On the way home our 75 F250 (390) started idling really low and wanting to die. In park it idles at 500 rpm and in drive/reverse it's 300 rpm. I put my hand over the carb, and from experience if it dies instantly then there is no vacuum leak, but when I put my hand on it it started to idle higher and smoother. Which makes me think air is getting in somewhere else (vacuum leak). Any ideas?

Engine:
Newly rebuilt 390 with headers, edelbrock carb (1405), electronic ignition conversion, mostly stock with nothing too crazy.

Symptoms
-Idles low, runs steady at higher rpms
-Revs up fine with no hesitation

What I checked/replaced:
- Vacuum is steady at 17 inHG (normal is 20inHG)
- No sign of vacuum leaks around carburetor or anywhere else (replaced carb mounting gaskets just in case).
- Rear intake vacuum port isn't leaking
- Capped off all vacuum lines with no change
- All vacuum lines were replaced recently
- Ignition timing and all other mechanical seems normal.
- What I noticed is when I disconnect the rear vacuum port, the engine will die, but if I disconnect the cap from the vacuum port on the carb it has no effect on idle. There is vacuum coming from that port, so it should cause the idle speed to change.
If I spray starting fluid around that port there is no effect on idle speed. This makes me think maybe the carb is leaking internally? The shafts for the throttle plates seem really loose.

 

Do you have factory installed A/C? If yes, check the vacuum actuated controls under the dash. An easy way to do this is to clamp the vacuum line that goes to them from the vacuum reservoir on the driver's side splash pan.
Loose fitting throttle shafts can be a source for leaks but that's usually something that happens gradually, not all of a sudden.
Is the carb new or recently rebuilt? Personally, from the sound of it, I would suspect it's time for a carb rebuild.

 

No A/C on the truck. The only vacuum lines are the brake booster, PCV, "low fuel economy" light, and transmission vacuum line. I disconnected all of them and capped them, so they all check out.

I have no idea how old it is, but not too long ago I got an edelbrock basic rebuild kit. It ran a lot better after that, and has been running fine until all the sudden. Is there an o-ring on the throttle shafts? Maybe I can just take those out and replace that. My first thought was that one of the vacuum lines came off or got a hold in it, but it seems more complicated.

 

find a buddy that will let you switch carbs for long enough to decide or rule out the carb as the problem.

 

Don't know anyone with one that will work. Plan B would be to buy one from the auto parts store, then just return it

 

One thing I noticed today, is when adjusting the idle mixture screw on the passenger side of the carb if I turn it all the way in the engine will die. Also if I take the screw out, then it will die. Then the driver's side I can actually screw it all the way in, or take the screw completely out without any affect on idle. I feel a suction with the screw out, but it seems to have no affect on the idle. The vacuum port (where I can take the plug off and it has no affect on idle) is also on the side of the carburetor so maybe there is something wrong with just that side. I checked the float level and readjusted but no change in idle quality.

 

Sounds like the driver side idle circuit is probably plugged. If that's the case, rebuild the carb and be very thorough about cleaning it - soak it for a while, spray carb cleaner through every passage, etc.

 

A quick call to Edelbrock and he told me to put compressed air into that passage and that should clear the air bleed. I did that and now it idles perfectly. I guess sometimes it's something simple.

 

Nice. Way easier than disassembling to clean it out!