Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem

'81 Z28 E4ME Issues

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MP81:
So I had a thread previously after rebuilding my '81 CCC carb a couple years ago, but I figured I'd start fresh, though I'll recap that one a bit.

Ran incredibly rich and I couldn't get the MCS dwell reading to go higher than 3. To be certain it wasn't the timing light I was using (that measures dwell), I went and bought the pinnacle of technology in 1981: the OTC Monitor 2000. Connected it up: still 3. Long story short, adjusted the idle air bleed a bunch and suddenly I had varying dwell and could tweak it to exactly where it needed to be. The Monitor 2000 also told me that my TPS was reading nearly 5V at idle, with no change at all no matter how you adjusted it, so I replaced that and was able to set that to the correct voltage as well. I had also replaced the O2 sensor with a new one, so that is new, as well, just in case.

So, that's where I am at now: New O2, new TPS, and have a tool that can confirm all necessary readings and thus tweak everything in closed loop to the proper spec, and have. However, all isn't exactly well.

One of the bigger issues I have is that when this thing is in open loop, it runs like absolute trash. If you have it warmed up and running in closed loop and it then falls back into open loop, it'll try to die. Suddenly starts idling lower and lower. If you bring engine speed up and bring the engine back into closed loop - back to perfect. But this also means cold starts usually require throttle input to keep the thing from dying. When you rev it so it doesn't stall, it sounds like it's breaking up. The throttle response is far from great during all this as well, so it affects drivability as well - end up doing a lot of chirping gears when trying to shift because you really don't know if the throttle is going to act the same was as 2 seconds prior.

The other issues are somewhat more minor. When warmed up and driving around, full throttle usually feels like two stages - it'll kinda just sit there and make a lot of noise and then it starts to pull. Sometimes this even happens with just moderate throttle (part of why driving the car can be so difficult sometimes, as it lacks any kind of consistency). Even when warm, blipping the throttle to rev match on a downshift produces varying results. Sometimes you get a great rev and things sound great - other times it basically does nothing and you end up chirping the tires as you let out the clutch. The full throttle issues may very well just be related to the secondaries being out of adjustment, but the throttle response varying so widely might be more related to the initial issue (or not).

MP81:
Had to split my post in two because I tend to ramble ;D:

Since I have the fancy Monitor 2000, I can see basically everything the computer can see, and that was a major help to fixing the initial issues I had in 2019 and 2020, but clearly I have a long way to go on this thing and really don't have a clue where to start. I have the idle mixture screws in the throttle plate set to the base of 4.5 turns out, and the service manual states those really shouldn't need adjustment unless you can't hit the varying dwell in closed loop (which I can). I've also noticed that the next time I'll drive the car after dialing it in...it won't be anymore. This should be a "set it and forget it" kind of thing, but it doesn't appear to be that way.

Any help would be appreciated - if we need to work through it in stages, that is fine as well. I have a feeling I am dealing with a few different issues here that may or may not be connected.

Cliff Ruggles:
Full throttle is basically not computer controlled other than ignition timing.  Issues there can be related to the choke pull-off release time for transition.  Once past all that it should pull strong clear across the engine load/speed range.

What size fuel inlet seat did you install during the "rebuild"? 

Did you install a new secondary plastic cam and spring?....

MP81:
I installed whichever seat was included in the rebuild kit I ordered from here, likewise with the secondary cam/spring.

Cliff Ruggles:
If you bought one of my kits I supplied the high-flow .135" non-windowed N/S assembly for the CCC carburetor.

So any issues once you are past transition at full throttle are going to be fuel delivery related.

I didn't see anything mentioned about the M/C solenoid.

It may be time for a simple test which is nothing more than removing the top and the primary jets, metering rods and setting it up to run straight off a smaller set of jets.

I've corrected issues with troubled CCC set-ups quite a few times with that move.  You'd be surprised how well the engine will run set up in that manner once you nail down the most ideal jet size for it........

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