Lark IV Cooling

[putback]

Bought a ’62 40 hp lark. 20+yrs in storage, looks very good and all original, not altered. 120 lbs compression on both cyls. Redid the fuel, ignition & cooling systems, even rewired it. Put it in a barrel started right up idled well, brought it up just off idle, nice and smooth. Showed a good water discharge. Then things changed. Water discharge slowed to a mist, then mostly steam. Shut it down, heads seemed to hot to me. Water intake clear. Checked the new impeller, case & key, ok. Rubber tube grommets look fine. Thermostat opens at about 130 in a pan of water on the stove. All back togather now ready to retry. This is my 1st vintage outboard don’t want to screw it up, any ideas or advice.

[chris-p]

That motor has a thermostatically controlled cooling system. When the stat is closed, only a small misting of water will be exiting the discharge hole. Then when the stat opens, it will increase in water output. It will continue to open and close as it needs to control temps.

However, if it is overheating, that is a problem.

[frankr]

Got it deep enough in the water?

[outbdnut2]

Splash a few drops of water on the head – if it boils and spits, it’s too hot. You will probably not see much discharge unless you have it on a boat or have a test prop and rev it up for awhile. All the Johnson/Evinrudes from 1959 up that have thermostats behave this way, along with the ’58 35 HP Lark and Super Sea Horse, and the 1958 Fat 50 HPs.
Dave

[fleetwin]

OK, you replaced the impeller, did you use an OEM impeller or aftermarket? You pulled the gearcase down again after you experienced trouble while running in the tank and found no obvious issues, do I have it right? Frank’s advice is spot on, make sure the water level is high enough. You checked thermostat operation, that seemed OK, how about the plastic housing and relief spring/cup at the bottom of the housing, are those pieces OK? Perhaps some debris got caught in the relief spring/cup causing water to bypass the powerhead. The engine won’t cool if the relief cup/spring/housing are not in place/working properly. Finally, have you confirmed that the engine is actually overheating? Like others have said, seeing some steam from the cooling outlet is normal, water spray will be intermittent depending on thermostat operation….

[outbdnut2]

Like Frank said – be sure it’s deep enough in the water – it has to be several inches above the cavitation plate to work. I’ve replaced several impellers (some brand new) that were destroyed because someone ran them with the intake screen barely covered with water.
Dave

[putback]

Its well over the cav plate in a 55gal barrell. Yes I pulled the gear case after and found no problems. The relief spring/plastic valve clean, free & not deformed. Did spend the $$$ for an OEM pump kit. No I don’t KNOW its overheating. There was minor splash water drops on the head/block no boil or spit there, could hold my hand on the head a second or two. On the 1st start had good water discharge for 30ish seconds then slowed to near nothing, then ran it maybe a minute. Not knowing I just shut her down. It sounds like what it did was normal, I just didn’t run it long enough. When the temp gets above freezing I’ll give it another try. I’d feel better if I could add a real temp gauge but don’t want to bore/thread the casting for a sending unit. I could wire a bulb to the overheat light sending unit in the head. Are they near accurate? What would a ballpark runtime be befor it shows a good discharge stream.

[fleetwin]

What is confusing to me is that there was good water discharge for the first 30 seconds, then it slowed to near nothing….
Usually it is the other way around, little or no spray on start up, then it sprays for a bit when the thermostat opens up…
I wouldn’t bother with the temperature gage…You might want to unplug the wiring from the hot light, then hook the leads up to a "hot horn" from a newer engine….

[frankr]

If the water temp is near freezing, the thermostat probably won’t open at all unless you run at high speed. Then it will close again when you slow down. When closed, there will be only a bit of spray out the blow hole in back. This is normal operation, even at warmer water temps.

If it put out significant spray when you first started it, I’d guess the ‘stat was stuck open, then snapped shut and worked normal. Just my guess.

Temp gauges often come with a Z-bracket to hold the sender in contact with the cylinder head. No drilling necessary. Or you could jury-rig a tee arrangement to put it in the existing port in the cylinder block that feeds the automatic choke. Actually, I’d prefer the head because that’s the hottest part.

https://www.amazon.com/Sierra-62748P-Wa … ture+gauge

[Monte NZ]

It might be worth while to try using a pistol type thermometer, where you can point it close to the head and see what the temperature is. You may be able to hire or get the loan of one.
It may also be worth running the motor without the thermostat and watching and feeling the discharge water to see how hot it gets.
Another method, which is shown in the workshop manuals of the 1960s, was to use two thermomelt sticks. The higher temperature one melted if the motor was running too hot.

MonteNZ

[Author]

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