(Pt 2) Possibly OT, ??s about a GM feedback Q-jet

[Shannen Durphey]

I failed to mention that setting these carburetors has always seemed a matter of
reverse tuning to me.  Opening the idle screws adds more fuel, so the engine runs
better.  Increasing timing makes the engine run better still.  Problem is that the
idle speed ends up around 1000-1500 when the engine really starts to smooth out. 
Been down this road enough times to take home a couple of tee-shirts.

Instead of opening the idle screws until the engine runs well, it always seems to
work better to open the throttle adjustment until I start to get vacuum on the
ported vacuum line to the distributor.  With the timing set to factory specs, the
idle mixture screws can be adjusted to obtain the best (not always good) idle. 
This usually leaves the engine idling at reasonable speeds in and out of gear, and
runs reasonably well.  Sometimes the throttle plates need to be opened slightly
more, but this can lead to high idle on fast decel if the vacuum advance is
"sticky".

The factory manuals are a good source of info, and so is a book by Doug Roe, called
Rochester Carburetors.
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.
>
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