6HP Johnson, stops pumping water at full throttle

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Author Topic:   6HP Johnson, stops pumping water at full throttle

weekendwarrior posted 12-22-2012 07:35 PM ET (US)
After a 20 year nap in the garage, my 1967 Johnson 6HP just had a new water pump professionally installed (and thermostat), and it now pumps lots of water, up until about 3/4 throttle. When the RPM's get up, it immediately stops pumping water. Even before the boat is on plane. If you back off of the throttle, then it immediately starts pumping again, and the water coming out is super hot for a couple of seconds, so it definitely wasn't pumping. The compression is 89# top cylinder, 90# bottom cylinder, and my understanding is that this is in the good range for this old motor. If I'm idling in forward and whack the throttle open, it pumps water until the RPM's get up (a few seconds), then it stops pumping. I can cruise around at 1/2 to almost 3/4 throttle and it does just fine, but full throttle and it stops pumping. Does anyone have any idea what might be the problem?
weekendwarrior posted 12-22-2012 07:37 PM ET (US)
When I say stops pumping, I mean that water stops flowing out of the above-water exhaust port. This old motor doesn't have a water stream, but this port is usually dumping a lot of water. This is the only way to visually confirm that it's pumping. I held my hand back there to feel the water while driving the boat to verify when it was and wasn't pumping.
seahorse posted 12-23-2012 01:54 PM ET (US)

Besides the possibility of a slipping hub inside the impeller, do not overlook the very common problem of a leaking head gasket or loose bolts. A slightly warped cylinderhead from overheating can also allow combustion gases into the cooling system.
On a small boat there could also be bottom or keel obstructions that could disrupt the flow and allow aeration at the water inlets as boat speed increases. A warped or corroded water pump housing can suck exhaust into it as the higher back pressure lowers the water level by the pump assembly.
HawaiianWhaler posted 12-24-2012 05:14 AM ET (US)
It appears that you are trying to use the auxiliary exhaust port as a telltale for water pump operation, not its intended purpose, hence not a good indicator of cooling water flow.
An outboard mechanic once told me that the auxiliary exhaust port is there to allow exhaust to exit above the water line when motor rpm is too low to sufficiently push all exhaust out the main port against the water pressure below the water line, or when operating in reverse. As you raise rpm, the below the water line exhaust pressure is sufficient that most if not all the exhaust goes out the main port, hence little or none goes out the auxiliary port. With that reduction in exhaust gas out of the auxiliary port there will be a concurrent reduction in water spray and spitting.
Given your reported professionally installed new water pump and thermostat, I suspect that there is nothing wrong and that the loss of exhaust and spitting of water from the auxiliary exhaust port is normal at higher RPM.
I would not expect a 6-hp outboard to have a temperature gauge output but it likely has an overtemperature alarm so you could run the motor at high rpm in a safe area to see if the alarm sounds. If no alarm, then you're good to go; if an alarm sounds no harm should be done since the alarm set point should be well below the temperature where any damage would occur. Or you could try to put a water pressure gauge somewhere in the line and see if it remains constant (or more likely rises) at high rpm, then you would know for sure that you have sufficient cooling water.
Just my humble opinion; good luck.
seahorse posted 12-24-2012 07:52 AM ET (US)

Unfortunately there is no type of over-temp warning system on a 1967 6hp, only the water flow from the exhaust relief.
weekendwarrior posted 12-24-2012 05:26 PM ET (US)
Thanks guys for the replies. Unfortunately it is most definitely not pumping, I found out because it seized up while I was running full speed and that's what prompted me to check. Fortunately after cooling for a short time, it fired right back up and the compression numbers are unchanged. I can also tell because when it stops pumping, when I back off the throttle and it starts pumping again, the water coming out is extremely hot for a couple of seconds.
To make a long story short, I've decided that as much as I like this old motor, it's just not worth throwing more money at it. So I just ordered a 6HP Tohatsu to replace it. I'm a little down about it, I wish I could have bought another new two cylinder 2-stroke, but they just don't seem to be available anymore. So thanks for the tips, but I'm giving up on it for now. :)

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Author	Topic: 6HP Johnson, stops pumping water at full throttle
weekendwarrior	posted 12-22-2012 07:35 PM ET (US) After a 20 year nap in the garage, my 1967 Johnson 6HP just had a new water pump professionally installed (and thermostat), and it now pumps lots of water, up until about 3/4 throttle. When the RPM's get up, it immediately stops pumping water. Even before the boat is on plane. If you back off of the throttle, then it immediately starts pumping again, and the water coming out is super hot for a couple of seconds, so it definitely wasn't pumping. The compression is 89# top cylinder, 90# bottom cylinder, and my understanding is that this is in the good range for this old motor. If I'm idling in forward and whack the throttle open, it pumps water until the RPM's get up (a few seconds), then it stops pumping. I can cruise around at 1/2 to almost 3/4 throttle and it does just fine, but full throttle and it stops pumping. Does anyone have any idea what might be the problem?
weekendwarrior	posted 12-22-2012 07:37 PM ET (US) When I say stops pumping, I mean that water stops flowing out of the above-water exhaust port. This old motor doesn't have a water stream, but this port is usually dumping a lot of water. This is the only way to visually confirm that it's pumping. I held my hand back there to feel the water while driving the boat to verify when it was and wasn't pumping.
seahorse	posted 12-23-2012 01:54 PM ET (US) Besides the possibility of a slipping hub inside the impeller, do not overlook the very common problem of a leaking head gasket or loose bolts. A slightly warped cylinderhead from overheating can also allow combustion gases into the cooling system. On a small boat there could also be bottom or keel obstructions that could disrupt the flow and allow aeration at the water inlets as boat speed increases. A warped or corroded water pump housing can suck exhaust into it as the higher back pressure lowers the water level by the pump assembly.
HawaiianWhaler	posted 12-24-2012 05:14 AM ET (US) It appears that you are trying to use the auxiliary exhaust port as a telltale for water pump operation, not its intended purpose, hence not a good indicator of cooling water flow. An outboard mechanic once told me that the auxiliary exhaust port is there to allow exhaust to exit above the water line when motor rpm is too low to sufficiently push all exhaust out the main port against the water pressure below the water line, or when operating in reverse. As you raise rpm, the below the water line exhaust pressure is sufficient that most if not all the exhaust goes out the main port, hence little or none goes out the auxiliary port. With that reduction in exhaust gas out of the auxiliary port there will be a concurrent reduction in water spray and spitting. Given your reported professionally installed new water pump and thermostat, I suspect that there is nothing wrong and that the loss of exhaust and spitting of water from the auxiliary exhaust port is normal at higher RPM. I would not expect a 6-hp outboard to have a temperature gauge output but it likely has an overtemperature alarm so you could run the motor at high rpm in a safe area to see if the alarm sounds. If no alarm, then you're good to go; if an alarm sounds no harm should be done since the alarm set point should be well below the temperature where any damage would occur. Or you could try to put a water pressure gauge somewhere in the line and see if it remains constant (or more likely rises) at high rpm, then you would know for sure that you have sufficient cooling water. Just my humble opinion; good luck.
seahorse	posted 12-24-2012 07:52 AM ET (US) Unfortunately there is no type of over-temp warning system on a 1967 6hp, only the water flow from the exhaust relief.
weekendwarrior	posted 12-24-2012 05:26 PM ET (US) Thanks guys for the replies. Unfortunately it is most definitely not pumping, I found out because it seized up while I was running full speed and that's what prompted me to check. Fortunately after cooling for a short time, it fired right back up and the compression numbers are unchanged. I can also tell because when it stops pumping, when I back off the throttle and it starts pumping again, the water coming out is extremely hot for a couple of seconds. To make a long story short, I've decided that as much as I like this old motor, it's just not worth throwing more money at it. So I just ordered a 6HP Tohatsu to replace it. I'm a little down about it, I wish I could have bought another new two cylinder 2-stroke, but they just don't seem to be available anymore. So thanks for the tips, but I'm giving up on it for now. :)