So my boat every now and then, not very often, but like if you stab the throtle, it like gives a backfire through the carb. That is caused buy the timing being to advanced correct??

Probably more like a momentary lean condition as the throttle opens quickly. What are you running? Is it a carb motor?

its a 502, that started life as a mpi, but now its a carb motor, we switched everything over, including the distribtor, plugs everything. I have a dart high rise and a holley 850 off a hp500 motor.

If the timing was advanced to much to where it was backfiring it would be doing it more often and you would most likely be hearing "pinging" while accelerating. From what you say I would expect it to more than likely be in the carb.

The HP500 carb is an 800cfm. Sounds like a lean spot as suggested. The Accelerator pumps may need adjusted or replaced or it could need more jet in the primaries so it transitions better to the secondaries.

Make sure the little plastic cam on the linkage is still there. Mine broke off the other day and was starting to open the butterflies before getting into the accelerator pump.

Like griff said. Probably the accelerator pumps. Pull the flame arresters, and have someone go from idle to say half or full throttle (engine off) and see if you get a good squirt of fuel from the primary and secondary nozzles. If its dribbly or weak, accelerator pumps probably toast. cheap fix. There is a adjustment for them too.

I would say pull the valve covers off and check the rockers or valve lash. If that isnt it I would than Check the Intake gasket or carb gasket it might of tore or isnt lined up right and your sucking air. Ive had it happen before. Also you might have crossed spark plug wires rushing to get the boat done.... Just my 2 cents

Accel pumps more than likely.

If you are still having the problem check the line in the fuel tank.. there is a rubber piece on some that pinches in as it gets older(drove me nuts till I found it)