A poppet valve is installed in the seawater portion of the cooling system of all emission controlled engines in order to control the
flow of the cooling seawater. There are two major functions of the poppet valve.
1. The poppet valve controls the amount of seawater supplied to the exhaust system from idle to wide‑open throttle.
2. The poppet valve prevents condensation at low speeds.
There are two specific spring pressures that are used to hold the poppet closed, depending on the engine model. Whenreplacing a spring, be certain to order one with the proper spring pressure rating.
Have you checked yours?
What type of boat is this? Do you also have thru hull water pick ups? Is this a SeaCore motor?
What height risers are on the engines?
I wouldn't recommend riser replacement until you have finished troubleshooting.