unburnt fuel out of exhaust

[motor dumb]

I need help with my 1984 50HP Mariner. When I am running my boat I start out at full power but after only a few minutes it sounds like I drop a cylinder. The motor doesn't miss or anything its just like I loose power. but I can shut it down for a bit to fish and when I start it back up it does the same all over again. I have checked compression while sitting in yard and all checks out fine. Also after I get home and after it sits a bit I get black unburnt fuel out of exhaust. I have also checked plugs and wires and coils and all seem to be good. Any help would be appreciated . THANK YOU ALL IN ADVANCE FOR ANY HELP

 

[Iceman66]

Re: unburnt fuel out of exhaust

You may have bad windings on the "high-end" of the stator, also check your rectifier.

Here are some good words( page 81 - 82) : ISSUU - CDI Electronics Practical Outboard Ignition Troubleshooting by CDI Electronics

 

[CharlieB]

Re: unburnt fuel out of exhaust

You need to determine if it is a fueling or ignition problem.

Air sucking into the fuel system can cause power loss as engine speed and load rises past what the fuel flow will support then you feel a loss in power as the float bowls begin to run low.

A simpplel test, replace a fuel line to the carbs from the fuel pump with a clear hose. Run the motor at a moderate idle and watch that hose to see if any air bubbles keep passing thru proving air sucking in somewhere before the fuel pump.

Remember to remove that clear hose once finished as it is NOT recommended for fuel use.

Ignition failing once up tp temp needs to be checked at temp while the condition occurs.

An inductive tach or timing light and a DVA meter will help.

Test stator voltage to each switchbox to see if the voltage reduces.

Use either inductive unit to see if spark or RPM of each cyl is affected.


I'm betting in an air leak into the fuel system.

 

[motor dumb]

Re: unburnt fuel out of exhaust

Thank you both. CharlieB My fuel pumps are attached to the sides of my carbs do you think I could be getting air from my fuel tanks to the pumps

 

[CharlieB]

Re: unburnt fuel out of exhaust

Those are a bit harder to test with no hose after the fuel pump. Closely inspect every inch of fuel hose, especially the fittingss. Any split or crack can be suspected of allowing air to suck in,same with the hose to fitting connections, loose clamp, if the hose can be 'turned' or rotated on the fitting air can suck in.

The quick disconnect fuel hose fittings to the motor and to the portable tank, the O-ring can age and allow air to suck in past, yet still seal well enough there is no leak when squeezing the primer bulb.

If your float bowls have a drain plug, set the trim to the angle you run at WOT, pump the primer till the carbs are full, Use a pie plate or something you can catch all the fuel from one carb. Drain that carb, pour that fuel into anything you can mark sort of like a measurement. Baby bottles are great for measuring small stuff.

Run the boat and when it 'drops' a cyl turn the key off, do NOT throttle back, just shut it down. Then drain a carb and compare measurements. We are only talking a teaspoon or two difference, and it is possible it could be either carb, or both carbs if the air leak is before the 'Y' in the fuel hoses.

 

[CharlieB]

Re: unburnt fuel out of exhaust

Unburned fuel out the exhaust can be caused by a number of things.

Float set too high and a cyl running too rich
Float failing/needle failing to seat flooding
Poor spark/ignition failing to fire a cyl at speed

Need serial #, # of Cyls, # of carbs to know which engine and ignition you have.