Low/rough idle

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Jul 25, 2019
Sumter, SC
Recently been getting intermittent low and sometimes rough idle of my 81 40...barely stays running at times...runs fine with just a little choke. Otherwise runs great...egr disconnected and FCS direct wired. I've had some previous issues with carb I've been able to correct with carb cleaner and fuel additive. So I suspect some partial blockage of slow circuit. Carb has never been rebuilt at least to my knowledge and I've owned it since 1988. So I figure that needs to be done sometime. Anyway, my question is what is the best way to clean carb without total rebuild? Or if anybody has other ideas let me know. Thanks in advance.
You have a vacuum leak. Connect a gauge to the intake manifold for a direct source for vacuum. Read the gauge and tell us what you see.

Since the engine starts and idles and you didn’t mention hesitation, your carb is likely fine, I’m sure a cleaning would help and overall inspection but what you’re saying sounds and is very likely a vacuum leak.
Ok sorry for the delay...had to dig out vacuum gauge and find the time. At any rate, hooked it up to "gas filter" vacuum supply and reading 15 and steady. Idle seemed okay but didn't have tach on to see RPMs. Responded normally to throttle blip. Only thing I noticed was my vacuum gauge connection was not completely tight on connector. My gauge scale says late ignition timing even though my timing light has my setting reading 17 degrees...I haven't messed with it since seems to run pretty good throughout range. I'm no expert though.
I see two issues.

1. 15inHg is too low, you have a 5-7inHg leak, usually intake manifold or a large leak off the PCV or brake booster.
2. Your timing doesn't make sense. 17* at idle is way too high, usually folks mask vacuum leaks by advancing timing but wow that is high! Bring it back to 7* then adjust the idle speed and mixture via a lean drop. For example the high altitude compensator on later 2F engines bumps timing 6* when at altitude but even then that is 13 degrees at idle with a fully smogged truck. A desmogged truck at sea level can usually go to about 9-10 degrees safely if using 91octane.
Yea I realize that is high (actually 15* advance I misspoke.) But it was running fine so i didn't adjust..."if it ain't broke." Anyway, I've got a previous post that discusses this with some additional comments from other members if you are interested. I will look for vac leak using carb cleaner. I have read some stuff though that says vacuum should read between 15 and 22 in idle...so that would put me on low end. BTW, right now idle is smooth although maybe a little lower than i remember without using tach. I probably could use a thorough reset of everything...carb hasn't been messed with/adjusted in 30 years. I kind of go by the philosophy of "if it is running OK...leave it alone." I have done basic stuff though new plugs, dist cap, rotor, wire, adjust valves, etc.
How you maintain your truck is up to you, I’m just telling you that 15* of advance is masking a large vacuum leak and/or carburetor issue. If you set the truck back to the correct 7* of advance at idle for sea level, you’ll likely see a worse running condition. If you choose to fix the leaks and properly tune the carb for the correct amount of advance is again up to you.

15inHg at idle is too low. The carb relies on strong vacuum to pull fuel through every circuit, you should be looking for 18-22inHg starting at 7* of timing. Bumping up beyond 7 makes the engine a little peppier if you have removed all smog equipment and increase octane to suit - then listen for pinging.

You have a lot of unknowns right now, since it runs, do a lean drop, put the timing to the correct 7* then fix the vacuum leaks. By choking the engine, you are essentially removing some air from the engine to help with the unmetered air leak. So fix that then dig into carb as well while you’re at it and ensure everything is in order. Then you’ll be aces.
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