Why does my marine outboard engine cut out under load occasionally after an hour of perfect use?

Question

Engine is a Mariner90hp- 2stroke 1996

When travelling at speed, it cuts down to tick over or will only travel at approx. 5knotts for a few minutes before returning to original speed again. This happens after about an hour of working fine!

The engine has been serviced yearly and winterised for past 10years.

Answer 1

This is a hard question to answer, because it happens so infrequently.  Like the other answerer's I'd look to fuel and ignition system.  The mere fact that this is a new, and self-correcting problem would point to an ignition problem.  I guess that something could block the fuel pick-up then fall off, but I'd be looking at an intermittent short in the electrical system.  I t could be anywhere, not just in the engine.  It could even be the battery.  It'll help if you can eliminate possible causes before you take it to a mechanic.  He/she will have a difficult time getting the engine to fail at the moment he is testing a component.  It could be as simple as a spark plug failing or shorting out due to a small metal flake shorting the electrode.  So the more you check it your self the less it'll cost when you take it in for service.. . I hope it's a simple problem, ...So Good Luck!!!!!!!

Answer 2

There are a couple of things to check.First is your air vent open on your fuel tank?(It must be)Second your fuel pump diaphragm could be getting weak and stretched.To check this

you can try squeezing the primer bulb when this is happening

to see if the engine picks up speed again.Make sure you have someone else with you when you try this.You may still need to remove it to visually check it.Make sure your in-line fuel filter is clean.You could also have a stuck anti-syphon valve on your fuel tank if it is not a removable type.Good luck.

Answer 3

cuold be 1 switch box cutting off and you lose hlf your cylinders  see if you getting spark to both sides when this happens

take off the cowl and oull a wire see if you have spark   be careful  or youll get quit a jolt

Answer 4

I also suspect an ignition component of failing when it gets hot.

To troubleshoot, you need to check all cylinders for a spark, while it's acting up.

Quick way to do that:  get to a dock or shore where you can safely remove the cowl.  You'll need a plastic or insulated tool (ask for "spark plug boot tool" at a car parts store - couple bucks).

with the engine idling, pull then replace the spark plug wires, one at a time.  Give the engine a couple seconds to respond.  If the RPMs don't drop noticeably, then that cylinder is not making power.

To confirm a spark problem (vs. fuel delivery), remove the plug, reattach it to the boot, and hold it against the cylinder head while someone turns the engine over with the key (unplug all the plugs for this test, so the engine won't start).

Look for a sharp blue spark.

If you get no spark, swap that ignition coil with one on  a working cylinder.  If the problem follows the coil, replace the coil.

If the problem stays with the cylinder, suspect switch box or trigger.  You can narrow it down further with a multimeter and DVA (direct voltage adapter).  Need to know how, just ask.

Answer 5

First check for unrestricted fuel flow to carbs, check filters and screen bowl.  Do you have a water seperating filter?  replace yearly, check for water in the fuel.  If all is good on the fuel side, you have an electrical/ignition problem. Could be switchbox, trigger, or coils.  These can only be diagnosed with Mercury/Mariner  tools.  Go to a dealer