Gas/oil leak at cav plate

Motor is a 2008 25hp Yamaha 2-stroke.

Never had much mess with this motor. Has only been on the water twice this year, last time being about 2 weeks ago. Boat ran perfectly with no issues. Yesterday the wife starts complaining about fuel smell in the garage so pull all fuel cells and jerry cans and everything looks legit. Make my way around behind the boat to find a basketball-size stain on the cement under the motor. There was a good amount of buildup on the top of the cav plate and it looks like the source is a few small holes on either side directly above the plate. Leaking almost identically on either side. I cleaned it all up, put down some litter on the ground, checked it a couple hours later and found it’s still actively leaking.

I know unburned fuel can come out the lower unit, but this seems like more than normal. Especially cropping up 2 weeks after the last use. Any ideas? We leave for a Canadian trip in 2 weeks so I’ve gotta get this sorted out ASAP. Many thanks!

(I’ll post pics from Before cleanup in this post, and from 2 hrs post-cleanup in the next post.)

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And here’s the photo two hours after I wiped it clean and let it sit

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Did you pop the cowling?

How did you run the boat last? Lots of slow trolling can build up oil/gas in the exhaust, but agree that it looks like more than just that (unless you never opened up the throttle for hours).

If you ran the boat normal or like you always do without issue, it could possibly be the crankshaft seal??

Yep. Everything looks pretty clean but didn’t dig in too deep. Jiggled hoses and nothing was obviously leaking. Very small amount of gas in the bottom in a few spots but nothing crazy. This is next to the tell-tale in the bottom.

When I ran the boat lastI ran it wide open from spot to spot and used TM spot-lock while fishing. Ended wide open running back to ramp.

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Gas tank properly vented?

Didn’t change oil the last time filling up did ya?

Gas tank was a little on the puffy side which is strange because I always keep that screw backed out a bit and venting.

It’s a two-smoker so no oil to change.

This morning I cleaned it up really good, inside and out. I did find a small pool of fuel on the top plate of the bottom carb. Soaked it up and cleaned off. Ran the motor on muffs for about 30 minutes. Motor is running like a top so that is encouraging. No signs of new leakage as of yet. Going to keep an eye on it throughout the day.

My theory is that on the first trip out this year I over-primed it, causing the carbs to leak slightly. A small amount of fuel pooled up and eventually gravity did its thing. I can see a direct line where it would find the little weep hole in the bottom of the housing by the carbs, then seep along the bottom the edge toward the shaft and drip directly down onto the top of the cav plate. I believe there was SOME leakage from the exhaust as well, because of the obvious darker color. I actually think the two leaks mixed together and I assumed it was all the same.

Anyone have any other ideas? I did talk to the service manager at Dan’s south side when I ran in to grab a set of muffs this morning. He basically told me he very rarely worries about this kind of leakage with a two-stroke. He said the heat spike could make the fumes a lot stronger as well.

Even in the gas/oil mix changing to a synthetic oil from a dirty Dino oil in a 2stroke can cause the exhaust system to flush itself out since it runs cleaner.

Carb float maybe didn’t sit just right, lil chunk of something something in there and with the tank bloated it pushed some gas up and out.

Either way, hope it isn’t a thing and all goes well on the trip

Thanks my man. Fingers crossed. Same oil for past 2 years so I’m good there.

I’m a year or two away from repowering with a 4stroke and adding power tilt & trim so really trying to avoid sinking cash into this.

dirtywater wrote:

Gas tank was a little on the puffy side which is strange because I always keep that screw backed out a bit and venting.

This is likely the issue. The pressure from the tank pushed gas through the carb and leaked out.

-J.

Jon Jordan wrote:

<div class=”d4p-bbt-quote-title”>dirtywater wrote:</div>
Gas tank was a little on the puffy side which is strange because I always keep that screw backed out a bit and venting.

This is likely the issue. The pressure from the tank pushed gas through the carb and leaked out.

-J.

It’s a solid theory. And increased pressure would coincide with the recent heat we’ve had.

Semi-related: Is it possible for boat or road vibrations to cause the vent screw to slowly tighten down? I know it was properly vented last time I started the boat and was surprised to find the tank under some pressure. The reverse happens to deck screws on a long towing trip so I guess it’s possible. Either that or I have a gremlin sabotaging my boat.