Both sides should tighten equally using the adjusting bolt that is attached to the plate the gearbox is bolted to as
the pulley mounted to the gearbox shaft holds both belts in place.
Which rear mower describes your V belt drive on page 121 of the owners manual; Figure 3 or Figure 4???
Obvious question - do you have the correct belts? I think mine takes a pair of 68" belts. When the gearbox is adjusted it tensions both belts at the same time.
EDIIT TO ADD - leonz beat me by less than a minute
i don't have an owners manual and both the belts are the same when i removed the old ones one was loose also pulleys dont seem to have any excess play in them
Obvious question - do you have the correct belts? I think mine takes a pair of 68" belts. When the gearbox is adjusted it tensions both belts at the same time.
EDIIT TO ADD - leonz beat me by less than a minute
I thought this was posted but apparently it was not;
Both V belts must be from the same lot number.
If both V belts are not from the same lot number
the rubber ingredients and the batch of nylon
or kevlar thread will not be the same and the
V belts will not show the correct amount of
deflection and act the same and tension correctly.
I hate to ask you to do this but you need to remove
V belt the guards and then the V belts and look for the
lot number on the wide section-the top of the V belt carcass
to find the lot number.
If the lot number is not the same the mower will not work
well and will work poorly and that will not help you in your
mowing chores.
You will need to return the V belts and ask for two identical
V belts with the same lot number.