What am I doing incorrectly with this IR/ Mass spec data?
What I was trying to say was that firstly the molecular formula has to be determined correctly. From the mass spectrum of the given molecule, the molecular ion peak ($\ce{M+}$) was determined as $\frac{m}{z}=142$, which corresponds to the molecular formula $\ce{C9H18O}$. Note that, it can also correspond to $\ce{C10H6O}$, but in your $^1\mathrm H$ NMR spectra, a total integration of at least $9$ is there, thus the latter consideration is anyway not possible to be the correct formula.

After determining the chemical formula, it's good to look for index of hydrogen deficiency(i.e. degree of unsaturation) of the molecule. If the formula is $\ce{C9H18O}$, the degree of unsaturation is $1$. From the IR spectra also, we can see a sharp distinct $\ce{C=O}$ stretch near $1700\ \mathrm{cm^{-1}}$, and also no aldehydic proton peak near $9.5{-}10\ \mathrm{ppm}$, therefore we can conclude there is a keto group present in the molecule. 

Now, we come to analyse the $^1\mathrm H$ NMR spectrum, which shows only total relative integral of $9$, but remember the integrals in proton NMR are always relative, they are never absolute. So, you can scale them up or down relative to your choice. Here to match the total protons in our system, we have to scale it up by a factor of $2$. Therefore the integrals now correspond to $4,8$ and $6$ protons respectively, and also by this scaling we can also say that the molecule is symmetric in nature.

Now peak at $\delta_\mathrm H=2.5\ \mathrm{ppm}$ is splitted into a triplet and these correspond to the protons attached to your carbonyl, resulting in the most downfield shift, near $\delta_\mathrm H =1.5\ \mathrm{ppm}$, two peaks merge and therefore they are actually the methylene protons, and lastly near $\delta_\mathrm H\approx0.9\ \mathrm{ppm}$, the triplets correspond to methyl proton peaks and thus we arrive at the final most likely possible structure as, 



Analysis of the splitting pattern to find out the $^3J$ values, I leave it to you...