Hey guys, newbie here asking for some help. I have a 1981 F150 4X4 Flareside with a 302 HO and a 3 Speed Automatic C6 Transmission that are not original to the truck. I have an Edelbrock Performer intake and 1406 Edelbrock Carb w/electric choke.

I've had problems keeping this truck running since I brought almost 10 years ago, but I love it too much to let it go.

I have both a Haynes and a Chilton manual and they mention vacuum leaks, timing, or carb can be some of the reasons for my current issue.

At any rate, I got it back from the mechanic last summer and noticed that it was acting like it wasn't getting fuel, hesitating a lot. When I got it home (the mechanic is 100 miles from my house), I noticed that it was leaking fuel. Sure enough, the fuel pump was bad. I had a new fuel pump put on and it seemed as though it ran better. There was some hesitation, but it ran fairly well other than that. The next thing I know, I heard what sounded like a balloon burst under my driver's side dash and I could hear what sounded like a loud hissing coming from there; especially when I stepped on the brakes.

I read up on this forum and checked some other online sites and it seemed to point to a vacuum leak caused by a bad power brake booster. I purchased one and installed it a week ago and the hissing sound went away. However, the engine revved up really high from a cold start right after I installed it and when I tapped the gas, it throttled down really low and started to rev up and down (kinda like it was lopping) until it died. It started up with some effort, but the lopping idle continued. When I put it into gear, it just stalled out.

I'm not a mechanic, so I don't know if its a vacuum leak, bad head gasket (which is why it was at the mechanics) carb issue, timing or what. I did replace the fuel filter to a clear one so that I could see if fuel was getting to the carb and the flow looked low to me.

Not sure what to do about increasing the fuel pressure.

Seems like some folks have had luck with just adjusting the distributor cap. I don't have a timing light, but it looks like the timing marks are correct and it appears to be set at 9-10 degrees BTDC.

Let's see, I had the catalytic converter removed, a rebuilt C6 transmission was installed along with the rebuilt 302 HO, new gas tank, fuel pump, fuel filter, plugs, wires, radiator, hoses and thermostat.

All this was done some time ago and it ran okay until one day when it just wouldn't crank. That's how it ended up with the mechanic. He said the compression was fine, heads weren't warped and he put new gaskets on it.

Pic is of the engine, however the air filter has been changed and the ignition coil is now on the driver's side; not mounted under the air filter housing's opening. The reason that I included this pic is because I noticed that the hoses mounted on the firewall (2 + 1 plugged) are now not there. There are no hoses there (not even plugged). Could that be an issue?

I'd appreciate any help and thanks for reading this long winded post.

 

Welcome to FTE.
Ok don't go making adjustments to anything just yet as you can make it worst.


As you know that inside hiss when using the brakes was the booster, you did good on the swap.


So in reading your post it sounds like this started after you changed out the power brake booster.
If so start looking over the motor for any vacuum lines you may have knocked off during the booster swap.
I would start at the booster and follow that hose checking for cracks/splits down to the intake where it connects.
Also back there you may find a vacuum tree that other lines could connect to that may have caps. Pull the caps and check the ends. Sometimes the vary end will be missing and cause a leak.


As for that vacuum "tree" on the firewall, that your picture? that you say is now missing. It does not look like the truck has AC and from what I know that "tree" was for the AC system.
In the picture the supply line is off the top of the PCV up to the "tree" I don't know where the other line goes to? Maybe to the trany module valve for shifting?
See if that line is on the PCV and if not plug that top fitting.
If you find that other hose going behind the motor is there but not hooked up to a supply take a picture where it is going and we can tell you where it needs to be hooked up to. That top fitting on the PCV is not really the right place.


There is also a smoke test you can use to look for vacuum leaks. Check Youtube for that smoke test for vacuum leaks to see how to run this test, it is not hard once you see it done.


Let us know how you make out and if still not right we can check other things.
Dave ----

 

Also got thinking if this also started after the booster swap try and pinch off the hose going to the "new" booster and see if that makes difference.
Dave ----

 

That's what I would do, pinch the line going to the booster or take it off the booster and put a bolt in it. See how it acts after that. Of course you will not have power brakes after that, but all we are doing is experimenting with it to try and pinpoint the problem.

 

Thanks FuzzFace2, yep the stalling issue happened after I put the brake booster on last week. I could start it and warm it up fine. Take it out on early morning road runs (15-20 miles) and it did okay. Some hesitation and a whole lot of hissing noise that made it difficult to hear anything, especially at highway speeds.

I'll start looking around and checking the connections and condition of my hose lines. Just thought it was strange that when I took this picture 5-6 years ago (maybe longer) that those hose lines on the firewall where there, but when I put the brake booster on last week, I noticed they weren't.

I'll get some pics of it tomorrow, so the differences are more obvious.

Man, I really appreciate the help.

Hoping its just a vacuum issue, but the gray ghost is old.

I'll post what I find when I get back in there tomorrow.

Thank you sir.

 

Thanks Franklin2, I'll post what happens when I plug the booster vac line and check the hoses tomorrow.

Really appreciate the help man. You guys are great!

 

Okay, I just came back in from checking the hose lines and disconnecting the brake booster vac hose line.

I started up the truck after taking the brake booster's vac line off and plugging it.

She fired up fine, throttled down to a normal idle after I tapped the pedal and warmed up fine.

I heard no audible hissing sounds (I did not use my extra hose to listen closely), but she was idling fine. At least until after she warmed up and I revved the engine a little, then she started that lopping idle again until she stalled out.

I took some pics of the where there had been hose lines going into the firewall. I'll include the pic from yesterday so there's a before and now view. But again, she ran better after I disconnected that brake booster vac line until the lopping began. Basically was running like I remember it, so I was happy; for a moment anyway

Thanks again guys, you really don't know how much I appreciate you alls help.

 

Okay, I detached and plugged the vac hose from the brake booster and started the truck. I didn't here any hissing sounds and all the hoses looked fine. She idled like she normally does, high idle from a cold start until I tap the gas and then idled normally. After she warmed up to normal operating temp, I revved the engine a couple of times and the lopping idle returned until she stalled out.

 

Red #2 was wrong anyway. Yellow #1 is just a vacuum distribution point. If the couple lines that were on it before are now hooked directly down on the intake, you do not need it. Originally the HVAC vacuum line and the automatic trans vacuum lines hooked up there.

Have you messed with the idle mixture screws on the carb to try and help it?

 

Here's a picture.

 

No. I just took the suggestion to disconnect the brake boost vac line.

1. Should I try adjust both idle mixture screws?
2. And how much should I turn them? I've read where some say use 1/4 turns. Does that sound right?
3. Also, should I adjust the idle speed screw? since idling seems to be an issue.

You said "the automatic trans vacuum lines hooked up there," so now I don't need that hose from the firewall going to the trans?

 

You need a vacuum gauge to properly adjust the idle mixture screws.
You can go by ear and adjust them till you just reach max engine RPM. It wont be as accurate but it will be close enough.

 

"You can go by ear and adjust them till you just reach max engine RPM. It wont be as accurate but it will be close enough."

Thanks matthewq4b, I don't have a vacuum gauge so I'll try it by ear and post what happens.

Thanks to everyone

 

It's not hard to adjust the mixture screws.

1st, make sure it's idling slow enough. In other words, totally warmed up no choke action.

After that, turn one screw in while listening to the engine. If it gets worse turn it out till it runs it's best. Then go to the other screw and do the same.

Then go back to the first screw and turn it in till it noticeably runs bad, then turn it out till it just runs it's best. Then do the same for the 2nd screw.

That will be it. You can play with them as the weather changes. I find on the holleys that if I adjust the screws till they are in the position where it runs the best that a extra 1/8 turn out after that on each screw keeps me from having to adjust it as the weather changes. You may want to try that on yours also. Screw them in and out till it runs it's best, and then 1/8 more turn out on each screw.

 

Turning the screws right is turning them in and turning them left is turning them out?