  We present an HCO$^{+}$ J=3-2 survey of Class 0+I and Flat SED young stellar
objects (YSOs) found in the Gould Belt clouds by surveys with Spitzer. Our goal
is to provide a uniform Stage 0+I source indicator for these embedded protostar
candidates. We made single point HCO$^{+}$ J = 3-2 measurements toward the
source positions at the CSO and APEX of 546 YSOs (89% of the Class 0+I + Flat
SED sample). Using the criteria from van Kempen et al. (2009), we classify
sources as Stage 0+I or bona fide protostars and find that 84% of detected
sources meet the criteria. We recommend a timescale for the evolution of Stage
0+I (embedded protostars) of 0.54 Myr. We find significant correlations of
HCO$^{+}$ integrated intensity with ${\alpha}$ and $T_{bol}$ but not with
$L_{bol}$. The detection fraction increases smoothly as a function of
${\alpha}$ and $L_{bol}$, while decreasing smoothly with $T_{bol}$. Using the
Stage 0+I sources tightens the relation between protostars and high extinction
regions of the cloud; 89% of Stage I sources lie in regions with $A_{V}$ >8
mag. Class 0+I and Flat SED YSOs that are not detected in HCO$^{+}$ have, on
average, a factor of ~2 higher $T_{bol}$ and a factor of ~5 lower $L_{bol}$
than YSOs with HCO$^{+}$ detections. We find less YSO contamination, defined as
the number of undetected YSOs divided by the total number surveyed, for sources
with $T_{bol}\lesssim$ 600 K and $L_{bol} \gtrsim$ 1 $L_{\odot}$. The
contamination percentage is >90% at $A_{V}$< 4 mag and decreases as $A_{V}$
increases.
