Christian: There is many variables you are asking. The little Sears is ok I used one for years for priming and such. Moved on to a HVLP unit from Eastwood I really like that unit.
As for thinning, house paint use about a 1 to 4 ratio. For automotive paint that depends, are you using a hardener? The secret to all this is do not be scared and the trick I like to use is this. Get everything mixed up and then practice on a piece of plywood, scrap steel, or anything that is handy. Shoot your paint and practice remembering to give it time to flash between coats (get tacky)once you see what you got move on to your parts.
The pot time (paint in your sprayer) will give you plenty of time to practice. Plus you can add more paint or thinner as needed. Practice is the key here my 1000 looks good but my next one will look better!
Guys I am very fortunate on my plugs since I do not use my 1000 and 1650 very much I have been running Marathon 92 octane gas in these guys with the plugs staying clean, of course being tuned up and all helps a lot also.
Blackend sparkplug (noticed I did not say oily) tip generally means too much fuel being dumped in combustion chamber, or letting the little guy sit there and idle way to long, or possibility your plug wire is breaking down.
Pops