Runs great then dies and wont start

1985 Mariner 25hp with an annoying problem I'm hoping somebody can shed some light on. Cranks over fine, fires right up, runs like a champ for 30 min or so both at idle and full open, then pops, dies abruptly and wont turn over at all. Let it sit for 45 min or so and crank it and it fires right up, rinse and repeat.

Checked spark with a tester: looks good both cyl
Checked compression: Good and same both cyl
Fuel pump: pumps full when cranked over
Pumped bulb: fuel system seems fine
Coil resistance checks out when cold.
Engage kill switch before it dies, then crank and it turns over fine

Anybody have an idea what's going on? It sure seems like it could be heat related as it runs great until it warms up.

It's been like this for a couple years and it's really driving me nuts. I've had to row or be towed in several times. Is there any greater shame than being towed in with your kids in the boat by some young punks in a ski boat blaring ACDC all the way to the dock (thanks punks)

Does your motor have the thermostat in the water jacket cover? The telltale hose (pisser) would be connected to it. You can try running it in a barrel of water with a gallon of saltaway in it for 1/2 an hour or so. Keep a close eye on the temps of the motor. It should never get too hot for you to hold your hand on it for a slow count to six. Make sure the water is at least six inches above the cavitation plate. Even fresh water will clog up the water circuits if not flushed properly. Lots of silt and minerals. How old is the impeller?

You need to IMMEDIATELY check for spark when it does this.

If none is there, I'd suspect a bad switch box.

Jeff

Forgot to mention that I replaced the impeller last year. Checked it after the most recent incident and a couple of the vanes are broken, but it's peeing fine.

Not sure about the thermostat. What would it look like?

I've had some trouble finding my motor through the various sites as well. My serial number doesn't appear to come up. Makes it hard to find a schematic to determine if I have a thermostat and where to look for it.

http://www.marineengine.com/parts/m...00101-thru-023895/crankcase-and-cylinder-head
Does this look like your motor the thermostat is on top of the head? If so pull the thermostat and get a new impeller. Adapt your garden hose to the water tube and see if you get good water flow out the thermostat hole. Replace the thermostat and put it back together and see if the head stays around 140 degrees when the engine is full warmed up. The exhaust cover on the side of the motor should stay fairly cool also. You should be able to hold your hand on the head for a slow count to six is a good way to tell, the motor is getting too hot if you have to pull your hand away.

That looks pretty close and I figured that might be the stat at the top of the tower at the back of the head. $82 for a new stat, is there a way to test the old one? Can I put it in water and heat it up and see if it goes wide open at a certain temperature?

How much do you spend for gas and tackle on a outing. It sounds like you had the issue even after replacing the impeller. You can put a string in the valve and hang it center of a pan of water and hold a meat thermometer right next to it should fall off of the string around 145 degrees. My main concern would be chunks of impeller plugging up circuits in the motor. A new thermostat will eliminate that variable. Overheating the motor can make the thermostat go goofy. The japs are really proud of their thermostats