Boat parts & accessories for Johnson, Evinrude, Mercury, Mariner, Force, or Yamaha outboard motor, & Mercruiser, OMC stern drive. 

  Tips On Boat Motor Problem Solving

The following is supplied by TMS as a brief reference guide only. TMS assumes no liability for loss, damages, or incorrect diagnoses resulting from usage of this information.

There is a golden rule to any gasoline powered motor whether 2 stroke or 4 stroke. When experiencing and looking for the cause of a bad running motor, first course of action is to know that compression, fire, and fuel, are the three required elements for a motor to run. If any of the three are lacking, you will obviously experience a problem.

The fuel system on an outboard, being a breed of its own, can confuse the average person during diagnosis. For example: an anticipated fuel problem, can sometimes actually be ignition related, or even compression related. This is why it is always a good idea to check the simple things first. If fire is good to ALL cylinders, and compression is within range showing no more than 15 lbs. difference from cylinder to cylinder, then the problem would most likely be fuel related. Either stopped up carbs, weak or bad fuel pump, low or no compression on cylinder supplying pulse to fuel pump, a bad squeeze bulb, tank not venting, stuck anti-syphon valve on tank, loose fuel line connections, etc., could all be causing a problem you may be encountering. Upon diagnosis, just remember to check all three required elements before you simply start replacing parts by trial and error. We receive many inquires on how to test the Johnson/Evinrude VRO oil injection system. Below are two methods to troubleshoot problems with them.

VRO oil flow test VRO troubleshoot chart

Outboard Ignition Parts can be the "tough" problems to figure out for the average person, and they can cause a very wide variety of symptoms. Outboard ignition systems are very different than those of the automotive type. What is causing your motor to not start, cut out, run good to a specific RPM, overall run poorly? First make sure to check that timing is advancing properly and fully, there are no broken damaged or bare wires, nothing shorted out, no rogue arching spark around plug wires, etc. If you are having an ignition component problem, special equipment is usually required for testing most system parts such as stators triggers and power packs, so it is normally advised to let a qualified technician diagnose these problems for you. Do not listen to advice from the guy next door of "try changing this, or try changing that". Trial and error replacement of ignition parts can get VERY EXPENSIVE if you don't get it right the first time, and dealers or parts houses WILL NOT take returns on electrical parts! That could easily wipe out the original idea of trying to save a buck. If you are dead set on diagnosing and repairing the ignition system yourself, and you have access to required testing equipment, then you might want to check out the following links that may provide valuable information to help you along.
Mercury/Mariner Outboard Motors:    Troubleshoot Guide Test specs
Johnson/Evinrude Outboard Motors:    Troubleshoot Guide Test specs
Chrysler/Force Outboard Motors:    Troubleshoot Guide Test specs

Compression is the heart of an outboard. If one or more cylinders are damaged due to improper timing settings, lean fuel condition, overheating, or stuck rings from carbon buildup, your outboard is about to become history. Does a compression test alone always determine that my outboard is in good shape? Suprisingly the answer is "no". A good compression reading does not completely determine internal conditions. To correctly diagnose condition of pistons, rings, and cylinders, a leakdown test should also be performed. Using a quality leakdown tester, each cylinder should not show more than 10% loss. If any do, there is certain to be an internal problem in the making. For example, a compression reading of 120 lbs would be viewed upon as a good cylinder by the average person. This may or may not be the case. If a leakdown test of the same cylinder indicates say...20% to 25% or more leakage, it would be a good bet that excessive wear, scored cylinder, the cylinder scratched, and/or the rings stuck or damaged in a way that does not show up with a simple compression check. In any case, wear or damage is present and advisedly requires attention before further damage and/or exceptionally poor performance and efficiency results. Most common effects from diagnosis of excessive leakdown would be in the idle range of the motor. If all other motor functions check out attempting to diagnose an idle problem, then its a good chance that blowby is the culprit. It is advised that leakdown tests should be performed at the beginning of the compression stroke (rings just above exhaust port opening, which is the critical point), and NOT at TDC. Make sure you have flywheel locked down securely when doing a leakdown test. If you try to hold the motor from turning with a socket and breaker bar, you are putting yourself in a dangerous position should the breaker bar slip from your hand. Purchase a compression gauge and leakdown tester to go in your boat's toolbox and check cylinders a couple of times a year to ensure there is nothing unusual going on internally. Using proper grade of gasoline and outboard oil is essential to preventing internal problems. Proper winterization procedures will also do wonders to prolong the life of the internals of your outboard. If compression check shows more than 15 lbs difference on any given cylinder from highest reading of other cylinder/s, or if leakdown test shows excessive loss, you have a problem in the making! DO NOT continue running the motor until problem is diagnosed and repaired! It could result in causing much more internal damage than you already have.

Cooling system is critical for an outboard to live. It only takes a few seconds, especially running at high RPM's, for an overheat situation to kill an outboard powerhead. Do not rely on a water hose to check your impeller. A water hose will pressurize the system, overriding the impeller, and giving a false sense that impeller is ok. Replace your water pump impeller EVERY TWO SEASONS regardless of how many hours are put on the motor. An impeller will go bad from dry rot due to non use, more so than ever wearing out from overuse.

Winterization is commonly ignored by many when it comes to outboards. "The water drains itself so why bother"? By spending an hour with your investment, you can prevent 90% of the common problems that occur with an outboard motor.
In the fall when you figure your out on the water for the last time for the season, add fuel conditioner to your fuel tank and let it circulate throughout your entire fuel system before you put the boat up for winter. This will help prevent stale gas and varnished carbs in the spring. Following this procedure will eliminate the need to run the motor out of gas (which you cannot do with todays oil injected motors). Also top off your fuel tank at the same time to prevent condensation from forming in the fuel tank.
When you get the boat home, remove the air box cover and with the throttle in wide open position, spray fogging oil in the carbs. Remove plug wires and crank the motor a few revolutions. This will distribute the oil through the internals of the powerhead and protect them from corrosion while stored out of use.
Questions? Feel free to visit our message board about anything you might need help with, and we will do our best to provide you with the information you need.


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