an alarm that comes up at half throttle sounds like fuel obstruction, BUT this motor shouldn't have the vacuum switch that would sound the alarm.
there is a host of things that could be the problem....but to be clear the issue is that it is hard to start and doesn't idle well?
1 - double check the primer solenoid woks.
a - disconnect the tiny hoses coming from the solenoid and pump primer bulb hard ....check for NO fuel coming out of the two tiny nipples.
b - with primer bulb pumped hard....turn key on, push key into activate solenoid....verify that fuel DOES come out of solenoid.
2 double check that ALL butterflies of the carbs are fully closed at idle , if not , if just ONE butterfly is slightly open it isn't going to idle right.
a - if not fully closed you'll have to do a link/synch procedure for your model motor.
3 - test the functionality of the fuel pump.
a - hook up a TEE at the fuel inlet and attach a vacuum pump/gauge....when you get the motor running the pump should draw 3inhg at idle...if more than 5 inhg, you have a fuel obstruction before the pump.
b - if you have less 3 inhg, then you may have a pump issue.
1 - check the pulse limiter is not fouled up (goes between the engine and the air motor inlet on the VRO)
2 - remove VRO from motor....and follow the testing procedures from boatpartstore<DOT>com/vrotable.asp
4 - Adjust the floats on the carbs according to the service manual...not sure if this motor is plastic carb or metal carb...nor do I know if these carbs have an idle mixture screw....if they do then find out the original number of counterclockwise turns from fully seated and try to start the motor..
to start the motor the steps should be like
1 - pump primer bulb hard
2 - turn key to ON and push in for 5 seconds or so
3 - push throttle to slightly more than 1/4 up (in neutral)
4 - turn key to START whilst holding in key.
5 - once motor is started hold key in until motor starts to die, then let it go. Let it warm up for a minute.
you may need to push throttle up more than 1/4.