Thank you ddgonzal for clarifying. Today after work I tried adjusting the idle-mix and primary slow jet plug, and I managed to get the engine back to its original state (idling, accelerating, everything). I even did a short test drive to be sure, and it still ran fine. About 200m later driving back home the idle became weak again but driving it was normal. Just before I reached home it died on me, had trouble starting, and even if it did start it would die off again.

*Update* I finger tightened the primary slow jet plug (oddly, loosening it kills the engine now) and loosened the idle mix screw quite a bit (I think it's now about 4-5 turns out from bottom, is that a lot?) and the engine is sort of stable. Idle is much better than this evening but still not as smooth as originally. It hesitates/jerks a little bit when accelerating sometimes.

Sounds like it is dirty inside. Probably a dirty fuel bowl, which is easy to fix.

Remove these bolts (engine cold), which lets the dirty fuel run out.



After you crank the engine some, it will fill the bowl again and start up. If if runs good for a few days, then acts up again, repeat the treatment. I had to do it three times over a month before it totally cleared up.

Some may prefer to remove the carburetor and dissassemble, cleaning it thoroughly.

If it's dirty inside then why was it running perfectly fine before me and my friend messed around with the idle-mix screw and primary slow jet plug? I'm pretty sure disturbing the screws is what's causing the engine to misbehave...

Btw, what exactly does the primary slow jet plug do, and if I loosen it what exactly happens inside the carb?

Most poor running problems usually start at the ignition system. The last thing to adjust is the carby.

#1/ check points, plugs.

#2/ check tension of inlet/exhaust manifold.

#3/ check ignition dwell, then timing @ correct idle.

#4/ consider adjusting carby.

As for the mixture screw, usually it will be around 1&1/2 turns out from seated. There should be a point around there that it will smooth out. Screw it in and it should stumble before stalling, outwards and it should also start to stumble. Find a nice happy medium. Do this with all accessories off.

When a car starts to run rough, it shouldn't be assumed that the carby is at fault. Even if there is black (fuel) smoke. It may be the case that a weak ignition system cannot burn the fuel correctly.

+1


Quote:
Possibley (1) loosening the jet let come crud get loose or (2) it's a coincidence.

Do what you want.

Today's progress:

Morning start was tricky and when it finally starts it stumbles a lot, but after warming up it became better. When driving there was quite some moderate jerking. I loosened the idle-mix screw about 1/2 turn, fiddled with the primary slow jet plug but finally re-seating it gently.

Afternoon start up was no problem (left the car for about 5 hours). Jerking was slightly less, idle was smooth, and no more black smoke.

Evening start up no problem (left it for another 4 hours+). Loosened the screw 1 1/2 turn, jerking had significantly reduced, idle was smooth and no black smoke.