Possibly OT, ??s about a GM feedback Q-jet

[Shannen Durphey]

That's not a computer controlled carburetor.  The connector you're seeing is an
auxilliary accelerator pump enrichment device.  The carbs are designed to run
relatively lean mixtures.  This introduces the problem of lean stumble during cold
engine operation.  To remove the stumble, the temp controlled accelerator pump
enrichment allows an "extra" shot of fuel.

You haven't mentioned it, but I'll bet your blazer was originally equipped with a
305.  The "ecm" you're seeing is only a spark retard box, designed to sense knock
and back off timing if it's present.

You're engine's not computer controlled.  You can remove the spark control box, and
a simple jumper between 2 wires to the spark control box will allow the engine to
run without replacing the module.  I've seen many of the spark control units (BTW,
does your fender have an ESC badge on it?) fail intermittently, stranding a vehicle
beside the road for a couple of hours each time.  The budget fix is to jumper the
two wires.  It's been a couple of years since I've done this, so I won't try to
remember which two.

Disconnecting the extra fuel circuit can cause some severe cold weather
hesitation.  Sometimes retuning can make up for this.  Sometimes not, as some of
these carbs had a hesitation from the factory.  Best bet if you don't want to work
with it is to get an older Q jet.

If the carburetor is truly dumping too much fuel you might try disassembling it and
looking around the metering well plugs for signs of fuel leakage.  It's very
common.  A good job with epoxy can repair this condition for many years.  

Shannen

ae2598 at wayne.edu wrote:
> 
> I'm kinda at a loss to figure out exactly what I have here, so I'm turning
> to you guys for advice.  The vehicle is an '85 K5-Blazer, with a
> computer-controlled (somewhat) Q-jet.  I got it for cheap because the gas
> stations that worked on this thing in the past apparently couldn't figure
> out what to do with it (other than replace everything underhood!) and it
> still runs lousy.
> 
> The first thing that I find odd is the HEI distributor.  It has advance
> weights and a vacuum can.. In fact, the only way in which it differs from
> the "standard" HEI is the use of a 5-pin module, and an extra harness
> hanging out the back.  The pickup coil apparently feeds a signal to the
> ECM as well as the module.. probably a timing reference signal.  This
> harness has been butchered by previous repair attempts, and probably isn't
> giving much data to the ECM anyway.
> 
> The next thing that's not exactly right (IMHO) is the carb itself.  It has
> a M/C solenoid, but no TPS or IAC.  Every description I read about the C3
> Q-jets shows at least the M/C and TPS, and usually an idle air control,
> too.
> 
> The ECM seems a bit un-sophisticated.. It is in a very small package, and
> has a single connector with only 7 wires connected!  No ALDL that I could
> find, and no provisions for a PROM.  The service number is 16042121.
> 
> Anyway, what I would like to do is make this a non-computerized motor.
> The distributor seems easy enough, I'll just plug in a 4-terminal module
> and all it a day.  As far as the carb, what will be the result of not
> connecting the M/C solenoid?  Are these carbs capable of metering fuel
> without the help of the ECM, or should I just find another Q-jet?
> 
> Anyone with ideas to share on this would be appreciated..  the carb seems
> to be dumping way too much fuel into the motor at this point, and I'd like
> to solve this as cheaply as possible!
> 
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