I took off the spark plugs, and each piston is black and possibly wet. It idles fine, but when I apply throttle it makes a loud clanking sound and it does not rev, even in neutral. There is also a small amount of oil coming from the valve cover. All pistons pass the compression test with flying colors. Checked for vacuum leaks by spraying starter fluid in various places. Carbs are bench synced. If I continue to open the throttle after clanking starts, it just gets louder and the engine can die. Also the spark plugs may have oil on them. The choke increases revs, but opening the throttle has the same result. Also, it was able to run with pilot screws all the way in and the choke off.
Below is similar to all plugs.
So, are the black pistons a problem? What about oily spark plugs? Or is this just a carburetion issue because I am too lazy to pinesol the carbs? Should I upload a video for sound? Thanks.