Won't idle when warm.
Tuned
Joined: Oct 2016
Posts: 255
Likes: 2
From: South dakota
Looks like your choke system is a Hot Air Type system without the Hot air tubes hooked up.There should be a heat stove on the right side exhaust manifold to hook into. If not just get an electric choke setup and tie into a Keyed 12volt power source. As far as stalling Try increasing your initial Idle speed. If that does not work I'd look into the carb Idle circuit, Maybe plugged?
Tuned
Joined: Jan 2017
Posts: 338
Likes: 8
From: North Idaho
USMC, it may be obvious but I don't see anyone else saying this... have you gone around the engine once it's warm and stumbling with a spray can of starting fluid (small spray hose attached)? That's how I look for vacuum leaks (the engine will race momentarily if you suck in any fluid from a leak. Just keep the spray away from the carb intake so you don't have any false positive results.
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Freshman User
Joined: Aug 2017
Posts: 33
Likes: 0
USMC, it may be obvious but I don't see anyone else saying this... have you gone around the engine once it's warm and stumbling with a spray can of starting fluid (small spray hose attached)? That's how I look for vacuum leaks (the engine will race momentarily if you suck in any fluid from a leak. Just keep the spray away from the carb intake so you don't have any false positive results.
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Freshman User
Joined: Aug 2017
Posts: 33
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I appreciate everyone's responses, I'm going to take the carb off check all gaskets and clean, adjust everything and see where that gets me. I'm thinking about going with an electric choke as the last owner removed the heat line. Is there any advantage/disadvantage in the two?
Hasn't been mentioned but is there a chance there is a misfire that is only noticed when the rpm's start to lower as it gets warm?
Hasn't been mentioned but is there a chance there is a misfire that is only noticed when the rpm's start to lower as it gets warm?
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Freshman User
Joined: Aug 2017
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it seems the carb does have a double spacer which I'm going to pull apart and ensure a gasket is sealing and I'm going to check the jets and float. Also plan on cleaning everything up.
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Freshman User
Joined: Aug 2017
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Post Fiend
Joined: Feb 2008
Posts: 5,831
Likes: 121
From: St Albert, Alberta
Watch "73 F250 390 Stumbling at idle." on YouTube
https://youtu.be/KfCO3xkRn0Q
https://youtu.be/KfCO3xkRn0Q
That sounds like a vacuum leak, Disconnect all the hoses off the vacuum tree then retest to rule out a faulty accessory.
Lead Driver
Joined: Jul 2017
Posts: 9,597
Likes: 1,172
From: San Jose, CA
I'm not sure about this, since I've never tried to use flammable gas on purpose as a method for hunting down vacuum leaks, but it seems sketchy at best. Not from a danger standpoint, but from the "gas" part of the equation.
At the low pressure that it's coming out of a torch it might just dissipate to quickly from the fan blowing it away from where you point it.
I would try that same test but with brake cleaner and see what you find.
Even a spray bottle of water should do the trick, but I've never used that either. Always Berryman's B12 Chem-tool was my go-to because that's what was in the garage. But it's not friendly to new paint, if anything is painted. But old cured paint couldn't care less about a little B12. Just don't spray a ton of it to the point it puddles. Then it could effect things.
Paul
At the low pressure that it's coming out of a torch it might just dissipate to quickly from the fan blowing it away from where you point it.
I would try that same test but with brake cleaner and see what you find.
Even a spray bottle of water should do the trick, but I've never used that either. Always Berryman's B12 Chem-tool was my go-to because that's what was in the garage. But it's not friendly to new paint, if anything is painted. But old cured paint couldn't care less about a little B12. Just don't spray a ton of it to the point it puddles. Then it could effect things.
Paul
Cargo Master
Joined: Mar 2002
Posts: 3,132
Likes: 218
From: W (BY GOD) V
I've got a 73 F250 with a motorcraft 2100 on top of a 390. Runs great when cold but won't idle when warm. As long as you are on the throttle it stays running but wants to die once stopped. So far I have replaced plugs and wires, fuel filter, fuel pump, timing is set at 12* (I've advanced it trying to find a sweet spot but that didn't work either), pulled all vacuum lines one at a time found nothing. With a vacuum gauge hooked up it it jumps between 12 and 14 in/hg at idle and stays steady at 22 with acceleration.
I've tried propane to find a leak as well as wd40. I'm not sure where to go from here. Did I miss something simple?
The threaded fitting on the choke housing is a vacuum leak. That is your choke stove riser tube. A brass block-off cap is need at the least, a proper choke stove hookup should be dome or go with a full electric choke.
Also verify proper curb idle.
Cargo Master
Joined: Mar 2002
Posts: 3,132
Likes: 218
From: W (BY GOD) V
SHOOTING COMBUSTABLE FLUIDS AT A HOT ENGINE
The proper way to search for vacuum leaks is with a propane bottle and regulated wand.
You put the wand into the carb opening and adjust the valve until the shaking stops. You then move the wand around the engine until idle stabilizes.
Carb/brake cleaner flare-ups are dangerous for arm/facial hair and eyebrows.
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Freshman User
Joined: Aug 2017
Posts: 33
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https://youtu.be/WJ2YDCkOVoE
this is what I was getting on the vacuum gauge today. Seemed to be better but not good. I'll be honest only thing I did different was tightened the carburetor bolts and the truck was on the deep side of E. I plugged the open ports on the carb but didn't make a difference.
this is what I was getting on the vacuum gauge today. Seemed to be better but not good. I'll be honest only thing I did different was tightened the carburetor bolts and the truck was on the deep side of E. I plugged the open ports on the carb but didn't make a difference.