Remove quotation marks and maintain comma separator in natbib agsm

Question

I'm using natbib and the agsm harvard style for my bibliography. I would like to modify the style, such that the quotation marks for articles disappear.

I found an apparent solution in this answer, which changes the agsm style file in line 698 from:

{ format.title quote "title" output.check }

to

{ format.title "title" output.check }

However I noticed that this way of removing quotations also results in a change of in-line citation lists.

So a former

Helgeson et al., 1981, Tanger and Helgeson, 1988

now becomes

Helgeson et al., 1981; Tanger and Helgeson, 1988

I'd like to keep the comma but have little clue where to start finding the reason for this. I hope somebody can help me!

Edit: MWE

\documentclass{article}
\usepackage[latin1]{inputenc}
\usepackage{natbib}
\bibliographystyle{agsm-nq}
\usepackage{filecontents}

\begin{filecontents}{test.bib}
@article{Helgeson.1981,
    author = {Helgeson, H. C. and Kirkham, D. H. and Flowers, G. C.},
    year = {1981},
    title = {{Theoretical prediction of sth}},
    pages = {1249--1516},
    volume = {281},
    number = {10},
    journal = {{American Journal of Science}},
}
@article{Tanger.1988,
    author = {Tanger, J. C. and Helgeson, H. C.},
    year = {1988},
    title = {{Calculation of the thermodynamic stuff}},
    pages = {19--98},
    volume = {288},
    number = {1},
    journal = {{American Journal of Science}},
}
\end{filecontents}

\begin{document}
The revised HKF formalism \citep{Helgeson.1981,Tanger.1988} ...
\bibliography{test}

\end{document}