Thank you for the question XXXXXXXXXXXX,
Does the engine shake only when in gear? If so we need to take a look at the prop and propshaft to see if either is bent....
If our line of thinking is that the engine is missing and low on power, then we first need to perform a compression test. On four stroke engines I much prefer a leakdown test (The leakdown test will work better with the valve system of the four stroke motor. Also a ignition spark test will need to be performed to prove the integrity of the system.
If the issue does not come to the surface (I believe it will when these tests are completed), then we need to perform a cylinder balance test by unplugging the injectors, one at a time, with 25% power on the engine. This test will prove that each cylinder is producing power and running. This test will certainly tell if any cylinders are not pulling their own load.....
We can perform the ignition spark test without any special tools....Just remove all the spark plugs and ground them to the block, now turn the engine over and observe each spark plug sparking to confirm that each is operating properly...
The compression test should come out: all cylinders being fairly even in ultimate pressure generated. And no more than a 10% difference between the highest and the lowest. I would expect the compression to be around 100 psi on this engine.
I am thinking that you are not running on all cylinders....Do the tests and the issue will come to light....
Marine Mechanic
21 Years Experience as Mechanic, 18 Yrs Boat Yard Owner/Mechanic, Factory Certified