I have a Honda GV 400 engine. Is there an after market carb that I can buy for it.

did you take the choke off after engine warmed up?

https://www.jackssmallengines.com/Products/HONDA/Ignition-Part/IGNITION-COIL

You could try the link below but they aren't cheap.

http://www.ereplacementparts.com/honda-g200-type-peafvin-g20010000012344556-small-engine-parts-c-37657_37658_37986_37678.html

Contact Bobs used cycle parts in Phoenix Az or your local parts dealer.

honda dealer and eBay come to mind

need a carburetor for a Honda f 400 tiller. where can I buy new or used?

sounds like it's time to rebuild the carbs you cantry running some sea foam cleaner thru the gas but you will prob. have to do the carbs

Sooted plugs proves tht the air-mixture is too rich.(too much fuel going to the combustion chamber)
You have to set the air-mixture screw on the carb/s.
Clean the carb/s, replace 'inline' filter , if you don't have one ..install one, reset float level.
It's better to also check the valve clearance too.
Fit in new plugs (gapping them to specs.) and run the engine to its normal temperature thn set the air-mixture screw...standard setting....all the way in i.e. tighten it slowly to the end making sure not to damage the tip of the screw, thn 2 and half turns outwards to 3.
Start the engine....run to normal temperature and thn reset the air-mixture screw +/- either way.
Hope this helps!