EA81 - Cause of Rough Idle?

[GLwagon1984]

My 1984 GL Wagon will not maintain a steady idle once it is warmed up (choke open), but idles fine cold (choke closed), and it sputters when I take my foot off the gas pedal and allow the revs to drop from a higher rpm. Just sitting idling, the needle on the tach bounces around between about 600 and 1,000 rpm, but just applying enough pressure to the gas pedal to bring it above 1,000 rpm stops this. Like the tach, the reading on my vacuum gauge is all over the place when hooked up at idle. Does anyone have any ideas as to the cause?

Here's the backstory / what I've done to it so far:

• The car was driven regularly until being traded in at an independent Subaru shop/used car dealer, then sat for about a year - only driven occasionally
• I bought it and drove maybe 100 miles over the course of a month, and it ran and idled great until it suddenly started dying every time I let off the gas pedal while out of gear (or clutch in)
• I replaced both fuel filters, and did not notice any bad smell to the gas or any junk in either filter
• That didn't fix the problem, so I converted to a Weber carb, and it ran great, but idled high around 1,000 rpm
• I adjusted the carb per Weber's instructions and to bring the idle down to around 800 rpm, and the rough idle issue started
• I couldn't fix the issue no matter how much I fiddled with the carb, so I thought it was a vacuum leak
• I searched for leaks using carb cleaner, then removed all vacuum hoses (including vacuum advance) and plugged all the ports, but the issue continued
• I replaced the PCV valve and intake manifold gaskets, but still no difference
• Just today I tried to start it after a month without running it, cranked it for 30ish seconds a few times (with breaks in between) and noticed the fuel filter at the firewall had very little, if any, gas in it (but I'm pretty sure the tank is over half full), so now I think I've got a fuel pump issue and am wondering if this has anything to do with idle issue.

Sorry for the long write-up, but I want to share as much relevant information as I can. Any suggestions will be greatly appreciated. Thanks!

[Steptoe's photos]

thanks for not writing up a stupid post - the more details - the better !

I now usually just ask for a compression test result - less typing  for me to start with.

I also had an 84 model and some 24 years ago I suspect it was for sale for an intermittent problem, similar, but not same as yours. Mine ran fine during the day temps but at 4am in winter bogged down and only idle, no take off power. Short story is that I used air compressed into a fire extinguisher vessel as I had no compressor myself. Isolated all steel fuel line sections by removing and replacing at same time, the rubber fuel hoses. With rubber hoses off, I blew air from front to back end of each section. When it came to fuel pump to tank - I got some resistance, then heard the blockage clear up on the floor of clean, empty fuel tank. It was rusty crud that I think was lining the tube or sock and shrunk in the cold, blocking off sufficient fuel delivery. Fixed my problem and the story has fixed a few more. Compression test might be easier, but near useless in this case

[GLwagon1984]

Thanks, Steptoe's photos! A compression test was the only remaining thing I could think to try. I had not even considered a clogged fuel line. As you said, a compression test might be easier, so I'm thinking I'll start there, then move on to the fuel lines and hoses and report back as soon as I've got results. Hopefully the problem presents itself in one of those two areas, and I'm hoping it's just a fuel line!

[Steptoe's photos]

and the lazier way to see if it is a fuel line thing - a quality fuel system cleaner additive for starters, then know that you likely need to replace fuel filters after a tank or two of fuel. 8mm nuts on the tank sender at the rear of fuel tank, with a pretty long life rubber seal. Remove it when tank is very close to empty or empty for safety to simply view the internal condition. Should be shiny new galvanised look about it. My worst looked like black and brown barnacle cladding. That was a throw away. People suggest rubber fuel lines can collapse inside causing blockages.