Lehman 120 engine restriction ?

[TJM]

We have noticed that our port engine surges about 500 RPM after about an hour of driving.
Wicked confused on this one.
Engine synchronizer is engaged and working. RPM gages show the port engine is 700 RPM slower than starboard (nothing new on these old gages). Speed is OK at 8 kts. After the surge the RPM are almost balanced at 1750 RPM and the engines sound smoother and more in synch with each other and speed picks up .50 kts.
Bottom has been cleaned, running gear looks fine.
All of this happens with the engine synchronizer working.

Could this be fuel related ? Filters ? Pumps ?

 

[kchace]

Surging RPMs are a classic symptom of a filter getting clogged. Changing the filter on that side would be the first place I would look especially if it hasn't been done in a while.

Ken