My 79 plow truck (which I desperately need) 400 2bbl (Motorcraft), stock everything, failed Calif. smog by enough that it was a "Gross Polluter". Now I have to take it to a special test station to get it registered. Test said high CO at idle. 2500 RPM test was fine. Tech said it was probably a carb adjustment problem. I had the screws out 4 turns to get it to idle smooth. When I turned them in, it stumbled and died. Rebuilt the carb and no change. Found a broken vacuum switch and replaced it. No change. I hooked up a vacuum gauge and got 16 steady. But...I live at 6500 feet so I'm not sure what normal is. It acts like a vacuum leak, but checking hoses, spraying carb cleaner around, and disassembling the EGR valve did not find the problem. Suggestions? Snow is on the way, and I make a few bucks helping preople out when they need a plow or a pull.

 

Did you check the power brake booster? plug off the vacuum line and see if that helps.

 

I plugged the line like you suggested.....no change. I checked the timing with the vac advance tube disconnected. 6 BTDC as per the specs. I plugged each and every vacuum hose going to the carb and it still would not ide unless the mixture screws were out 4 turns or more.
Frustrating. I can pay the registration fee to keep from getting a late penalty, but I can't take it on the road until it passes smog. A question...there is a bimetallic valve on top of the carb that makes a loud hissing noise. If I close it with a screwdriver, it quiets down. No change in vacuum or idle though. Just wondering what it does.

Another question....I found a vacuum hose diagram at AutoZone, but it only shows a schematic with letters associtated with the vacuum ports on the carb. But...I have no idea which ports are which letters. Any suggestions?

Sorry for the long post, but I'm out of ideas. Everything looks fine, but it just won't run if I lean out the idle mixture. GRRRRR!

 

my 400 would not idle worth crap and i couldn't find a vacuum leak what soever. i rebuilt the carb twice, then gave up and had it professionally done and still nothing. in the end it was fixed when she got a reman. carb. I was never able to find the problem but now idles fine.
just my .02