RPM drops briefly and new prop

[Wakeboarder141]

1990 Bayliner Capri 1850 4.3 Carb Alpha One

New boat to me had a 21 pitch 3 blade prop which was very slow out of the hole, and only running 3900 RPM WOT according to the tach at 40mph. Replaced it with a 17 pitch 4 blade. Much faster out of the hole, and now the tach shows 4100 RPM at 34mph. When running at WOT, I would occasionally get a very brief, but noticeable drop of maybe 200-300 rpm that it would then take a few seconds to recover from and climb back up. This only happened with the throttle totally pinned. If I backed off to 90% or so, it would still be 4100 RPM and never did the RPM dip. It also feels like it sometimes bogs down slightly when going from 90% throttle to fully pinned. Instead of gaining RPM, it would drop slightly and then recover to 4100. I didn't drive the boat enough before the prop change to know if that RPM drop happened before. The motor was replaced by the previous owner with a carb motor, and they installed an automotive fuel pump instead of a marine setup. Is it possible the tach can be wrong? It seems to me that I should have gained more RPM with my prop change, and I don't know why it is bogging down. I am thinking possibly a fuel delivery or electrical problem? Anyone have an experience like this before? Thanks.

 

[Scott Danforth]

you have another problem. with the 17p you should be spinning much higher RPM. start by looking at the fuel system

 

[Wakeboarder141]

you have another problem. with the 17p you should be spinning much higher RPM. start by looking at the fuel system

Any suggestion where to start? Would you suspect that automotive fuel pump isn't up to the task? It starts and runs great, and will rev higher in neutral, just won't get higher RPM under load.

 

[Scott Danforth]

the automotive fuel pump is a bomb waiting to explode

your are running out fuel at the top end. so start at the tank, clean everything until you stop at the intake manifold.

could be the anti-siphon valve is plugged
could be the filter in the pickup tube is plugged
could be the filter head is plugged
could be the inlet filter at the carb is plugged
could be internally there is items plugged in the carb.

 

[Wakeboarder141]

start at the tank, clean everything until you stop at the intake manifold.
.

I will start there and report back. Thank you very much.

 

[Rick Stephens]

Also consider whether your tachometer is accurate. Might test it with a shop tach.