1978 Johnson 15 stalls under load

[MKMG]

My 1978?, stalls under load. It starts Ok, idles good. Runs great at WOT with just me in the boat but when I start pulling kids around on water skis etc it stalls and won’t start. I paddle home and it starts and runs fine 30 mins later. The motor does feel warm but I have never taken note to what is normal. The motor is on a 12’ Lund, I’m 150lbs, and usually have one 8 year old in the boat and one on skis so not very much weight involved. When I was a kid we would go all day with 4 in the boat and 2 out the back no problem. The carb and impeller were done a few years back, and not used much since. It has a strong water stream. New gas, gas tank and hose. It will start to slow down and sputter for a few seconds before it stalls.
Thanks in advance.

 

[racerone]

I think a compression test is a place to start.------The motor really does the same work ( spins a propeller in the water ) if you are in the boat by yourself or pulling a skier.-----The boat behaves different !!----But you might consider a lower pitch ( 8" ) prop for the towing work.

 

[MKMG]

I think a compression test is a place to start.------The motor really does the same work ( spins a propeller in the water ) if you are in the boat by yourself or pulling a skier.-----The boat behaves different !!----But you might consider a lower pitch ( 8" ) prop for the towing work.

I will check the compression this week. I don’t think it’s a load issue, once the skier is up I’m only 3/4 throttle. Maybe more of a run time thing. I only go out by myself at WOT for 10-15mins but when pulling kids it’s usually 30-40mins of use before issue starts.
Does a solid water stream indicate my impeller is fine? Or could some pathway that cools the motor be plugged?

 

[racerone]

The telltale only indicates that impeller is working / pumping.-----It does NOT INDICATE that motor is cooling properly.-----If impeller is more than 5 years old you need a new one.

 

[oldboat1]

Compression test as recommended. Popping a skier up takes more power than towing her around. Get an IR temp tester (surface temp at top of the head.)

 

[MKMG]

Compression test: top-92, bottom-100. I’ll check the impeller but it only has a few hours on it since replaced. Maybe it was installed incorrect or defective since the problem started soon after that tune up.