What does a necchi bf nova cost now a days vintage

This may be an option. http://pages.sewing-machine-manuals.com/173/PictPage/3923732159.html

Sometimes there is a 'vintage website that has free manuals, but i do not have the website.

http://www.fixya.com/support/t9472526-necchi_alco_a709_3c_06906_made_in https://www.ebay.com/p/Vintage-Necchi-alco-A709-3c-Sewing-Machine-With-Case-Made-in-Japan/2197773494

Back when the internet was first made 'Free" by it's inventors, a friend purchased a machine, made it the late 19th century, with a horizontal bobbin. She found it's manual, online! Ergo, you can find your Necchi's manual online!

Looking at pix online, I'd guess perhaps 1950's (which one of the links also suggests).

Necchi BU Mira Sewing Demonstration Comparison to Pfaff 130

NIFTYTHRIFTYGIRL Part 1 of 2 NECCHI BU MIRA SEWING MACHINE WITH WONDER...

Necchi Sewing Machine eBay

Necchi BU Mira

Value is very subjective. Boils down to what a buyer is willing to pay for the machine, its condition, and included accessories.

...

uneven thread tension between upper and lower thread.

Necchi BF is a straight stitch machine
Necchi BU is zig zag machine
Necchi BU Supernova had cams for additional stitches

Try http://www.sewingstyle.com/Necchi-Sewing-Needle-Plates.html - the BU Nova is a 50's era machine so you may have to hunt down a second hand spare part or see if one of the other model plates will fit this model.

Also there are 3 or 4 Necchi Italian machine owner groups on Yahoo, I'd suggest that you join the one called Necchisewingmachineclub and then check out any posts on spare parts and see what you can glean there. Just read the group rules as often they like members to "share" information and don't like newbies to drop in and post a question then disappear. Or ask the same question on several groups (crossposting).

I know that many Super Nova owners love their machines and highly regard them.

Good luck tracking down the part for yours.

It is quite common for some sparks to be seen in the armature but id it is excessive you may need to replace the carbon brushes if you can get them

http://www.sewingstyle.com/Necchi-Sewing-Manuals.html