401k requires earned income, and if you convert rental income to earned income you need to pay self employment tax, 15.3%, then you can invest in solo 401k, up to 57,000.
Keep in mind that self-employement tax is taxed on all of your income at the SAME rate, unlike the tax bracket where first 20K is taxed at 10%, 20K-80K taxed at 12% etc etc. So if you convert your passive income to active income, you are paying a higher tax rate already, before you play with solo 401K.
Remember your depreciation will be recaptured whether or not you claim them or not, so you might not have enough net profit to fund your solo 401k after depreciation.
