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cooling line for carburator manifold.

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On my 75 620 my thermostat housing cracked at the temperature sensor, so I ordered a new housing. the issue I have is on the original housing there is the connection for the carburetor cooling hose, the new housing I got does not have the port for that hose. So my question is do I need to run that hose or can I block off that hose and not have to worry about cooling of the carburetor manifold.

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The coolant lines are to warm when cold and cool when hot. This keeps the air inside the intake at a more regulated temperature so it runs more evenly no matter the outside temperatures. It has no effect on the carb as it is well above these warming/cooling effects. To prevent carb icing the air filter has an air temp control (ATC) system that mixes exhaust manifold heated are with outside air at about 100F.


The '75 620 shouldn't have the coolant intake. (maybe the engine was swapped?) The intake and exhaust manifolds are bolted together and it's heated only by exhaust directed up against the under side.


In any event the thermostat housing should have a water by pass outlet on it. Your 620 should have a hose from the thermostat housing down in front of the engine to the lower rad hose inlet to the water pump. The later '78-'79 L20Bs Had a coolant outlet from the intake that joined the by pass from the thermostat housing and then went down to the lower rad inlet.


I would return it for the correct one.

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is that bypass outlet right next to the temp sensor, could I disregard bypass if I had to. or is recommended to run that bypass line. I ordered that part on ebay, and I cannot find the the housing I need, all the housing I come across only have the temp sensor hole on them.

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I don't have a picture right now, the way it looks is there right nest to the temp sensor there is a hose that coonects to a fitting right next to it and runs down to the lower part of the motor. I was wondering if I can bypass it temporary till I can get the correct one or take the one I have to a machine shop and have the shop tap a line for that fitting.

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Drill a 1/8 inch hole in your thermostat. This will, work but not the best.


What the by pass does is constantly pass warmed coolant past the thermostat. Why? Because the thermostat is outside the engine and on cold start up it takes too long for the heat to move by convection, out to and warm the thermostat. It will actually over heat while the water around the thermostat is stagnant and cold. By passing coolant past the thermostat and back into the water pump the thermostat samples how hot the water is almost instantly and can open as soon as needed.

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Yup you could.


I think early L series may have done without the bypass relying in the 'jiggle valve', a small hole in the thermostat. This would also allow a flow of coolant from the engine past the thermostat but into the rad. The bypass is better as the warmed coolant does not displace cold rad water back into the warming engine but recycles it directly back into the engine at the input to the water pump. 

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