Bogs when accelerating

I've got a 77 Ford LTD II with 351M. Figured I would start a new thread for this. Some folks have made suggestions, but I wanted to take a systematic approach to this. I am by no means a mechanic. I've messed with cars, done tune-ups, etc, and can usually handle something, as long as I've got some decent guidance. But, have never dug into troubleshooting something like this.

First, the symptoms.

The car bogs when you first accelerate. If you accelerate too fast, it will die. It does this whether it's cold or at full operating temperature. It does this whether you are in park or in drive. After you get past this initial bog, it runs fine, most of the time. Intermittently (not very often), you can fill it slip/skip while at cruising speed. One thing I've noticed is that I hear a very light "whistling" noise come from under the dash, when I press the acclerator and experience the issue.

Sidenote: When I test drove another 77 LTD II wagon a couple of weeks ago, it experienced a very similar issue, and I also noticed the same "whistling" noise. Although, it seemed to clear up as the car got warmer.

One of the most prominent suggestions has been to replace the accelerator pump. I ordered a new one, went to replace it, to discover that the existing one looked great. Most likely, it had been replaced recently in an attempt to correct this issue. I proceeded with replacing it with the new one. No change, still have the same symptoms.

Any input would be greatly appreciated. Especially a systematic approach with the easiest/cheapest fix listed first.

 

A. Rebuild the whole carb i belive there are 2 pumps in there and they are super easy

B. do you have cruise control?

C. Pull the check valve out of the power brake unit after you shut the car off and see if it hisses like crazy. if it doesn't it's bad


the last 2 could be the whistle under the dash and a vacuum leak=Bog

 

No cruise control. Yeah, I was kinda thinking vacuum leak. I'm thinking I should pursue that before rebuilding the carb. I've never troubleshot a vacuum issue. Other than checking the power brake check valve, how else can I t-shoot vacuum? That's one thing I've never messed with before.

 

The whistling noise tells me you have a vacuum leak. Are all of the HVAC functions working correctly?
Find the port on the intake that serves the under dash vacuum operated devices and cap off the port to see if your symptoms abate.
A brake booster is another possibility for a vac leak you might hear inside the car.
Also see if the carb's accelerator pump is working.
These are my first thoughts and where I'd begin looking.

 

Hiss under the dash is almost always a vacuum leak in the HVAC system. If you have the automatic climate control, I am betting that when hit the gas you get hot air out of the vents, and on a long hill, the system switches to hot air in defrost. Sure signs of vacuum leaking. It is a pain to find, but follow the hiss and you will get there eventually.

 

Hmmm. I haven't noticed any issues with the HVAC system. Everything seems to blow (hot vs cold) as it should, when it should, and where it should. But, I'll check it out thouroughly.

What do you mean by "automatic climate control"?

Thanks for the suggestions!

 

The best way to check for vacuum leaks is with a Mityvac tool, type with the dial gauge on it, otherwise you have to run the engine and play hit and miss disconnecting hoses. They're relatively inexpensive and you can check all kinds of stuff for pressure and vacuum and also drain fluids and bleed brakes with it. The parts stores may loan the things? I know I'd be lost without mine.

I'd concentrate on the carb first then timing/distributor/timing chain issues. A little vacuum leak shouldn't cause that much of a problem, but you never know? Borrowing a carb you know is okay off of a car that runs good is the fastest way to check that I've found but that's not always possible. A rebuilt carb is good if it's rebuilt right. You can do it yourself but when throttle shaft bushings are leaking and stuff like that only a pro can do it correctly.
Sure sounds like carb missing at speed?
Do the timing marks slip on Ford balancers like GM do from the rubber in- between the pieces loosing adhesion?? The timing mark will really mean nothing and you search like crazy everywhere else thinking the timing is okay, timing by ear and the feeling in the seat of your pants isn't such a bad thing sometimes.

 

Bluevista thats the ONLY way my older vehicles are timed! I still have other derby guys that will not depend on a timing light they want me to do it "by ear"

 

Change the accelerator pump setting. It's on the lever on the side connecting to the throttle. Put the rod into a hole further from the lever point. Might be worth a try.
Dave

 

My car also does this, it has a 302. I have been through the vacuum lines. Installed a brand new carb, fuel pump, timing chain, vacuum advance, and complete tune up. I look forward to seeing where this thread goes.

 

Good tip. I will definitely play around with this on mine and see what happens.

 

OK let start from the beginning. When it does bog, is it a long slow bog that eventually gains momentum or is it a short bog that kills the engine? I need to know the basics of your car. I.E. Gears and transmission. Cam size and timing. Carb. or throttle body size. And engine timing. Lets get down to the brass tacks and we can follow the trail to your problem.
Dave.

 

Ok stupid me but before you do anything. Something VERY common i forgot about! CHECK your vaccum reserve can! Looks like an old time coffee or large soup can with 1 or 2 vaccum hoses on it. sometimes they are in plain view other times they are "hidden" up under fender.Usually on the firewall or inner fender. Pull it off and check it very well for leaks or vaccum test it. These things will rot out on the bottom and get pinholes in them and usually are positioned so the the holes that do develop will mask the sound of a vaccum leak. Worth a try

 

Okay, so there is nothing special about the engine and trans in this wagon. It is the factory ford 302 v8 with the aod trans (c4 trans maybe?) its a 4spd. So far I have done nothing to upgrade. It has a new fuel pump, and timing chain/gears. I started having cold start issues and the car sat for a year. I replaced the ford 7200 series variable venturi carb with an aftermarket motorcraft 2150 2bbl carb. I have been through and replaced all vacuum lines (time consuming) Timing is spot on. Today I turned in the mixture screws and leaned it out a bit and also moved the accelerator pump linkage to a lower hole. There is still a noticeable hesitation, but is accelerating way faster and better, idling smoother and the exhaust is not near as gassy. Also, it no longer stalls when taking off from a stop. Before, I had to lightly tap the gas then hit it again, making sure to accelerate beyond the dead zone. Seems to me the carb was dumping way too much fuel. Guess that's what I get for assuming a brand new carb would be tuned close enough to work on this ol wagon without any adjustments. I will be driving the ol Cream Soda, another 100 mile commute tomorrow. I will update everyone on the performance. Thanks for the tips, I think relocating the accelerator pump linkage made a big difference. (More tips, advise, and opinions are always welcome)

Thanks All

 

It's a short bog that can kill the engine. If I take it slow, it gets past the bog, and works fine. Everything on my car is stock. It's a 77 Ford LTD II with 351M. I haven't checked the timing yet.