A systematic search for carbon-chain cumulenones beyond H2C4O has been undertaken using microwave spectral taxonomy in combination with a pulsed jet discharge source. No evidence was found for the C2υ isomer of H2C5O or its longer derivatives, but HC(O)C4H, the longer variant of propynal, HC(O)CCH, was identified instead. Its rotational and leading centrifugal distortion constants have been derived to high accuracy from detection of both a- and b-type lines; those below 40 GHz were measured using a Fabry-Perot cavity, while lines between 40 and 72 GHz were recorded by double resonance techniques. Overwhelming evidence for the identification is provided by detection of HC(O)C4D, DC(O)C4H, and HC(18O)C4H at the expected frequencies using isotopically enriched samples. Because HC(18O)C4H is produced with comparable abundance when using either 18O2 or C18O as the source of oxygen, and because H13C(O)C4H is not preferentially formed when starting from 13CO, atomic oxygen appears to be a key reactant in formation, plausibly via O insertion, e.g., H2CC2n+2H + O → HC(O)C2n+2H + H. Under the same experimental conditions, HC(O)CCH is more than 10 times more abundant than H2C3O, regardless of the source of oxygen, and no evidence is found for cyclopropenone, c-C3H2O. Taken together, these observations indicate that propynal and longer chains with an odd number of carbon atoms are either energetically more stable than cumulenones of the same size, are kinetically favored products, or both. On the basis of the HC(O)C4H discovery, searches for the isovalent sulfur species, HC(S)C4H, and HC(O)C6H have been conducted. Guided by new quantum chemical calculations, the rotational spectra of both were observed in the centimeter-wave band with the same spectrometer.