Slight vibration at 1300 RPMs

[charlg]

Dropped the boat in the water and noticed a slight vibration, not a skip, at 1300 RPMs while underway. Got to the dock and one at a time put each motor in neutral and increased RPMs to 1300 and the vibration (felt through the floor) only existed from the port motor. Only changes in the spring were new plugs, wires, cap, and rotor.

Had my mechanic come back and he replaced the new cap and rotor. He said he was getting shocked when he touched the side of the cap. That didn't work. He checked the firing order, and said it was right. Then he started removing the new plugs with only 2 hours on them. 7 out of 8 were completely black, not wet, except for 1. 1 looked brand new. He also did a compression check on some of the cylinders and they were +/- 150. Nothing helped.

Also, at WOT port motor will only go to 2500 RPMs with drives down. Last 2 years this motor would only get to 3500 RPMs (no one can figure that out). If you trim the outdrive very slightly, RPMs will increase, but props don't seem to bite.

If I have to go another year with problems, I'm going to have to get out of this boat.

 

[ardeprint]

Have you checked your drive for play. Too much play or damaged actuator joints could change the position of your trim. Or, causing your drive to move up and down creating cavitation on your props. It might be far fetched, but take a look.

 

[charlg]

Had the compression checked. Mechanic said possible head gasket? The heads were just done at the end of last year? How do these numbers look?
160 185
155 190
150 165
170 160

 

[kevin27]

Those numbers are not real good to be honest. If this engine has only gone 3500RPM WTO for two years and now 2500RPM with those numbers you may be better off with a replacement long block engine. Are these engines raw water cooled(saltwater)? If it was me I would look into a complete reman engine.

 

[KayKook]

Sell it now while it is still running.

 

[charlg]

Looks like my only 2 options are a new engine or new boat. Can you fix Bayliners? Maybe I should switch brands.

 

[techmitch]

A cylinder leak down test should be the next step. Are those compression test numbers wet or dry?

 

[charlg]

Dry.

 

[techmitch]

The compression test should also be done wet, IE a good squirt or two of engine oil in each cylinder before each cylinder is tested. Also, all spark plugs should be removed during any compression test and the throttle should be held wide open.

 

[dpmulvey]

Your problems started at least 2 years ago. Those engines should spin up to 5K at WOT.

 

[techmitch]

Your problems started at least 2 years ago. Those engines should spin up to 5K at WOT.

Agreed

 

[Gunn]

Are your engine's fuel injected or carbed? Plugs may indicate running too rich; have you checked the carburetor? What about the choke; is it opening fully?

 

[charlg]

It is fuel injected. I bought a spark tester to try, and will have another compression test done.

Also got another opinion that said to check back pressure from the exhaust.

Keep the ideas coming. I appreciate it.

 

[Bob & Sue]

Getting another compression test done wet like prior post states is a good start you really need to do a leak down test to determine what the problem is even though your compression test showed 150-190 that is within mercs specs service manual quote: Minimum recorded compression in any one cylinder should not be less than 70 percent of the highest recorded
cylinder. I would prefer to see them within 10%. Here's a good link on how and why to do compression and leak down test you may have something else wrong in the fuel or electrical system causing low rpms.

http://www.perfprotech.com/blog/articles/marine-engine-condition-2