Just rebuilt carbs, now when throttling up, one motor tends to backfire a bit. Any suggestions?? New plugs were installed...

Backfire or engine miss???
If backfire, is it through carb or out exhaust???

Lean miss?
Accelerator pump. It's gotta squirt the instant the plates open.

ck for cracked spark plug . might have cracked one changing them . does sound like lean miss though

Accelerator pump failure usually causes a hesitation going from the idle circuit to initial acceleration.
Its basically a transition circuit that gives a brief squirt of fuel until the jets flow the fuel.

Chances are you messed up on the carb rebuild or you have a wire on the wrong plug.

I have a backfire issue too..through the carb..but it will do it when Im running..at like 3000 rpm it will backfire..then lose power almost as if someone shuts the fuel off.. but it will start right back up..

If the boat is running fine for a while and then it starts acting up, it could be your ignition coil getting hot. Your tachometer may feak out as well. Ignition coils don't last forever. I see you have a 1988 Baja. Time to change your ignition coil. Also, take off your distibutor cap and see what type of condition it is in.