Yamaha 90 slows down after about 15 mins.

[Backdoorz]

I have a 1999 Yamaha C90TLRX two stroke. I purchased this motor new attached to a 17 ft. Sea Pro.

About a year ago, I experienced problems with the motor. I was running at WOT and the motor started slowing down, just like I was slowing it down with the throttle. The throttle was still wide open. The motor would finally slow down to around 1000-1200 rpms and run fine at that speed. Everytime I would push the throttle forward nothing would happen. It ran at idle. I have no alarms during when this happens. Throttle cables are OK. Motor is peeing great. This problem is still occurring today (7/1/2022). Motor starts fine, idles fine and runs great for about 10 to 15 mins. and then it gradually slows down to about idle speed. It runs fine at that speed. No sputtering or coughing.

If I turn the motor off for about 15 mins. and then restart, it will take right off and go to WOT, but it will only last about 5 mins., then it slows down like before.

Things that I have done.
- Rebuilt all three Carbs
- Tested coils with volt/ohm meter (tested ok)
- Replaced CDI
- New fuel filter
- Examined and cleaned fuel pump and lines
- Tried a portable tank
- New plugs
- New thermostat

I have never experienced a boat motor running in limp mode, but is sure seems like that may be what its doing. Although I don’t have a clue why it would. Buzzer for overheating works.

Thanks in advance for any help.

 

[99yam40]

testing things while the problem is there is the only way you will find it.
spark on all 3 cylinders is needed.
timing monitored also to make sure the timing is not dropping back when this happens.

does pushing on the primer bulb do any to help?

 

[Backdoorz]

Forgot to mention new primer bulb. Bulb stays hard and pumping it does not affect the situation.

 

[Backdoorz]

If the spark was bad on any cylinder, wouldn't the motor run rough or spit and sputter?

 

[99yam40]

I would think so,
but with an inductive timing light hooked to the individual plug wires you will be able to make sure they are getting steady spark,
and see what the timing is doing as long as you mark the flywheel good enough to see where it should be lighting up at.
all while the problem is happening
flywheel should have TDC marked for all 3 cylinders and only #1 will have the other degrees, but you can measure #1 distance from TDC and mark the others if you wanted to go that far.
I am not sure how Yamaha limits RPM when it thinks there is a problem, but adjusting timing would be one way to effect RPM.
have you looked to make sure the carbs are opening up as they should when this happens?
I would think you would hear a change in the sound of the motor as you opened the throttle even if the RPM did not go up

 

[racerone]

Have you installed a new water pump impeller ??-----Tested the overheat horn ?-----Fuel mixed at 50:1 ?----Checked gear oil ?------Exhaust housing is clear , not partially plugged ?

 

[Backdoorz]

Water pump was replaced about 3 years ago. Pees good.
Overheat horn works.
50:1 on fuel.
Gear oil changed December of 2021.
No blockage on exhaust.

 

[99yam40]

when did you test the horn last?
my C40 has 2 temp switches on it ,
one to tell the CDi when the motor is warmed up
and one to tell when it overheated
make sure they both work.

but I would check to make sure the timing is not retarding when it should be advanced

 

[robert graham]

An exhaust leak under the cowling could cause what you describe…maybe run motor with top cowling off to see?

 

[R&O123]

I have a 1999 Yamaha C90TLRX two stroke. I purchased this motor new attached to a 17 ft. Sea Pro.

About a year ago, I experienced problems with the motor. I was running at WOT and the motor started slowing down, just like I was slowing it down with the throttle. The throttle was still wide open. The motor would finally slow down to around 1000-1200 rpms and run fine at that speed. Everytime I would push the throttle forward nothing would happen. It ran at idle. I have no alarms during when this happens. Throttle cables are OK. Motor is peeing great. This problem is still occurring today (7/1/2022). Motor starts fine, idles fine and runs great for about 10 to 15 mins. and then it gradually slows down to about idle speed. It runs fine at that speed. No sputtering or coughing.

If I turn the motor off for about 15 mins. and then restart, it will take right off and go to WOT, but it will only last about 5 mins., then it slows down like before.

Things that I have done.
- Rebuilt all three Carbs
- Tested coils with volt/ohm meter (tested ok)
- Replaced CDI
- New fuel filter
- Examined and cleaned fuel pump and lines
- Tried a portable tank
- New plugs
- New thermostat

I have never experienced a boat motor running in limp mode, but is sure seems like that may be what its doing. Although I don’t have a clue why it would. Buzzer for overheating works.

Thanks in advance for any help.

i had that problem 8 years ago i removed my main oil reserve tank and
found that the small filter behind the tank needed to be cleaned so that small tank near the fly wheel oil level stays full, if not the engine goes into limp
mode so you don't damage your motor.

 

[99yam40]

i had that problem 8 years ago i removed my main oil reserve tank and
found that the small filter behind the tank needed to be cleaned so that small tank near the fly wheel oil level stays full, if not the engine goes into limp
mode so you don't damage your motor.

a C90 does not have oil injection

 

[QBhoy]

Hi. Is the new primer bulb fitted since the issue or before ?