Bought a used Honda Harmony 1011 and works well. But, even with the mulching deflector in place on the right (discharge) side, all the cut grass just flies out the left side, regardless of deck height. Would a mulching blade cure this?

Hi,
There is a mulching blade that comes with the kit. It is #72511-763-R00 (Honda Code #3812773) that normally comes with the kit. There are more than one version of the H1011 (9 as a matter of fact) and denoted on the model number tag. To see the parts breakdowns (and parts manuals) for the various models see:
Outdoor Distributors - Lawn Mower Parts
Peace,
Clarke
PS An upside down blade will also do what you allude to!