Help finding vacuum leak?

[tbratz]

After a recent trip in my '69 for some reason it won't idle below 1300 rpm. It seems to be running smooth other than it won't idle down. It also now has a very strong exhaust smell so I wonder if it is leaning out. Even with the carb main idle adjustment turned all the way out it still does not affect the idle rpm. I also noticed that I can adjust the idle adjustment screws on both sides of the carb and it also has no affect or change in the idle. I also threw on a spare carb just to eliminate some kind of carb issue, but the other carb does the same thing and idles very high. There are no leaking vacuum hoses and when I spray a little carb cleaner around the base of it there is also no change in idle speed. Also, if I pull the pcv valve from the valve cover and plug the valve with my finger, although I can feel a slight vacuum, if I am not mistaken doing that in the past would cause the engine to stumble and want to stall. I checked the distributer in case maybe it came loose and turned a little, but it is tight and where it is supposed to be. I am thinking that it is probably a vacuum leak, and since I can't find a leak around the engine I wonder if it can be an intake leak inside the engine. I have never dealt with a leaking manifold, so I am not familiar if an internal leak would cause this. I am running a Holley 4010 and Edlbrock performer intake. Any ideas or suggestions before I pull the manifold would be appreciated.

 

[DirtDonk]

Back to the top.
If you had not tried the second carb, I would have said to check the choke linkage. Even if the choke plate is all the way open sometimes the linkage sticks on the high-idle cam and causes your symptoms.
Even with two carbs however, it's very much worth checking out. Make sure nothing on the intake is out of place and jamming it up (or the carb base gasket for that matter) and that the linkage is moving freely all the way down.

Should be easy enough to confirm by eye if the fast-idle cam is releasing fully, or if your throttle shaft is riding on the cam.

Paul

 

[tbratz]

Thanks Paul for the suggestion. I double checked and the choke linkage is free and non-binding, I even tried new base gasket while diagnosing everything. The only thing left I guess is to pull the manifold and re-gasket it.

 

[DirtDonk]

Maybe, but sure seems drastic. However, with the sheer number of issues people have had with this exact thing, it’s not out of line to suspect an intake gasket.

What happens when you slowly cover the carburetor with your hand? Does it change as you get closer to fully covered? Or does it remain the same until it finally chokes off?
Or does it even choke off?

Can you look down the Venturi with a flashlight to see if the throttle blades are closing all the way? Engine off is fine for this test.
Your other tests with the idle speed screw and idle mixture screws sound very much like the throttle plates are open past the idle circuits, or transfer slots.

And speaking of transfer slots, when it’s idling high like this do you have a vacuum signal on the ported vacuum fitting?

 

[Timmy390]

More info......Is this a fresh build? Did the idle issue just start happening as is was it OK before X day or X change and now is an issue? What modifications do you have?

Bad brake booster, vacuum advance...... I think the issue with the intake gaskets was contained to just one part number. Unless you used them I wouldn't expect a failure,

Tim

 

[tbratz]

It's not a fresh build. Have owned it since '81 when I graduated high school. No brake booster, switched to hydra-boost a long time ago. It has been running fine until after a recent drive and the issue showed up. The only vacuum lines is to the pcv valve and the vacuum wipers, which at least the wiper port I blocked off. With the throttle plates all closed on both carbs, it doesn't affect the rpm. Also, Paul, with the carb completely covered/ closed off with my hand, it has no affect on the Engine rpm. That's why I think it has be sucking air from somewhere, but I can be very wrong. On the vacuum circuit (open not to the distributer) it has about 18 psi. Thanks again guys for your attention.

 

[DirtDonk]

A booster leak would’ve been a good call!
Too bad there isn’t one of those.

Just the fact that nothing changes when you put your hand over the carburetor is a huge clue.
I have to think that it’s a intake leak. Though it should’ve showed up immediately while spraying stuff around the compartment.
What did you use as your spray tester?

You might even consider using the water hose just dripping it heavily everywhere to see if you notice a change. If not it’s gotta be inside.

But back to the timing and distributor, as was said you can’t just assume. You have to test.
Find somebody with a timing light if you don’t have your own and verify the timing.
Although I don’t think that normally can keep the engine from dying with your hand over the carburetor, you still need to verify all the peripherals.

 

[tbratz]

I used both a spray water bottle and a can of carb cleaner to test for leaks. I also checked the timing after I saw the distributor was tight with a light and nothing unusual. I have a 74 f100 with the same 4010 I run on the Bronco, so even though I tried a spare carb with the same results I want to triple check before I think about pulling the intake as a last result. Although I am at a loss I am going to switch those carbs around to further the process of elimination.

 

[73azbronco]

Exactly what is your vacuum at idle? Simple cheap tool.

 

[tbratz]

It is normally around 20 psi.