Vacuum leak???

[Super Blue 72]

Hi guys, thanks in advance for any input!    This is a problem I thought I might through out there first as I don't have a chance to look at the car soon. 

My car usually starts up fine, needs to warm up a bit with the foot playing with the gas to keep it running, no biggie.

A few days ago, when it started up (cold, about 20 degrees F) it went into a fast idle about 2000 RPM which it usually doesn't do. (I usually have to play with the gas to keep it running for a bit.)  I tried to back out all the adjuster screws but the idle stayed right up there at about 2000 RPM.

The car would drop to about 1500 RPM when it got a little warmed up, but wouldn't drop to idle which is usually about 900 RPM. 

Then the car started stumbling and stalled.  It would start right up but then would stall at anything less than 2000 RPM.  Car still had good power, though if that makes any difference.

I rebuilt the carb (TQ) a few years ago but the car basically stays in the driveway and is just moved around occasionally.  It's a '72 340, AT, Thrmoquad carb.

Any idea?    Does this sound like a vacuum leak? 

Thanks guys! 

[hemiken]

Have you checked the choke butterfly, it may be closed or sticking close. 

[MEK-Dangerfield]

It does sound like a vacuum leak. Mine was fine idling and driving, but would stall at red lights if I didn't keep my foot on the gas.  I'm not familiar with that carb, but if the idle/mixture screw(s) do nothing, there's a leak.


   Mike

[wally426ci]

if its at the carb, you can take the aircleaner off and hold your hand over the opening. if it runs better, you definitely have a leak, if it stalls out then id say no leaky.....

[hemiken]

It could even be a split hose from the severe weather changes you guys get.

[Bullitt-]

When the fast idle is engaged is not the time to adjust the mixture screws as the throttle plates are open negating the effect of the idle circuit. Try resetting your mixture screws, Base idle mixture setting: screw both idle mixture screws in (clockwise) until they seat lightly, back both screws out  1.5 turns each. Warm up the engine & set your rpm to 900, adjust mixture for max.rpm, re-adjust idle to 900. Repeat until no increase in rpm is attainable then turn mixture screw in equally until idle is 700 rpm.

good article here: http://australianfordhardtopclub.org/Carter_Thermoquad_info.htm

[Super Blue 72]

Cool, thanks for all the great replies guys! 

The choke was moving around fine.  If I opened it up or closed it, the idle was unchanged.

I didn't touch the mixture screw in a while, I will use that adjustment procedure when I can get the car running better.  The screws I backed out were just the idle screws on the driver's side of the carb in an effort to lower the RPMs.

I'll try the idea with covering up the carb with my hand to see if the engine is sucking air for other than the carb.

Yeah, temps were cold that day, could've spit a hose or something, or else it was a critter under the hood!    Grrr, them mice!!!!   Although they make nice pets. 

Well, it might be time to replace those vacuum hoses under the hood anyhow, I think they are the factory originals... 

Thanks again for the help!    Maybe I can get to it in a couple of days.