What could cause this and how hard would it be to fix?

?????

Im looking into a 280zx turbo that has a burnt valve

What is a burnt valve?

I believe its when the valve no longer seats/seals correctly

Valve lash set too tight or eroded valve seats from unleaded gasoline could cause a burned valve. When the valve doesn't seal completely hot exhaust gases squeeze through and erode the valve and valve seat material or warp the valve from uneven heating. Then the valve doesn't seal well at all, you get compression loss and it's all down hill. If the valve is burned the valve seat is probably bad also. You would have to remove the head, replace the valve and install new valve seats. If you have one burned valve, unless you made a mistake setting valve lash on a single valve, it's probably time to rebuild the head.

Is it possible that something else could be mistaken for a burnt valve?

The main sympton of a burnt valve is the cylinder isn't firing and / or smoke is coming out of the exhaust. Upon inspection the compression value is either very low or there is no compression. One could suspect piston ring failure but that would make noise and you would hear it. Either way, it's time for the head to come off.

I actually thought I had a bad valve due to intake/exhaust misfiring and it turned out the problem was my carbs were set too lean.

The cylinder was skipping would that be a burnt valve or something else?

A leak down test would identify a leaking valve. At least do a cylinder pressure (compression) check. You're jumping way ahead to an expensive conclusion. You might just need a simple tuneup - plugs, rotor and cap.

Is it possible that something else could be mistaken for a burnt valve?

Yes, a worn or scored valve guide. Either way, the head has to come off for the repair.

Zed Head is 100% correct. Right now a leak-down test (differential compression test) is your friend.

Edited September 14, 201212 yr by Gary in NJ