Starboard engine runs at lower RPM than port

Rick Young

Rick Young

Member
Jun 2, 2008
57
Katy, TX
Boat Info
Boatless
Engines
Boatless
We have a 2007 320DA with twin 350 engines and V-Drives. Last fall the starboard engine would not run up to full RPM like the starboard engine. Both engines seemed to run fine at lower RPM's. We thought it might be electrical as we were to the point of needing new batteries. All four batteries were recently replaced and we took the boat out this past Saturday to see if that corrected the problem and it did not. At WOT the starboard engine still ran approx. 700 rpm slower than the port engine. I will be replacing the fuel filters next as part of my annual spring maintenance program so I am hoping a dirty filter on that side may be the problem. Any other thoughts and what else I might take a look at as a cause for this problem?
 
I have personally seen at least a half dozen situations where a bad cap and rotor caused this type of problem. I recommend using Merc Marine parts, particularly for ignition components but some here recommend using automotive parts. I guess taking that chance or not is up to you. A set of plugs and cables would not hurt either but I would start with cap/rotor. Merc makes a kit that includes both for around $75/ea. They had some problems with them a couple years back but they seem to be of higher quality now. I just installed a set for a friend a month ago and they were fine.

There are all kinds of other possibilities, fuel filters, problem with outdrive just to name a couple but I would replace the cap/rotor anyway as a first step if you have no idea when they were last done. It seems like people on our lakes tend to skip these for some reason but they do corrode.
 
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My Port side 350 mag runs 200 to 300 rpm lower than the starboard. I did full tuneups as well as new filters and new fuel pumps.

I am going to put new plug wires on both engines in the next week or two and see if that solves the problem.

When my boat was surveyed two years ago, I did confirm that my tachs were accurate. The surveyor hooked up his timing gun to each engine as we cruised around the lake, this was how we confirmed my tachometer readings at the bridge where the same as what he was getting with the gun (ignition timing light gun).
 
Rick Young said:
Dirty bottom - STB - 4300 rpm - Port 3600 rpm
Click to expand...
That's much lower than mine... I'm getting 5,000 rpm (which is the correct WOT for our setup) on my starboard and 4,500 rpm on my port.
 
Different boat and engines, but I had a similar issue last year on my '310. Starboard was lagging behind port above 3000 RPM, just didn't have the guts to push the boat. I started replacing the plugs and wires (couldn't get to the outboard side of the motor) and it mostly cleared up after just doing the inboard side. New plugs and wires going on this year, and I expect it to be resolved. On the other hand, it wouldn't hurt to do a compression or leak-down test to be sure everything is solid.
 
Found a fouled plug on the STBD engine. Replaced all eight plugs and it seems to run fine now. Actually runs a couple of hundered rpm's better than port engine now. Going to replace the port plugs too and see if that evens them out.
 
Rick Young said:
Found a fouled plug on the STBD engine. Replaced all eight plugs and it seems to run fine now. Actually runs a couple of hundered rpm's better than port engine now. Going to replace the port plugs too and see if that evens them out.
Click to expand...

Change your cap and rotors too if they have not been done in the last few years. Trust me.
 
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