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This is a follow-up to a previous post ('78 402 cooling system leaks). I've been studying the problem - seems that there is too much pressure in the cooling system. After a new pump install a I started the motor using a cooling muff & at a high idle I had a very strong tell tale stream, leakage past the poppet valve diaphragm (no rupture in diaphragm), and high pressure leakage past the cyl block cover gasket onto the top spark plug. I ran the motor only a couple minutes like this. I've been studying the cooling system and it looks like the water from the pump enters the cylinder block under the bottom cylinder, fills the water jacket around the two cylinders, then exits at the top of the block through the thermostat & poppet valve housed in the coil mounting bracket. The tell tale is fed through a port in the water jacket, connected to an external hose on the L/H side (base of the exhaust manifold cover) that leads to the fuel pump casting on the R/H side (transfer port cover), then on to the tell tale nozzle. Water that flows past the thermostat/poppet valve is directed to a port in the block under the coil mounting bracket. That port seems rather small - 3/8" dia. or less. From there I assume the water flows around the exhaust manifold, then somehow is directed through the ex. man. divider plate/ex. man. cover, then into the exhaust pipe, down through the lower unit and out the prop hub. I have removed the cylinder block cover, poppet valve diaphragm cover, & coil mounting bracket; an inspection revealed no blockages or anything abnormal.
If my assumptions concerning water flow through the engine are correct, then the symptoms indicate that the blockage (if there is one) is down stream from the thermostat/poppet valve. I did use pressurized air to blow down through the small port on the block and had no indication of a blockage. Next I put a funnel in the port & poured water down it and again there was no indication of a blockage. I am hesitant to pull the exhaust manifold cover because it would require pulling the power head out of the lower cowl to gain access to all the bolts.
So here is my question; is it plausible that there is a blockage in the exhaust manifold area, or is a strong tell tale normal on a cold motor and my leaks are due to a long period of non-use that can be fixed with some new gaskets & seals? I'm thinking I'll just replace the cyl block cover gasket, the poppet valve diaphragm, the thermostat, and the coil bracket gasket & see if it does ok. Am I asking for failure by not pulling the power head now & going through the exhaust cooling passages?
Appreciate your help,
Joe in St Louis
If my assumptions concerning water flow through the engine are correct, then the symptoms indicate that the blockage (if there is one) is down stream from the thermostat/poppet valve. I did use pressurized air to blow down through the small port on the block and had no indication of a blockage. Next I put a funnel in the port & poured water down it and again there was no indication of a blockage. I am hesitant to pull the exhaust manifold cover because it would require pulling the power head out of the lower cowl to gain access to all the bolts.
So here is my question; is it plausible that there is a blockage in the exhaust manifold area, or is a strong tell tale normal on a cold motor and my leaks are due to a long period of non-use that can be fixed with some new gaskets & seals? I'm thinking I'll just replace the cyl block cover gasket, the poppet valve diaphragm, the thermostat, and the coil bracket gasket & see if it does ok. Am I asking for failure by not pulling the power head now & going through the exhaust cooling passages?
Appreciate your help,
Joe in St Louis
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