This isn't f-body related, but general engine related... I hope this is technical enough to be here. If not, please move it.

I have a GM 292 inline 6 (in a boat). While out cruising, I had a nasty backfire, Ever since, the engine backfires through the carb above 2000 rpm while under load. Checking the plugs shows an extreme lean condition. Timing seems WAY advanced, but I can't check it because the mark on the balancer is no longer visible.

The carb jets function properly and the float is good. I richened the carb up a TON and it's going through a LOT of fuel (almost twice as much as normal), but the plugs still show a lean condition. I've also marked the distributor and backed timing off by about 5 degrees. It still backfires above 3000 rpm and doesn't have the power it used to.

The only thought that comes to mind is that the timing gears may have jumped a tooth and the cam is a little more advanced than it should be. Are my symptoms inline with this?

PS. This motor has a timing gear set and does not come with a chain from the factory. The timing set looks like this:

 

Do this test;
Turn motor over clockwise to the mark at the balancer 0.
Remove distributor cap, slowly turn motor counterclockwise & while watching the rotor of distributor stop when the rotor just starts to move.
This is the amount of play the timing chain has in crankshaft degree's.
If it exceeds 10 you got problems.
Also try this, open throttle warm & rev motor to high r's & slam the choke plate closed with your finger.
Let it choke down to a 1000 R's still at WOT & let up on the choke plate.
Keep the trottle open to unload the motor & let the R's climb & choke it again. Repeat 3-4 times.
This cleans out the main venturi's 'specially after a backfire.
You can also sea-foam the carb .
While idling add this thru the bowl vent.(anyway to get it inside the carb)
Let it run (poorly) of of it & do the choke trick again.
It will run bad backfire& belch flames but it will clean the $h!t out of that carb.
Your problem sounds fuel related big time.