Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem

'81 Z28 E4ME Issues

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MP81:
What specifically regarding the MCS?

When I installed it, I set it to backed out 3.5 turns, and I think it only requires adjustment if you can't hit the correct varying dwell in closed loop?

Just for my own refresher - the lean-stop is that "screw" mentioned above, and the further you back it out, the less the metering rods can sit in the jets and thus the "less lean" it can go, overall. Likewise, the idle air bleed screw is the rich stop, which only allows the metering rods to come up and out of the jets a certain amount (so screwing that in would make the mixture less rich).

The MCS seems to function properly, at least in closed loop when you're able to tell - I can get varying dwell within the range it needs to be. But in open loop, I'm not sure it's possible to tell. I don't think it clicks rapidly, unless you have the "OBD" pins jumpered to read codes, but it might once the car is in open loop. I don't remember for sure, but I don't expect it'd do anything in open loop, since it doesn't factor in the O2 reading there, so I assume it's some kind of "base" setting?

Cliff Ruggles:
The M/C solenoids can be troublesome.  I would set it up to run straight off the jets (about 7-8 numbers smaller than what's in it and see if it helps some?......

MP81:
Sorry, I went on vacation and totally forgot about this!  ;D

So would there be any kind of signs to look for, specifically, that would tell me the solenoid is having issues? Is it possible that maybe those two idle screws on the bottom don't like where they're at, so even though I can "hit" the intended varying dwell in closed loop, when it falls back to open look, it's not getting enough air or something?

I also need to do something very simple, but important, and make sure I'm not just chasing my tail here (or dealing with a second issue), and check my plugs. It ran really rich for a while there when I first rebuilt the carb, so it's entirely possible I fouled a plug or 7. I checked one and it was fine, so it's not likely that's the case, but it's a relatively easy thing to check.

MP81:
Out of due diligence, I checked 7 of the 8 plugs (didn't get to number 6 due to it looking like more of a pain in the ass than even number 5 was, due to the primary, though one plug out of eight won't cause the car to try and die at idle) and they all seemed to be pretty clean. Nice tan color - plug 7 did smell a little funky, but looked to be the same color. None of the others had the same smell.

So plugs do not appear to be an issue and thus likely aren't influencing anything.

Figured I would update on those, at least.

So as far as going with a jet change test, where would I find myself those - are they specific to the Qjet? Do they just unscrew? I didn't mess with those when I rebuilt the carb, aside from poking them with some wire to make sure they weren't blocked (they looked clean - the whole inside of the carb did), since I had read it was very easy to screw em up.

Cliff Ruggles:
The CCC carbs used a different style of jet but they are same thread as earlier models so any jet from any year will screw into the holes and tighten up fine......

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