81' stalls when warmed up?

[varrick]

Hi picked up a 720 a while ago and have slowly been fixing it up. It has been running good but all of a sudden it started stalling on me.
It runs fine when starting up or cold but as soon as the revs drop down to idle speed it starts to run rough and dies. I can drive around town no problem but when i go to stop or take my foot of the gas it drops revs immediately and stalls. I can keep it going with my foot on the gas but it is tricky and anything under 1000 rpms are rough.
Any help or advice would be great. Thanks!

[Rev_D21]

Sounds like it could be running a bit rich on you. Is it belching any black smoke? I would check performance by hooking up a vacuum gauge. See how the gauge reacts to idle and off idle. It will help find the reason very quickly.

[varrick]

No black smoke, i will give that a try. I suppose i just hook the gauge up to one of the vaccum lines going to the air filter or something that shouldn't effect running it eh.
I picked up some tune up stuff. plugs wires etc, that i need to do anyways but i don't think that is the problem since it just started stalling all of a sudden and wasn't a gradual issue.

thanks

[PEZi]

Probably a vacuum leak, but more info would help!

[varrick]

So i checked for vacuum, checked electrical systems etc. After a bunch of tune up stuff truck is running mutch better but still had the same problem. Eventually it turned out that my Idle Circut in the carb is clogged. Therefore it runs fine during warm up with the choke closed sucking in a rich mix but then as it warms up the choke opens, mixture thins out and it eventually dies.

Havn't been able to clear the jam yet with high pressure air, cleaners etc so next step i will try i it with the carb off and if that doesn't work i have a spare one i will slap on real quick to try it out. Still working progress

[Tpanda]

did you ever get this fixed? My 82 does the exact same thing. Cold starts fine, warms up fine. It will idle all day until you give it some throttle. Once it idles down, the trucks dies and I can hear the carb sucking air.

All of my vacuum lines are capped except the one dist. advance. Im pretty sure its carb related but havent dug deep into it. I want to put a weber on it anyway, but still be nice to have a daily driver while I save up.

[Davezilla]

Sounds like your idle circuit is way too lean, and since you found a blockage in your idle circuit, that confirms it. Most idle quality issues are due to people tuning the carbs way too lean on the idle mix by following manufacturer's recommendations where they tell you to adjust the idle mix screw in until the engine starts to stumble then back off just until the stumble goes away, this is good to satisfy the EPA, but not so good for your engine... especially when the temperature outside drops, making the mixture even leaner.
The best way to adjust is with the help of a wideband and set your mix to 14.7:1 or somewhere close to that number. Without a wideband, set your idle with the help of a vacuum gauge and set the mix to the highest vacuum you can achieve. No vacuum gauge? No problem... Set your idle by turning the screw in until the engine starts to stumble or lose rpm, then turn the screw out counting the number of turns it takes to make it lose rpm or stumble on the rich side, next you turn the screw back in exactly half the number of turns between the stumble pooints and you're set. The engine will idle much smoother and you can set the idle rpm pretty much as low as you want without it stalling or roughing out... I've been able to get idle speeds as low as 500 rpm smoothly using this method, of course, you don't want to set it that low for every day but it should be able to do it... You should idle at around 650 to 800 rpm smoothly once properly tuned. The condition of your plugs, wires, cap and rotor also play a part in idle quality as well as timing advance, but your idle mix is very critical to idle smoothness and it's resistance to stalling out.

[Tpanda]

My issue seems to be related to the brake booster.

If I pump the brakes while the truck is idling the truck stalls. I may have two issues causing the same problem but Im going to start with the brake booster and trace down its vacuum issues first.