US Marine Engines that are making excessive noises can be many parts inside and out. The following is a general information to help you determine the problem.
Internal engines noises related to bearings, pistons and other components will increase, decrease or stay the same as temperatures change from cold to hot.
Rod bearings, main bearings and piston pins will become louder as engine temperatures increases. This is from oils becoming thinner. There is no external fix for this.
Nominal oil pressure hot idle is 15 LBS and 35 LBS at 3000 RPM however what is more critical is the same idle oil pressure as before. Example if you had 10 LBS at hot idle you should always have that amount every time you run your boat. Your engine requires 2 LBS minimum to keep lifters pumped up.
Pistons noises decrease in noise as temperatures increase from expansion. There is no external fix for this however performance engines will always have some cold noise because they require extra piston to cylinder wall clearance.
To test which cylinder if any has your knocking try a cylinder kill balance. Removing the spark from a knocking cylinder stops combustion your noise will go away or decrease. If it just decreases that's a good sign you have multiple damaged cylinders. Look for piston destination by removing the cylinder head you should see pieces missing and aluminum flakes on your spark plugs.
You can do a cylinder kill test using your 12 volt test light. Put your alligator end on a good ground no paint. Good place is the 4 bolts holding down your carburetor. Next place one hand on your boats body not engine or you will get shocked. Now push that pointy end straight into the top of one spark plug wire at the distributor cap. Your engine will run rough and if that cylinder is your problem your knock will change.
If your engine did not run ruff once you shorted out each cylinder your test light is bad try it by touching the positive on your battery that little light should glow. You did start that engine and put on the water muffs on your out drive. NEVER run your marine engine without water running through it.
When your doing your cylinder balance test and you find some cylinders do not run rough you have a run-ability problem each cylinder should cause your engine to drop the same amount of RPMS we will get into that next.
Now do a compression check in the noisy cylinder if its low you have a bad piston. If its normal than you have a bearing problem or crossed spark plug wires. What I mean by normal compression is all your cylinders should be the same readings using 5 revolutions with your fully charged battery
Compressions gages are never correct so if you have 100 PSI so should all cylinders. It is not how much you have but equal amounts in each. A good gage will give you 150 PSI.
Stay tuned for more topics
We Offer New Installations as Well
For US Marine Engines not listed on our website call our sales department at Email Us Here. List Prices Do Not Include Hardware, Accessories or Installation.
Delivery for Rebuilt US Marine Engines
US Marine Engines will coordinate shipping throughout the 50 states - saving you time and money, and helping you get your boat back in service.
Installation for Rebuilt US Marine Engines
Professional Marine Engine Installations are available in-house. Please call for availability and specific price rates for each vehicle.