Hello, I've read thru these forums many many times to diagnose and repair my vehicles yet this is the first time I've posted. I've recently acquired a 1975 f250 custom and I'm having an issue with stalling on decel, or while braking, or while going downhill. Most times the idle is perfect and then for some reason drops low and barely runs. New in the truck are the cap, rotor, plugs, carb, air filter, belts and battery. The carb is a Holley manufactured for motorcraft and doesn't appear to have an air fuel adjustment at least not in the meter block. Any help/ideas are greatly appreciated!

Before messing with that, (the screws are capped) I would verify no vacuum leaks, including pcv, and then also check your distributor timing advances to be sure they work properly, as well as base timing.

Ok I did find a vacuum leak on the manifold when I first purchased the truck. I capped it but will check closer. I didn't think about timing as its an intermittent problem but will pick up a light today. Thanks for the reply. I'll post my findings.

The three incidents you describe all involve letting off the accelerator. You didn't say if the truck is an automatic, but if it is, you might have a look at the antistall dashpot on the carburetor. It may be that the rpm's are returning to an idle speed too soon, causing it to stall.

Could be something goofy with the fuel pickup in the tank....does it change with fuel level?

It doesn't seem to be caused by fuel level however the tank doesn't take fuel well. It tends to spit back if filled to quickly. I haven't checked the vent yet.

*UPDATE*
So I replaced the PCV, installed a digital tach and set the curb idle to roughly 800 rpm, the truck ran great for the last 3 weeks or so and then today began to show the same symptoms. Most notably the idle while at a stop dropped down to around 500 and i had to feather the pedal to keep her from dying. This intermittent crap is driving me nuts...

More often than not...a drop in rpm on these trucks is the result of a vacuum leak. If it happens most notably while stopped or stopping I'd venture to believe your booster is going bad, or at least the valve.

^^^ That will be one of the biggest leaks typically, as it requires the most vacuum. If you hear any kind of air sucking sound inside the cab, that is a likely culprit.

I've used a whole can of carb cleaner and can't find anything. You're referring to the brake booster ya?

The brake booster is what the guys are talking about, I had a bad brake booster on my 78 F-150 Ranger Lariat, for mine at times acting like I was trying to stop an aircraft carrier like The USS Intrepid, and at other times the truck would stop normally. Does your brake pedal feel hard like a rock a times? Have you got factory A/C on your truck? If you do you might want to check behind your HVAC control panel, if you have cruise control there is a can under the hood either on the left inner fender, or near the firewall that is about the size a one pound coffee can and it looks like one with a single vacuum line coming out of it, you might want to check that, and your brake booster I had my booster replaced at the local Ford dealer, and they also replaced my master cylinder when they replaced my brake booster. It is my advice to you replace your master cylinder when you brake booster, for they both are the original parts my braking problems were caused by a vacuum leak on my brake booster, caused by age and rust. Hope you find this helpful, btw welcome to the ole FTF .

Hammssammich said:

I've used a whole can of carb cleaner and can't find anything. You're referring to the brake booster ya?

Click to expand...

I'm thinking you should use a propane torch to find your vaccuum leaks.
NOT lit of course.

Typically the brake booster won't have much of an external leak, just through where the pedal rod goes through the firewall. The is a big diaphragm inside that can get a tear in it, then leak vacuum. It does tend to come with a stiff pedal as well.