Yamaha 40 hp 1986 - loss of power

[PhiloBeddoe]

Thanks for checking out my posting. I have a 1986 Yamaha 40 HP outboard that is having a problem which I appreciate any guidance.

This is my first boat and I would like very much to learn why this problem is happening and not just drop it off and write a check. (although that would be easier!)

Here's the problem:
After 20 minutes of operation at 70% throttle, the engine loses power and feels like the motor is throttling back to 20% automatically. I do not hear any alarm coming from the control unit. After everything cools off, it runs well again and then after warming up, the problem comes back.

We have changed the plugs, checked the fuel line and pump.

Also, I have changed the water pump impeller, but have not tested the motor since swapping it out. I doubted this will be the solution to the problem, but I was told to make the impeller change part of my annual routine.

Thanks for any information or experience you can provide.

Philo Beddoe
1986 Arima Sea Explorer "ole ole"
Yamaha 40Hp
EZ loader

Marina Del Rey, Ventura

 

[99yam40]

Re: Yamaha 40 hp 1986 - loss of power

Does this motor have a model # you could share with us?

http://forums.iboats.com/showthread.php?t=250876

Have you checked to see if your alarms system is working if it has one?

It sounds like your motor is going into a safe mode. maybe because of over temp

Have you changed or checked the operation of the thermostat along with changing the water pump?

You need to buy a Yamaha service manual for your motor , it will help you troubleshoot and repair your motor and perform the necessary preventative maintenance needed

 

[brogden31]

Re: Yamaha 40 hp 1986 - loss of power

i hade one that did this, but it was my carbs dirty and they would suck some thing in the jet ,then when it sat for a min the trash would float away from the jet, then when running fast again it would get sucked it jet again so on so on

 

[PhiloBeddoe]

Re: Yamaha 40 hp 1986 - loss of power

Thanks for the suggestions.

THE MODEL IS A Yamaha 40 ETLJ


99yam40, I have tested the alarm, and it works, but I have not tested the sending unit for the alarm. When the motor slows down, its not very hot. There's no steam or anything and i can touch the motor with the cover off. I pulled off the thermostat cover and boy was that thing corroded! Ive replaced it. Also, changed out the impeller. I bought the manual. All great suggestions thanks,


Another question now that I've changed the thermostat. I can't seem to find a replacement gasket. Can I trace the old one out of cardboard from a legal pad? I've read yes, but just want another opinion. Also, do I need to use gasket seal (from autozone) with this new gasket?


Im taking it out this weekend hopefully and will spray some carb cleaning in there as well.

I haven't tested it since Ive put in the new impeller, thermostat - am hopeful that the problem is solved...

thanks for all the tips

 

[99yam40]

Re: Yamaha 40 hp 1986 - loss of power

Spraying chemicals into the motor while it is running is not a good idea. It will not clean the carbs and it gets the chemicals into the crank case and bearings which need lubrication not chemicals. If you need to clean carbs pull them and take apart and clean.You can test the fuel supply system with a pressure/ vacuum gauge teed into the line at different points to make sure it is not starving for fuel. If it runs lean on fuel the pistons will melt and hurt the cylinders also

If you had a lot of corrosion at the thermostat then the cooling passages maybe full of corrosion and salt also not allowing proper water flow to all parts of motor.

How did you test the alarm system? hopefully you pulled wire off temp sending unit and grounded to block not just put 12 v on horn?

Testing temp switch is simple. pull it off motor, hook to a ohm meter, and place into a pot of water with a thermometer and start heating water. Watch to see what temp the switch closes and then turn off heat and add small amounts of cool water and see what temp it opens back up. At least then you would know that if it goes off something is causing it to heat up past where it should

If this has oil injection that could cause safe mode also and you need to make sure it is functioning properly or it will hurt the motor.

Not sure on that old of a motor but on the newer ones A timing light will show if it is going into safe mode. It causes the timing to not advance or retards timing thus limiting the RPM. Read up on it in your manual to see what it says.

did they not have a gasket where you bought the other Yamaha parts? Can they get you one?
Cardboard may seep, but a sealer might help.

Good luck and have fun.
Read and learn about everything you can from your manual and these online forums

 

[PhiloBeddoe]

Re: Yamaha 40 hp 1986 - loss of power

Just tested the boat out last weekend and want to post an update with additional information about the problem we're having. We had just replaced the thermostat (got the correct gasket - thanks for the suggestion) and impeller.

We used the boat for 1 hour at 70% throttle, which is our normal cruising speed. We started out slow, just to make sure the new impeller and thermostat were functioning properly. They seemed fine, so we eased up the throttle. No problems at all. We were motoring all near the harbor entrance in Ventura. We went fast, slow and everything in between.

We decided we should mess with the trim angle so we throttled back, moved the motor up and then resumed speed. Within a minute, the motor throttled back on its own and was having the same problem as last time. We took the cover off, messed around with it for a few minutes, checked the temp (it was fine) and nosed around. Then, we raised the motor all the way up and then all the way down back into the water.

The problem immediately went away and didnt come back for the next 15 minutes. Then, it was time to go home.

We have not tested it again since then and are anxious if our mystery is solved???!

I am wondering if this is just a coincidence or if messing with the tilt is somehow affecting the motor.

As always, I really appreciate the guidance I've gotten in this forum!

 

[99yam40]

Re: Yamaha 40 hp 1986 - loss of power

Did you test temp switch and alarms system as I said to make sure they are functioning properly?
You could have picked up something that restricted the water inlet(plastic, sea weed, etc) causing over heat and when you tilted it all the way up it came off or by tilting you moved trash/water around in the carb bowls that restricted fuel flow in jets. May have to pull carbs and clean and inspect to be sure.