Ford Muscle Cars Tech Forum banner

1 - 11 of 11 Posts

·
Registered
Joined
·
230 Posts
Discussion Starter #1
As of yesterday, I had it running at a surge-y 800-1000rpm in park/neutral, which dropped to like 500-600 in gear.

Best I could do was a bumpy 17 inHg of vacuum at idle. Seemed low on a (AFAIK) stock z-code 390. I assumed I had a vacuum leak somewhere, so I plugged all the ports, but I couldn't do better than about 800 in neutral and 500 in gear. And that 500 in gear has a pretty rough, weak feel.

Figuring it was the last thing left that could be leaking, I replaced the gaskets between the carb and phenolic spacer and spacer-to-manifold.

Now everything's worse.

Now the car refuses to idle below about 1000rpm in neutral and also surges between 1000-1200. Attempts to drop it down cause it to surge downward, then die. Even idling at like 1100, if I put it in gear it dies. Even at 1-1.2k rpm, it's only pulling 15-16inHg with all the vacuum ports plugged.

Yes, I scraped off the old gaskets. No, I didn't put any silicone or other crap on them.

At this point, I'm suspecting any of the following:
-Carb base isn't flat (thus, a leak)
-Spacer isn't flat or is porous
-I f___ed something up on the carb when I pulled it to do the gaskets (doesn't seem like it)
-My problem's not vacuum, but valve related (which is what most vacuum diagnostic guides say is the case for a surge-y vacuum reading). What should I look for for this?

What would you recommend I look for/fix next?

Other background:
'67 Country Sedan
z-code 390
625cfm carter/edelbrock AFB, believe I've got the idle mix ok.
Timing 12* base
Car's been sitting on and off for the last 2 years
 

·
Registered
Joined
·
8 Posts
Go back to the basics. Do a compression check, make sure it's getting fuel, consistent spark. Once you get her to idle, check for vacuum leaks again with carb cleaner or ether starting fluid. Carb float level could be off. Keep looking, you will find it.
 

·
Registered
Joined
·
16 Posts
A low-tech check for intake vacuum leaks: with the car idling, smother the carb with your hands. If it stumbles or dies, probably no leak. If it doesn't die and especially if it idles up or smooths out, it's definitely getting air from somewhere... that's how I discovered the leaky power brake booster on my Gal.
 

·
Registered
Joined
·
230 Posts
Discussion Starter #6
Try a different carburetor for a first hand 30 minute or less way to double check a performance issue like you're having.
...assuming I have a second 4 barrel carb sitting around that's in running order.

Which I do not.

If it doesn't die and especially if it idles up or smooths out, it's definitely getting air from somewhere... that's how I discovered the leaky power brake booster on my Gal.
That's me. Will Double check the booster.
 

·
Registered
Joined
·
16 Posts
Try disconnecting your booster vacuum lines and plugging the holes then see if it idles correctly.Make sure you plug it with something that WONT GET SUCKED IN :)

If it still doesn't idle correctly you could then try fiftybuckbunny's test from earlier (with the booster still disconnected). Spray carb cleaner or starting fluid around the perimeter of the intake manifold and at the base of the carb and spacer. If it idles up or smooths out with the carb cleaner... new gasket.

I've also read about using propane instead of carb cleaner but I've never tried it.
 

·
Registered
Joined
·
230 Posts
Discussion Starter #8
To give the whole story...

I had already pulled the brake booster on and off vacuum, and it wasn't making a difference.

Since this thread started, I removed the factory (?) phenolic spacer under the carb, replaced the under-carb gasket and the gasket between the top and bottom halves of the carb, then drove it a bunch, and it seemed to be running a lot better.

Idling like ~800 in P/N, ~500-600 in gear...bumpy, but not stalling. Seemed to creep up higher after I drove and it warmed up, but not a lot, maybe 50-100rpm both in and out of gear.

Figured the carb gaskets had solved it. Declared it good.

Then, on the way over to a buddy's place, I drive like 40 miles on the freeway with it running like a champ, then halfway down the ~4 miles of stoplights (Beach Blvd in Huntington Beach for reference), it decides it doesn't want to idle.

...well, won't idle below 1100-1200 rpm, that is.

My brakes have a weird feel to them: they don't really spring back up after you step on them. Not that they stick and grab, but when you pull your foot off, it's like there's almost nothing returning the pedal back up.

I'd swear it's the booster that's failing and causing the leak, but it made no difference when I pulled it out of the circuit while debugging before.

Any reason to suspect the vacuum modulator on the tranny? Plugging the line didn't seem to make any difference before, dunno how failure prone those are.

Grrr...

It kept seeming like "oh yeah, I'll just replace _____", but at this point there are no more "just"s. Like so many have said, I really need to go back to basics here: Fuel, Compression, Spark. Plug all the carb vacuum lines, then add back in, chase the gaskets with starter fluid.
 

·
Registered
Joined
·
230 Posts
Discussion Starter #10
That DOES sound like a booster seal going bad on you.
Agreed.

In spite of what I just said about not diving at every quick solution, I just ordered one online. Vacuum leak or no, the pedal feel is all screwy (even for '67 power drums).

I'll still do the full debug as well. Wouldn't surprise me to learn there's more than one vacuum leak going on here.
 
1 - 11 of 11 Posts
Top