Re: maybe i need to check it with a tach?

[jerry-ahrens]

The two connectors on the left are aftermarket for sure. If you are using one of those, do yourself a favor and toss it! Just my humble opinion… Not sure about the third. The fourth is an OMC I believe. Try sucking air through [on the barb end] the one on the right [the OMC connector]. If you can pass any air through, then the o ring is hard and needs replaced, or replace the whole thing for $8 at the dealer.
The gray hose on your fuel pick up on the tank head looks just like the old quicksilver hose that I mentioned earlier. That’s certainly not original, and should be removed and checked inside. I’ll bet you have a vacume leak somewhere in that system. The fuel pump can’t do it’s job if air is being drawn in where the hose connects under those zip ties, or if air is being drawn into one of the fuel coonectors o rings. Does your fuel supply hose [from tank to engine] look like the gray one in the pic? If so, better have a look inside.
The fuel cap gasket, as well as the tank top gasket, will have no effect on the engine, besides a fuel leak in the boat. Of course they both should be replaced.
I always think of the vacume side of a fuel system, the same as drinking soda through a straw. If you poke a hole through the straw, above the liquid line, it will be more difficult to suck [vacume] the soda through it. If the hole, or leak, is to great, it will be impossible! A good test for this is to use a nice new [flexible] piece of clear tubing between the fuel pump and carburetor. Once the engine is ran up to full throttle, to clear out the air pockets in the test hose, there should be no bubbles present in the clear tubing. If there are, then you have air being introduced into the vacume side of the system, somewhere. These are just some things that I’ve ran into over the years.