25 hp four stoke timing question

Can someone help with the timing specs for a 2004 25 hp four stroke tiller steer motor? The motor idles smoothly, but seems to backfire from the carb when you goose it. We set the timing by centering the timing marks at 0, then lining up the red timing ticks and slipping the timing belt back on (my interpretation is that this is 0 degrees btdc??). I have searched high and low on tricks for setting the timing on this motor, but with no luck. Any help or advice is surley appreciated.

[boats.net]

If the belt timing marks are aligned it will be at TDC, The ignition timing is not adjustable on that engine.
Could be low speed circuit in carbs plugged up, may need to clean carbs or could be valves not seating. Run aleak down test to check valves.