miles. Runs awesome with choke partially closed when cold. When warm the sluggishness appears and is accompanied by a slight hissing under the hood when accelerating. Recent tune-up, timing set to specs, EGR disconnected-no change. I think I was the first to rebuild the carb. I tried not to change any factory settings....good and bad maybe. Everything else underhood is pure factory.

Where do I look next?

 

Vacuum leak.

Check the complete vacuum system,all hoses and the throttle rod that holds the butterflies . spray some carb cleaner right where the rod sticks out on both sides.
Also the vacuum line that runs to the transmission.

Check the index spot for the accelerator rod.

FIg #19.

http://www.autozone.com/az/cds/en_us...rInfoPages.htm

 

Thanks Mil1ion!

Maybe that explains the sucking sound I hear. Will give everything a check.

BTW: What is the round, black device on the rear passenger side of the intake manifold with numerous vacuum lines attached? It is beside the transmission dipstick. What is it and what does it do? Almost every vacuum-related component runs to this "thing". I have been around Fords for a while, but have no clue what this "thing" is.

With the low mileage I doubt it is anything serious. I found a stray vacuum cap laying in one of the intake manifold depressions when I got it. Looked new. I couldn't find anywhere for it to go. I assumed a facory worker dropped it, but maybe I will look around a bit.

The pump rod is in hole 4. Was in there when I got it. Carb kit says it should be in 3. I left it where is was (4). I don't think anyone ever messed with this truck from what I can see.

 

I believe that round black device you are describing is the vacuum amplifier.

 

You mean the thing in this illustration?




or this one?

 

i must have a vacum leak to ... mine makes the same exact noise , ill have to check that out tomorow ....

 

If it still has the TAC (thermostatic air cleaner) the first thing I'd check is to see if it's operating properly. IE: look in the snorkel to see if it's open after it's warm. If not, check the vacuum motor that operates the TAC (for it's ability to hold vacuum) or the thermal switch that signals it. Both could be suspect to not only the vacuum leak you hear but the problem you describe.
Please report.

 

Thanks guys!

That's it! So it's an amplifier. What does it do?

This evening I got home late, but had a chance to check a few things. I checked the vacuum advance at the distributor. It is fine. I unplugged the hose at the distributor vacuum port on the carb. Hooked up my vacuum pump to the hose and it does not hold vacuum good at all. Maybe this is why my gas mileage is almost non-existent. And the truck runs sluggish. The hose tees when it leaves the carb. One end goes to the "amplifier" and the other end goes to one of the ported vacuum switches. I will need to just take some time and check all of the components. Vacuum hoses look great. Very tight fitting and pliable. I still need to check all of the other manifold supplied devices, air cleaner snorkel, etc.

Thanks for everyone's help. Will update my progress.

 

It amplifies vacuum

Due to its design I would think it makes a more efficient use of available vacuum.

 

Two different Vacuum Amplifiers were used in 1978 ~ CA or non CA. Both have Ford engineering numbers on the parts that are used to identify them.

D5FZ-9E451-B .. EGR Vacuum Amplifier CA only (D52E-9E451-B1A; B2A; B3A)

D5TZ-9E451-A .. EGR Vacuum Amplifier non CA (D5TE-9E451-A1A; A2A)

 

Thanks again to all for the info.

Okay. This evening I had a little time to look for leaks. So far it seems that the only component I have tested so far that held vacuum was the vacuum advance unit. My handle broke off my plastic vacuum pump, so I am having to use pliers to work the pump. Hopefully my new metal one will be in this week.

First I checked the ported vacuum switches. Both leak. On one the plastic tree actually spins in the metal housing.

The air cleaner duct vacuum unit doesn't hold vacuum at all. Should it? The pump just leaks down. The temp sensor in the top of the air cleaner makes a buzzing sound when running.

I test drove the truck with the vacuum advance hooked up straight to the carb port and everything else plugged. Truck feels much better, but still has a slight bog at mid-throttle. When you depress the throttle it picks up for a split second then bogs a bit. "Air leak" sound still there. Maybe it's equipped with a governor I can't find.....I think with all of your help I have made a good start, though.

Should most all vacuum operated components hold vacuum? I will be able to test them better with my new pump. Can you test the vacuum amplifier? Will keep posting results.

To NumberDummy: How have you worked with Ford part numbers and kept your sanity? I still haven't gotten the hang of the parts catalogs! Thanks for the numbers...I dread looking them up!

 

Some components will have small leaks due to their construction.

IIRC, Thermal vacuum switches will allow air one way but not another.

Thus, a Cold & Hot test is required.

 

New vacuum pump arrived today. Hopefully will have time to check things tomorrow evening. The thermal vacuum switches leaked even with the other ports blocked off. And one is really loose. So I think they need replacement. What about the air cleaner duct motor? Should it loose vacuum? Mine does.

 

I strongly suggest replacing the rubber vacuum hoses. They have been there for 30 years under extreme conditions. I have found most of them are either 5/32 or 7/32. I'm removing and discarding all the old rubber hoses when I get the routing figured out.

 

Ok guys. I put my new vacuum pump to work this evening and here is what I found.

1. Choke pull-off leaking bad. Real bad. Would not even attempt to hold vacuum.

2. Air cleaner duct motor leaking.

3. One ported (thermal) vacuum switch leaking where plastic meets metal. Couple dabs of sealer...holds great now.

Plugged choke pull-off and air cleaner motor vacuum hoses. Road-tested truck. Truck is VERY strong now. Especially at WOT. Off idle hesitation gone completely. The truck even has a pronounced "moan" during WOT that it never attempted to do before. I'm quite happy. Hope MPG improve.

Only other "leak" I could find was that the Vacuum Amplifier leaks vacuum when vacuum is applied to the port going to the EGR valve. The vacuum leaks from an unused port on the amplifier. Is this a vent? That's my guess.

I still have a slight hesitation that occurs a second after I accelerate during cruise. I think it is the EGR kicking in....but I guess this is normal.

If anyone can tell me if the amplifier leak is normal I would appreciate it. Thanks for all of your help.