Two things will limit RPM and power.
Fuel flow. If there is any type of restriction so the fuel pump cannot pump a strong stream. Even a zip tie too tight holding the cables, wiring to the fuel hose and pinching the hose, to a poor sealing O-ring at a quick disconnect fitting allowing air to suck into the system. Install a clear hose in place of the fuel hose to the carbs. Run the motor and watch for any air bubbles passing through. If found, there is an air leak, find it and seal it up. Remember to remove that clear hose once finished, it is not designed for fuel use, only as a very short term testing tool.
The second thing that will limit power is limited spark voltage. Voltage must be high enough to throw a hot blue arc across a 7/16 inch gap. There is a low speed windings and high speed windings. There is NO RELIABLE Ohms test to prove high speed voltage. A DVA Meter or a DVA Adapter for use with any Volt/Ohm meter must be used to verify high speed stator voltage.
NEVER assume any electrical parts is at fault without PROVING the fault. There is NO RETURN on electrical parts, some get very expensive.
CDI Electronics has a very good Ignition Troubleshooting Guide available FREE
CDI Electronics Practical Outboard Ignition Troubleshooting
It covers ALL merc ignitions, find yours and follow the directions, test and be sure what is or isn't working correctly.
Repost your progress and any questions you may have.
