Yamaha 75 CETOL power drop

[robert.babnik@gmail.com]

Hello, My uncle has an issue with power drop if he runs his Yamaha at max rpm for 3-4 minutes. The power drops to idling for around 2 minutes and after that he can run it again at max power for 3-4 minutes. The motor is not more hot than normal. If he is using the engine at 3/4 of max power it works without problems for hours.
He was suggested to disconnect thermostats from CDI but it did not makes any difference. The water pump works fine. He cleaned the carburetor and replace the air filters.

I am thankful for any advice helping to solve that issue.
Regards
Robert

[99yam40]

Thermostats are not hooked up to the CDI ,
so I am not sure you know what is really going on an what has been checked.

Does the motor have gauges and a working alarm system that would tell it what alarm came in?
Is this motor a pre mix or oil injected?

[robert graham]

The power drop after running wide open would point to a possible fuel delivery or fuel system venting issue....is fuel tank vent open and clear?....is fuel pump in good working condition?....Could there be fuel restrictions in the hoses, primer bulb, other?....

[madharchod]

20 dollar fix replace your fuel line

[robert.babnik@gmail.com]

Thermostats are not hooked up to the CDI ,
so I am not sure you know what is really going on an what has been checked.

Does the motor have gauges and a working alarm system that would tell it what alarm came in?
Is this motor a pre mix or oil injected?

Thank you very much for your answer. Your are right that my uncle at his ages might not understand the issue. Thank you for your patience.
Here are more info's.
The motor has only gauges: RPM meter and oil level indicator. Probably it would alert low oil level alarm.
The motor is oil injected.

Thank you for further advice's.

[robert.babnik@gmail.com]

The power drop after running wide open would point to a possible fuel delivery or fuel system venting issue....is fuel tank vent open and clear?....is fuel pump in good working condition?....Could there be fuel restrictions in the hoses, primer bulb, other?....

Thank you for your reply.
This weekend we replaced complete hoses from fuel tank to motor with primer bulb and we tried with another fuel tank. We clean the fuel tank vent. Fuel pump we do not know if it is OK. Then we try to drive with 75% of power for 20 minutes and everything was OK. After we start to use max RPM for 3 minutes the power drop to idle occur again.

Thanks for any further advice.

[robert.babnik@gmail.com]

20 dollar fix replace your fuel line

We did but it did not help.
Could someone explain to me what could be the reasons for power drop?
Is CDI responsible for power drop after it gets some temp sensor or temp switch out of range?
Can an non proper cooling make power drop?
This motor has 75HP two stroke and 3 cylinders.
Best regards and thanks in advance.

[99yam40]

Over temperature and low oil level should sound alarm and limit RPM along with getting indicators on the tach which is causing the problem.Does any of this occur? if not repair that.

Guessing that water pump and thermostat and temperature overheat switch are working properly is the wrong way to deal with this.
Test things or replace
More RPMs uses more fuel which creates more heat, more heat means more water is needed to keep it cool. when was the pump and thermostat serviced last?
If the problem is overheat then either the switch is working in spec or it is not, test it.
possible blockage in cooling passages causing not enough flow or pump just cannot pump enough.

Lean fuel to air ratio will also cause more heat than normal and can cause piston/cylinder damage