4 stroke 50hp 1997 Yamaha outboard black smoke rough runnning

[JLarimar]

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

You are correct, there is a solenoid and prime starter unit and connecting linkage to the four carbs. I'm going to pull off the prime starter and test it as the manual shows.

Checking The Prime Start; Assembling The Carburetor - Yamaha F50A Service Manual [Page 119]

Yamaha F50A Manual Online: checking the prime start, Assembling The Carburetor. 1. Measure The Length Of The Prime Start Plunger. 2. Connect The Positive Prime Start Blue (L) Lead And The Negative Prime Start Black (B) Lead To A Battery As Shown. 3. Measure The Plunger Length A After...

www.manualslib.com

this is the main thing, the motor was running GREAT for the last year, no issues or early signs of issues. After some cold weather in January got out on the water late in the month. The motor starts and idles fantastic, after putting it in gear and throttling up it starts choking badly and if run over 4 mph for a few minutes the motor will stall out when put back into neutral. Will only restart with WIDE OPEN THROTTLE. The plugs get black with carbon and it puffs black smoke on acceleration.

 

[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 cylinders show black carbon build up on the plugs

 

[JLarimar]

You are correct, there is a solenoid and prime starter unit and connecting linkage to the four carbs. I'm going to pull off the prime starter and test it as the manual shows.

Checking The Prime Start; Assembling The Carburetor - Yamaha F50A Service Manual [Page 119]

Yamaha F50A Manual Online: checking the prime start, Assembling The Carburetor. 1. Measure The Length Of The Prime Start Plunger. 2. Connect The Positive Prime Start Blue (L) Lead And The Negative Prime Start Black (B) Lead To A Battery As Shown. 3. Measure The Plunger Length A After...

www.manualslib.com

this is the main thing, the motor was running GREAT for the last year, no issues or early signs of issues. After some cold weather in January got out on the water late in the month. The motor starts and idles fantastic, after putting it in gear and throttling up it starts choking badly and if run over 4 mph for a few minutes the motor will stall out when put back into neutral. Will only restart with WIDE OPEN THROTTLE. The plugs get black with carbon and it puffs black smoke on acceleration.

Confirmed Prime Starter is operating as expected.

 

[99yam40]

so what does the solenoid do?
is it connected to all 4 carbs?
what does that linkage attach to and move?
almost looks like a choke plates in the front of carbs.
and the linkage in the parts breakdown call the linkage choke linkage.

is it possible those are not opening back up like they should?

 

[JLarimar]

so what does the solenoid do?
is it connected to all 4 carbs?
what does that linkage attach to and move?
almost looks like a choke plates in the front of carbs.
and the linkage in the parts breakdown call the linkage choke linkage.

is it possible those are not opening back up like they should?

not 100% clear on actual function beyond what the parts are titled
there is a primer starter that extends and opens the front carb choke plate with linkage
there is a solenoid above that the extends with the same linkage, its unclear to me how that works? likely tied to the choke and appears to remain closed during running
may be malfunctioning on wide open throttle and will test once reassembled

 

[cyclops222]

Check and adjust the AUTOMATIC CHOKE system. Carburetor or F I. It is normally done with a cold engine.

 

[99yam40]

can you run the boat with the cowling and silencer off so you can see what all is moving and opening/or not opening as you open the throttle when the problem shows up.
something is affecting all 4 carbs /cylinders

 

[99yam40]

all the motors I have messed with did not have chokes if they were prime start or Fuel injected, so I am very confused

 

[JLarimar]

all the motors I have messed with did not have chokes if they were prime start or Fuel injected, so I am very confused

ran the motor with the cowling off, none of the linkage attached to the prime starter or solenoid moved (see pic of carb breakdown, same problem persists
not fuel injected

 

[JLarimar]

Check and adjust the AUTOMATIC CHOKE system. Carburetor or F I. It is normally done with a cold engine.

confident from the parts drawings and manual search is does not have an automatic choke system?
the only reference to "choke" is the linkage on the carbs, this all appears to be operating similarly to how it has been
the carburetor has been gone through - new floats/needles and gaskets
new fuel pump (mechanical)
maybe I'm missing something with the linkage although it hasn't changed much since before the problem was present

 

[JLarimar]

not 100% clear on actual function beyond what the parts are titled
there is a primer starter that extends and opens the front carb choke plate with linkage
there is a solenoid above that the extends with the same linkage, its unclear to me how that works? likely tied to the choke and appears to remain closed during running
may be malfunctioning on wide open throttle and will test once reassembled

 

[99yam40]

is there a switch that operates the solenoid or maybe pushing in on the key switch would operate the solenoid?

are there butterfly plates in the front of each carb that the linkage is tied to?
if so what operates those.
they should not be closed when running

 

[JLarimar]

is there a switch that operates the solenoid or maybe pushing in on the key switch would operate the solenoid?

are there butterfly plates in the front of each carb that the linkage is tied to?
if so what operates those.
they should not be closed when running

thanks for this, I just checked the solenoid when starting and the choke linkage moves with start up, I'm going to unplug the solenoid and test run this afternoon, I may be dealing with an "always on" solenoid

interesting a few months back the spring mechanism in the key stoped working, pushing the key IN did activate the choke, now it seems like pushing in the key does nothing

Also, the only way I've been able to restart after the motor runs, is to open the throttle wide and back it down after starting, this leads me to believe the choke is stuck on at start up...

 

[JLarimar]

thanks for this, I just checked the solenoid when starting and the choke linkage moves with start up, I'm going to unplug the solenoid and test run this afternoon, I may be dealing with an "always on" solenoid

interesting a few months back the spring mechanism in the key stoped working, pushing the key IN did activate the choke, now it seems like pushing in the key does nothing

Also, the only way I've been able to restart after the motor runs, is to open the throttle wide and back it down after starting, this leads me to believe the choke is stuck on at start up...

No luck, the choke is not the issue, unplugging the choke solenoid and visually inspecting it not operating during running and the problem persists

 

[JLarimar]

Where I'm at and none of this has fixed the problem:
all the carbs have been refreshed with new floats, needles, gaskets and seals
new spark plug coils
new fuel pump
Seafoam intake treatment

Basically the cylinders have black carbon build up and the motor runs rough when increasing acceleration, like its choking on fuel
Confirmed the choke is not stuck on, no automatic choke only carb linkage
Motor starts and idles great and will stay running just fine
Once the motor is warm and has been running for awhile if I shut the motor off many times I can only restart if I open the throttle up wide and back it down once running, it will then idle fine

Could this be a CDI problem? (yike$)
Could this be a timing issue?
Can I check the valves with a camera bore scope through the intake manifold? What should I be looking for?
Is there a way to clean all the carbon out of the cylinders and intake? Could a build up of carbon cause a fast onset of this problem (runs good one day, the next massive acceleration problems)?

 

[cyclops222]

Can not help at all....... Really a super odd ball system. Call a dealer seems logical.

 

[JLarimar]

Can not help at all....... Really a super odd ball system. Call a dealer seems logical.

OK I think I've solved the issue!

After checking compression (145-144-149-145) and getting fresh ethanol free gas I was playing with the linkage on the carbs while running. I noticed the intake side of the linkage would not open till FULL THROTTLE. So I held that linkage open while accelerating and the motor stopped choking!

The image attached shows the piece of linkage that was bent and preventing the intake throttle body from opening during early acceleration. I've bent that linkage piece back to the original position and will test run on the water shortly...fingers crossed.