  Context: Submillimeter galaxies (SMGs) are distant, dusty galaxies undergoing
star formation at prodigious rates. Recently there has been major progress in
understanding the nature of the bright SMGs (i.e. S(850um)>5mJy). The samples
for the fainter SMGs are small and are currently in a phase of being built up
through identification studies. Aims: We study the molecular gas content in two
SMGs, SMMJ163555 and SMMJ163541, at z=1.034 and z=3.187 with unlensed submm
fluxes of 0.4mJy and 6.0mJy. Both SMGs are gravitationally lensed by the
foreground cluster A2218. Methods: IRAM Plateau de Bure Interferometry
observations at 3mm were obtained for the lines CO(2-1) for SMMJ163555 and
CO(3-2) for SMMJ163541. Additionally we obtained CO(4-3) for the candidate
z=4.048 SMMJ163556 with an unlensed submm flux of 2.7mJy. Results: CO(2-1) was
detected for SMMJ163555 at z=1.0313 with an integrated line intensity of
1.2+-0.2Jy km/s and a line width of 410+-120 km/s. From this a gas mass of
1.6x10^9 Msun is derived and a star formation efficiency of 440Lsun/Msun is
estimated. CO(3-2) was detected for SMMJ163541 at z=3.1824, possibly with a
second component at z=3.1883, with an integrated line intensity of 1.0+-0.1 Jy
km/s and a line width of 280+-50 km/s. From this a gas mass of 2.2x10^10 Msun
is derived and a star formation efficiency of 1000 Lsun/Msun is estimated. For
SMMJ163556 the CO(4-3) is undetected within the redshift range 4.035-4.082 down
to a sensitivity of 0.15 Jy km/s. Conclusions: Our CO line observations confirm
the optical redshifts for SMMJ163555 and SMMJ163541. The CO line luminosity
L'_CO for both galaxies is consistent with the L_FIR-L'_CO relation. SMMJ163555
has the lowest FIR luminosity of all SMGs with a known redshift and is one of
the few high redshift LIRGs whose properties can be estimated prior to ALMA.
