Yamaha 15hp Enduro 2-Stroke Overheating ?!?!

[Tspringer]

Hello Gang!

Once again, my intrepid and much abused but never dead Yamaha is giving me some fits. Last year we were in the Bahamas when COVID hit - same back to the States and in all the change of plans and craziness going on the engine was hung back in its spot on the aft rail and pretty much left until the past week. Yes, not loving care I know but sometimes crap happens.

So when I went to get it running again, the throttle and gearshift were basically locked up. Lots of WD40, cleaning and working things loose got it all going and she fired up and ran great. So off to the raw bar we went seeking food and drink. 3/4 of the way there just putting at little over idle (no wake zones) she started running a bit rough and it because clear overheating. The place on the back of the engine up just below the top cover where water should spit had no spitting going on. Hmmmmmmmmm

So I began to try and figure out the problem. Not so easy.

I have no manual. I cannot find one online anywhere. The engine dates to around 1994 or 1995 I think but there is no real telling as there has not been a data plate on it ever in my ownership.

Where are the water pickups? Where is the impeller located? How would one check or replace the impeller? What other possible issue could cause this?

While asking questions.... how does one check the crankcase oil? LOL

Any ideas are greatly appreciated. Gotta get the little beast going again so we can head back to the Bahamas!


Terry

[tkeithlu]

It's a two-stroke. There isn't any crankcase oil.

Here's one page from the 1994 15 HP Yamaha parts manual. Find the pump. See how it's all assembled from the exploded view of the parts, and go at it. Good luck with it.

https://www.boats.net/catalog/yamaha...casing-drive-1

[Tspringer]

tkeithlu -- thanks.


So it seems the lower unit has to be removed and partially disassembled to replace the impeller. Yuck but oh well.

[tkeithlu]

Oh, yeah. They didn't design that sucker with your convenience in mind.

As my memory has deteriorated with age to the point that I am merely brilliant, I have found it wise to place disassembled parts on an old sheet in the same orientation as the exploded view.....

[Scubaseas]

seloc online has the repair manual for a fee. Youtube has lots of videos on impellor replacement.

[s/v michaela]

that's the easiest motor to change impellers on. just drop the lower unit by 4 bolts and undo the shift rod connector. After the lower unit is dropped(15 minutes) then pull the cover off the impeller housing and swap out the impeller... doesn't get more simple that .....

[Shrew]

Change plug, change impellor, change zinc, clean the carb.

It's more than likely the impellor. Also, find a better lubricant than WD-40. WD-40 is a penetrant and a solvent, but not really a lubricant. It's great for breaking the seized parts, but now you need to put a lubricant in there.

[Tspringer]

Ok - got the impeller out, super easy. Free the shifter connection - 1 10mm nut - remove 4 12mm bolts - presto.

Pic attached.

This is clearly worn, the blades are permanently bent over and weak but enough to cause no water to spit at all and overheating? Hmmmmm

Also - here is a great video of a 9.9hp vintage Yamaha 2-stroke being serviced - same motor as the 15hp just different carb jets

https://www.youtube.com/watch?v=ec_7_CntB0k

Now to find a replacement... always fun when you have no means of determining what the actual year or model number of the engine is!


Terry

[Tspringer]

Trying again to upload/attach an image of the impeller....

Getting an image to show in a post should not be this hard. I run online companies, mine are not this difficult. Incompetence sucks.

Are you really telling me that a site that does the traffic that this site does will not let you simply attach/upload an image file to a post and have it show without that image being hosting elsewhere? That is just SORRY.

[Tspringer]

WOW - 20 minutes spent trying to simply upload and attach an image to a post and still not happening. This site has serious UI issues.

I give up. I run very successful websites. Not being able to just attach and upload an image to show in a forum post is so 2004. Pathetic.

[GoneDiving]

Originally Posted by Tspringer

Ok - got the impeller out, super easy. Free the shifter connection - 1 10mm nut - remove 4 12mm bolts - presto.

Pic attached.

This is clearly worn, the blades are permanently bent over and weak but enough to cause no water to spit at all and overheating? Hmmmmm

Also - here is a great video of a 9.9hp vintage Yamaha 2-stroke being serviced - same motor as the 15hp just different carb jets

Now to find a replacement... always fun when you have no means of determining what the actual year or model number of the engine is!


Terry

So, did the new impeller fix the over heating? If the rubber has lost the tension against the water pump housing it can bypass enough to cause over heating regardless of how it looks.

If that hasn't fixed it you probably have cooling passages full of sand, corrosion, crap or pet Labradors. Pull the thermostat cover off, run the unit at a high idle and let water and crap flush out everywhere. If still no good, its pull the head and clean all the passages. Beware of seized and snapped bolts that will dampen your day.

Is the pee hole actually open or full of crap too? A shot of air will tell you.

While your doing all this why not spring for a new thermostat? They don't last forever.

Most of the parts we mentioned are generic across models and years. Guess it within a decade and you will likely receive the correct part.

Enjoy.

[s/v michaela]

a hardened impellor will push no water,,,,, but the blades will usually break off. Also if you have left your motor without flushing it regularly ,
salt water can form salt dams which will plug the passages and cause over heating.

[PhilC]

When you have it all back together with a new impellor you should have a telltale jetting water and if it has one (and as soon as up to temp) water flowing out the larger hole in the back of the leg (mixed with exhaust gas). If you have both of these great! If you don't then you need to start trying to isolate where the blockage is. Start by cleaning out with a piece of wire any accessible parts of the telltale. Then remove the thermostat housing, test thermostat and clean out all you can reach from there. If that dosent work then feel around the head and exhaust jacket area with your hands (or using digital thermometer) after a short run. Raw water cooled engines need to run cool <70deg C to avoid forming salt crystals - if you can hold your hand on any part of the powerhead for few seconds before it becomes uncomfortable that's around 70deg. This is important because you need to figure out which bolts to undo to access a site of a potential blockage (the head or the exhaust jacket) if it's a 90's motor you are likely to break a fair percentage of the bolts undoing them so the less you undo the better.

[Tspringer]

Originally Posted by GoneDiving

So, did the new impeller fix the over heating? If the rubber has lost the tension against the water pump housing it can bypass enough to cause over heating regardless of how it looks.

If that hasn't fixed it you probably have cooling passages full of sand, corrosion, crap or pet Labradors. Pull the thermostat cover off, run the unit at a high idle and let water and crap flush out everywhere. If still no good, its pull the head and clean all the passages. Beware of seized and snapped bolts that will dampen your day.

Is the pee hole actually open or full of crap too? A shot of air will tell you.

While your doing all this why not spring for a new thermostat? They don't last forever.

Most of the parts we mentioned are generic across models and years. Guess it within a decade and you will likely receive the correct part.

Enjoy.

I have not replaced the impeller yet -- have to source a new one. We are getting into Cooley's Landing marina tomorrow and I will get a new one then.

The impeller I removed is not missing any blades but they are very weak and bent all the way over. I could certainly see it as a strong possibility that water is bypassing between the housing and impeller blades when it spins.

If replacing the impeller does not fix it I will do as you suggest regarding the thermostat. I thought that if the impeller was working it should pee water when first started even if the thermostat is preventing coolant flow?

[GoneDiving]

Originally Posted by Tspringer

If replacing the impeller does not fix it I will do as you suggest regarding the thermostat. I thought that if the impeller was working it should pee water when first started even if the thermostat is preventing coolant flow?

If the thermostat hasn't been replaced in a while, I'd get a new one anyway. They are not expensive and it would give peace of mind moving forward.

Correct. The pre stream shows cooling water being supplied not making its way past the thermostat and through all the passages.

https://youtu.be/joEQChcZl2A