On my boat, (and many others on the lake), i've noticed that there's a lot of black stuff on the transom.

I did a tune up this year, rebuilt the carb, and i've checked the plugs both at cruising speed (shut her off at 3500 rpms, kind of a wierd feeling) and at idle, and everything seems to be tuned well. I've noticed that sometimes I get the black stuff on the back of the boat, and sometimes I don't. One ride will do it, it seems.

I noticed a lot of other performance boats with the same issue, and a lot with clean transoms.

What gives? Should I let the boat warm up more? Should I idle for longer after running hard? Is it an indication of something going wrong?

Wax it and use Swipes

Even FI boats get some soot under certain conditions, it is almost impossible to stop it, just find a good way to clean it-like Swipes.

It is running too rich. See the post below on Dominators on a 540. Regardless of the type of carb, the process of dialing it in is the same.
We might get you close if you provide your engine specs and current carb number and set-up like PV#'s and jets.

It's a stock 502 mag, rebuilt about 160 hours ago. I rebuilt the carb this winter, and verified that the jets are stock, and I set the float levels appropriately, according to the clymers or seloc manual that I have.

Think mixture varies enough by temperature to cause this in some conditions but not others? Also, it seems more prevalent from the starboard side than the port side, but sometimes I take the boat out and don't get any soot, sometimes one run and it's pretty dark.

Ok...20 questions. Why did you rebuild the carb? Was anything changed besides the gaskets? Are you sure the gaskets we the right ones for your specific carb #? Have you checked the float levels with the boat running/idling?

Technically, you should NEVER wax a transom. There is something about wax that seems to attract soot / smoke etc. from even the best / cleanest running engine.

If you wax, soot will stick. Strip the wax if you can, and it should go away.

Get longer exhaust tips.

Strip the wax and it turns yellow.Been there..... BOB

I rebuilt the carb because the oil analysis I had done in the fall showed a bit more fuel in the exhaust than i'd like. I wanted to make sure there was nothing wrong internally with the carb.

The gaskets I installed matched the ones that came out, and it looked like the carb had never been rebuilt before.

I don't think it's possible to check the float level with the engine running, it's a weber / carter / edelbrock, the only way I know to check the float levels is to take the carb apart and use a ruler or a machinists scale.

When replacing a carburetor on an HP500, I found that I needed to adjust the idle air screws on the replcement Holley that I had purchased. The performance Holleys and Barry Grants come with four corner idle air adjustments. As I recall, I used a vacuum gauge and screwdriver to make the adjustments. I think most carb suppliers will say that they have backed out the idle air screws a full turn and a half from all the way in and that fine tuning was up to the user.

I was always able to make the black transom go away.

Don't know a thing about the Weber/ Edelbrock/AFB unit and how to make such adjustments. Perhaps Edelbrock will offer some advice.