1999 2 stroke 90 yamaha overheats at top end only

[birdseye418]

Fuel is pre mix no oil pump

Maybe 25 hours on new impeller and peeing just fine.

last two trips out it would overheat at top end 4,300 rpm after 5 minutes or so
run it at 3,500 to 3,800 rpms all day with no problems
anyone have this issue before?

 

[99yam40]

Re: 1999 2 stroke 90 yamaha overheats at top end only

Water pump including housing,
thermostat, temperature switch,
or salt/corrosion build up in passages are things that I would be looking at.

WOT RPMs should be around 5500 not 4300, sounds like you are propped too high

 

[Capt Ken]

Re: 1999 2 stroke 90 yamaha overheats at top end only

Correct, maybe prop'ed way to heavy and engine is in a strain.

 

[birdseye418]

Re: 1999 2 stroke 90 yamaha overheats at top end only

Correct, maybe prop'ed way to heavy and engine is in a strain.

prop is 13 1/4 x 17 stainless on a 1720 keywest I bought the boat used about 5 years ago and this prop was on it.
the boat plaines good and runs about 32 mph top end.
about 2 years ago I replaced the head gasket abd had an exaust manifold gasket replaced by a yamaha mechanic.
when I had the head off I did examin the water jackets and they looked ok, to me anyway

in 5 years the only time the motor would over heat is when passing through heavy patches of weeds

is it possible the rpm gage is inaccurate, if my prop is imoproper for boat and motor I still don't think the issue is related but i'm no mechanic
thanks for the info

 

[schematic]

Re: 1999 2 stroke 90 yamaha overheats at top end only

you are about 1200 RPM low. This puts you in a grey area of having one weak cylinder or overpropped. 17p shouldn't be that far off the mark unless that is one heavy boat. Has this always been your max RPM?

 

[99yam40]

Re: 1999 2 stroke 90 yamaha overheats at top end only

yea 17 on a 17 should be about right depending on the load, but your WOT RPM is way low.

check this site for a set up close to yours to see what they did http://www.yamahaoutboards.com/owner-resources/performance-bulletins

Sounds like you have a sick motor, motor and boat are set up wrong, boat is water logged, or your tach is wrong.

Take to Yamaha shop and have them test with a test prop to see if it can reach proper RPM and have them use a shop tach to see if yours is correct

Anytime you run motor with water pick up clogged enough to overheat motor you should be looking at water pump

 

[birdseye418]

Re: 1999 2 stroke 90 yamaha overheats at top end only

yes it's always ran it that rpm
it got a hole in the inhull 30 gallon tank so I now use a top side tank 18 gallon
it's possible the inhull tank has allowed rain water to enter causing extra weight but it jumps on a plane like it always has and runs skinny, just started having the overheat issue rescently

forgot to mention when the impeller was replaced the foot was rebuilt, is the water pump seperate from the impellar?
sounds like I need to have it tested before i blow it up

and rename her "Migraine"

 

[99yam40]

Re: 1999 2 stroke 90 yamaha overheats at top end only

Water pump housing does not come with the water pump kit and the impeller can be bought without buying the kit.
So, no telling what was replaced .

I always replace with the kit and inspect housing for any damage