fuel pump or wrong prop?

[countvlad]

i purchesed a new ss prop(stiletto advantage) same pitch with the old one.... but the new prop dropped my rpms from 5800 down to 4600...

so today i took the boat back out and bypassed the filter and gained 200 rpms up to 4800 trimmed down/in...

once i started to trim the motor up/in the rpms jumped up to 5400.... unfortunately i was by myself in the boat and couldnt pump the bulb to see if there's any change...

could the fuel pump be going bad and cant provide enough fuel ???... im still confuzed by the aprox 600 RPM jump and shouldn't i get around 5800 rpms before trimming up/in?
thank you

 

[jtexas]

Re: fuel pump or wrong prop?

what kind of motor we talkin bout here?

If the fuel pump is starving your engine for gas, in my experience, I expect to hear it misfiring. Course, it could be that I just haven't experienced a motor like yours.

couple possibilities that come to mind:
* new prop has more cup.
* new prop is out of balance or the blades aren't consistently pitched.
* new prop is larger diameter.
* tachometer malfunction.

 

[countvlad]

Re: fuel pump or wrong prop?

what kind of motor we talkin bout here?

If the fuel pump is starving your engine for gas, in my experience, I expect to hear it misfiring. Course, it could be that I just haven't experienced a motor like yours.

couple possibilities that come to mind:
* new prop has more cup.
* new prop is out of balance or the blades aren't consistently pitched.
* new prop is larger diameter.
* tachometer malfunction.

its a 120Hp 1989 Johnson...

i think i might of got my answer: the prop is so much more efficient than the old beat up prop.... i dont think there's anything wrong with the new prop, its same diameter, same pitch... just much better... and the tach works fine

im still confused the way to set or find your WOT: should you reach WOT with the motor trimmed in or out?

i'm going to install a gauge today and check the vacuum on the fuel pump... 3-4 inHG or arpox 40-50 inH2O is what im looking for right?