Yamaha 40 nearly stops every 300 yards

[broberts]

I have a Yamaha 40 Hp tiller steer that is about 8 years old. Had a 115 on a bonefish boat in Florida a while back too.

This spring at first running it began a pattern of running about 300 yards at normal speed, coughing once and loosing power. It would continue to idle and after about a minute or two of idling, I could run up to full power and truck along another 300 yards or so, then lose power again.

First thoughts were fuel starvation. Checked the tank for debris and water. None. Strainer on the intake is clear. In line fuel filter in the engine is also clear.

I tried squeezing the primer bulb but that did not help, so my thoughts went to cooling.

Some water peeing out the indicator so there is some cooling pumping.

This all seems much like what would happen in Florida on the bonefish boat when it ate too much marl from the flats. Got pretty good at cleaning the impeller and clearing the pee tube standing in water and watching for critters at the same time.

But this boat has never been in sand or dirty water. Only clean sweetwater.

I'm at a loss to figure if has a fuel pump problem or a cooling problem. Being a tiller steer there are no buzzers or other instruments.

The manual is hiding in places unknown. I suspect this engine has a high temp throttle cut back like the 115 I had in Florida, but I'm not sure.

If the thermostat is stuck closed, I could try to free it up if I knew where it was.

Anyone got any suggestions to trouble shoot it further.

 

[broberts]

Re: Yamaha 40 nearly stops every 300 yards

Update:
today I located and removed the thermostat. Took it for a run and it quit just like before.

Located what I thought was the overheat thermocouple, unplugged it and the motor ran without the slow down to idle after a few hundred yards.

So I think now the issue is poor water flow. Either plugged up somewhere or the impeller is mostly gone. We've had the zebra mussels here and if they started growing in the impeller housing their shells may have chewed it up.

So how do I get at the impeller on this unit ???

 

[broberts]

Re: Yamaha 40 nearly stops every 300 yards

bump

 

[Silvertip]

Re: Yamaha 40 nearly stops every 300 yards

the impeller is located in the lower unit. You drop the lower unit and it sits around the drive shaft at the top of the housing. I strongly suggest you obtain a service manual for this job. E-bay or your local Yammy dealer can fix you up. Otherwise try your local library. Even an after market manual (Seloc or Clymer) are better than no manual at all. Any of them help you avoid what can be very costly mistakes and help you troubleshoot the problem you are having which may or may not be cooling related.