4 stroke 50hp 1997 Yamaha outboard black smoke rough runnning

[JLarimar]

Open for some help on this one. My four stroke Yamaha began running rough while in drive and pushing black smoke from the exhaust by the prop. Shortly after the motors stalled at Idol. Could only restart with wide-open throttle to keep the engine at high RPMs continue to run rough with black smoke. Change the spark plugs recently. Also put on a new regulator rectifier. Any simple diagnostics is welcome or areas to look to track down the issue.

 

[racerone]

Possibly an issue with carburetor float/s and float valves.----Too much fuel.

 

[JLarimar]

Possibly an issue with carburetor float/s and float valves.----Too much fuel.

Copy that. Thanks for the reply. Sounds like a possible carb rebuild in my future.

 

[JLarimar]

Copy that. Thanks for the reply. Sounds like a possible carb rebuild in my future.

All the floats and float valves are in good shape. Changing out all the gaskets for good measure many were very flat.

 

[JLarimar]

Possibly an issue with carburetor float/s and float valves.----Too much fuel.

Problem is persisting. Balanced the air/fuel ratio on carbs per manual and it runs great on the muffs. The second the motor is in the water it runs really rough and gets worse with higher acceleration. Could it be bad coils?

 

[racerone]

They all run great on muffs.----Any water getting into the oil ?-----Post results of a compression test as well.----Perhaps running on 3 of 4 cylinders.----Check spark.---Check valve clearance.

 

[JLarimar]

Compression test 150 130 150 140
No water in the oil
Spark plugs have black chucky carbon build up
will check spark and replace coils by end of week
If I check valve clearance do I need to replace the valve cover gasket?

 

[flashback]

Black smoke is too much fuel, I'd go back to the carbs.

 

[JLarimar]

Black smoke is too much fuel, I'd go back to the carbs.

Copy that, going to adjust the pilot screws down to limit fuel flow.

 

[JLarimar]

Copy that, going to adjust the pilot screws down to limit fuel flow.

Cleaned carbs, no issues with floats and no float pin wear. Adjusted the fuel pilot screw all the way in to 1/2 turn out limiting fuel from the carb, same problem, running rough when accelerating.

Replaced the coils packs for the spark plugs, same problem no improvement.

 

[JLarimar]

Compression test 150 130 150 140
No water in the oil
Spark plugs have black chucky carbon build up
will check spark and replace coils by end of week
If I check valve clearance do I need to replace the valve cover gasket?

They all run great on muffs.----Any water getting into the oil ?-----Post results of a compression test as well.----Perhaps running on 3 of 4 cylinders.----Check spark.---Check valve clearance.

spark is good, replaced coil packs for plugs and problem did not go away
valve clearance is next I suppose...

 

[JLarimar]

Cleaned carbs, no issues with floats and no float pin wear. Adjusted the fuel pilot screw all the way in to 1/2 turn out limiting fuel from the carb, same problem, running rough when accelerating.

Replaced the coils packs for the spark plugs, same problem no improvement.

got back into the carbs, looks like the float is out of spec in the closed position by 1/16" replacing with new floats, also the o-ring for the plug on the main jet was cracked and old replacing those as well

 

[flashback]

The float is adjustable, might not need new ones. Get them set proper, float level is critical to the metering of fuel.

 

[99yam40]

do all cylinders/spark plugs show signs of over rich? ( black carbon build up)
if just some which cylinders?
how many fuel pumps does this thing have and what cylinder crankcase does it get its pulse from

 

[racerone]

Most 4 strokes use a pump driven by a lobe on the camshaft.

 

[99yam40]

your right, forgot it was a 4 stroke.
but what would affect all 4 cylinders if that is what is happening
each one have its own carb

 

[racerone]

Thermostat in place and working ?----The 130 PSI cylinder is suspect.-----Some will argue but I would remove the head for inspection.-

 

[JLarimar]

do all cylinders/spark plugs show signs of over rich? ( black carbon build up)
if just some which cylinders?
how many fuel pumps does this thing have and what cylinder crankcase does it get its pulse from

Yes all plugs show signs of over rich
as mentioned its a mechanical fuel pump that I replaced with a new one

 

[JLarimar]

Thermostat in place and working ?----The 130 PSI cylinder is suspect.-----Some will argue but I would remove the head for inspection.-

new thermostat replaced last year
gonna try a few other things before I remove the head...

 

[99yam40]

pull the stat and test in a pot of water as you heat it up, make sure it opens according to spec.

The only other thing that I can think of that could affect fuel to all Cylinders would be the prime start enrichment system.
make sure it is allowing the extra fuel when motor is cold and then closes off once the motor warms up.

Looking at a partes breakdown ,it lists a primestart and also some choke linkage.
so I am not sure what all that motor has on it.


just for the heck of it I will tell you my C40tlrx had the main jets drilled out by someone before I got it.
took me a long time to finally figure out why it had problems running high RPMs.
I replace the main jets and all was well.

if you have not owned this motor from new, you can not say if someone did or did not do stupid things to it before you got it.

Did this motor and boat set up ever run well for you?
if it has then you can rule out jet modifications, and just look for failing devices