Hi,
I know
NOTHING about this particular model so just taking a
GUESS.
Ok?
Many modern engines, including several Honda cars and outboards utilize a device inside the
intake manifold (see link below) called an "air. runner control" or what Honda calls the "air bypass".
The air bypass is basically a moveable air "dam" inside the intake manifold to divert and change the quantity and velocity of air going into the cylinders at certain operating speeds. This action improves the way air mixes with fuel in the cylinders to improve performance and efficiency.
The "dam" component is typically made of a plastic arm or blade controlled by a vacuum diaphragm mounted on the exterior of the manifold.
This diaphragm is controlled on your particular model by an electric solenoid (item 16 in the link)
Your description of the surging occurring between 2500rpm and 4,000rpm is what makes me
GUESS that the air bypass might be faulty.
Things that
MIGHT go wrong with the air bypass:
The solenoid
The wiring to the solenoid.
The vacuum supply or control tubes (items 17 and 18)
The diaphragm (integral to the manifold and not sold separately)
Possibly any diaphragm linkage...I don't know. (this is all a
GUESS remember!)
The dam arm or blade might be broken or has come loose inside the manifold. This is typically not accessible but might be seen with the manifold removed. At any rate, it would have to come with a new .manifold.
That's all I have.
If this isn't accurate I apologize.
Just tossing it up since no one else has answered you yet.
Good luck.
