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Restoring 1977 Mercury 500 50 HP

N

nauticalsun

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Outboard starts fast and runs fine but after a few minutes water then as well steam appears out the telltale pipe and upper ports so am suspecting a clog or thermostat or blown gasket problem in the water jacket system, but the manual isn't very clear about even if this model has a thermostat? Compression is good at all 4 cylinders with no sign of dark exhaust smoke down below. Any further suggestions, tips before breaking this motor down would be greatly appreciated?
 
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Doesn't have a thermostat. I would suggest taking the lower unit off and checking the water pump impeller and try to flush out the large water supply tube with a garden hose. Check your lower unit lube for water leaks also. It will turn it milky. I have the same motor. Start there and I will give you all the help I can. There are many more on this site that have vastly superior technical help.
 
On mine, the water tube was not sealing tight enough in the rubber seal in the LU and it did this nonsense. (Drove me nuts!) The tubes sometimes get bent up and be too short to seal properly. The tube must stick down at least 3/4 of an inch below the tower, and that rubber seal needs to be in good shape.

Jeff
 
Thanks guys, these tips are useful -- already have installed a new water pump a complete kit with new rubber seals, impeller, gaskets, and o rings and all, but will take the lower unit off again and flush water thru and check the circulation, and seals, still studying the manual on this model so hopefully will discover something soon. At least now not looking to change the thermostat :cool:
 
Okay took lower unit off and attached a garden hose with attachment to telltale hose and back flooded the system as water came gushing out the drive shaft housing but neither the exhaust tube or the water pump copper pipe riser has any water coming out at all. With a flashlight looking into the housing can see water gushing into chamber from the bottom upper left hand corner of motor and running out the bottom of drive shaft housing. This doesn't seem normal any suggestions? Thinking maybe a plug has popped out or something gasket?
 
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nauticalsun said:
Okay took lower unit off and attached a garden hose with attachment to telltale hose and back flooded the system as water came gushing out the drive shaft housing but neither the exhaust tube or the water pump copper pipe riser has any water coming out at all. With a flashlight looking into the housing can see water gushing into chamber from the bottom upper left hand corner of motor and running out the bottom of drive shaft housing. This doesn't seem normal any suggestions? Thinking maybe a plug has popped out or something gasket?
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I just shot the water up the copper pipe after trying to put some insulated thin wire through the pee hole system. Did it about 5 or six times and then hoped for the best. Pee stream increased about double.
 
Took the drive shaft housing off, then rigged up a water hose onto the water pump copper inlet tube and water is coming out multiple places around the edges of the exhaust extension plate and from above the water inlet housing so now planning removing that and replacing all the gaskets down there next week and hopefully nothing has cracked from the heat.
 
Plan is to replace all the gaskets, seals and such as possible that are still available while apart and flush and clean all the channels. It did seem as though the nuts were on pretty loose so maybe the water wasn't circulating correctly with some lost compression and leaks thus causing the steam ( A theory)? Does anyone know what is the correct torque for the nuts that hold all this together?
 
I have an OEM Service Manual and it lists hundreds of specific torque specifications. I'm not sure which you need. Pictures would be helpful. I bought my manual on Ebay.
 
A follow up report, replaced all the gaskets in the water jacket system with new ones, reassembled the Exhaust Extension Plate & Shift Linkage, plugs etc -then torqued the bolts- reinstalled lower unit and started the motor, - Fired right up and didn't steam up either ran cool with telltale pipe running fine with nice strong flow, so took the boat out today on the lake for a test run. She ran out fine Wow! Powerful !! but on the way back to port she lost power and stalled, but starts back up fine but when levering up the gas she stalls out again. Figured maybe Carburetors, she got me back to port on low speed? I am glad at least isn't a steam engine any longer. Any further ideas on the stalls would be helpful.[h=1][/h]
 
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Okay got a different gas tank with a new gas line, filter, and pump bulb, - cleaned and flushed out the carbs with cleaner and added some sea foam to the mix. Motor is running out well with acceleration much improved but jerky but am just testing in a barrel of water. We did find out that one of the cylinders # 4 was not firing and the coil tested bad so now am awaiting the parts to fix that as may smooth out the motor? Hopefully we can do another lake run soon.
 
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