1970 85 hp Johnson - dies after 15 minutes

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cascadeboater

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Help! I have this old Johnson motor that will now go about 15-20 minutes and then start to miss, and eventually quit. Once it cools - will run good again for awhile. Have previously replaced water impeller & housing, the points, fuel pump, powerpack (was bad originally). Noted that the flywheel was extremely hot. Will replace the thermostat now. Does anybody know why the flywheel would be getting so hot?<br /><br />Thanks
 
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itstippy

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Re: 1970 85 hp Johnson - dies after 15 minutes

The flywheel is very hot. What about the rest of the powerhead? Either:<br /><br />The flywheel is rubbing against something and generating heat through friction.<br />OR<br />Your powerhead is not getting cooling water through it's passages and you are frying (or have already fried) it to death. The heat is transferred throughout the powerhead, including the flywheel.
 
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cascadeboater

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Re: 1970 85 hp Johnson - dies after 15 minutes

Thanks for your reply. The powerheads (a V4) appear to be very hot as I cannot hold my hand on them for 2 seconds or so. Hard to believer the flywheel could heat up this much by jsut radiant heat from the engine block. Maybe the flywheel is touching the stator (generator coils)? Or.. could a defective rectifier cause the stator to get hot?
 
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itstippy

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Re: 1970 85 hp Johnson - dies after 15 minutes

The flywheel's bolted to the crankshaft so it'll heat up. Is the flywheel hotter than the block? By "powerhead" I mean the entire motor, not just the cylinder heads. If your thermostat's stuck closed you've got water going through the cooling channels in the heads, but not through the rest of the powerhead.
 
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cascadeboater

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Re: 1970 85 hp Johnson - dies after 15 minutes

I would say the temp is about the same on the flywheel and the powerhead (hard to tell definitely. I took the thermostat out and heated it with a torch (quickly) and the plunger appeared to move up but am unsure at what temp it would open - will replace anyway. When I replaced the points - the flywheel appeared not to contact the stator so kind of eliminating that as a possibility. Could there be a restriction to water flow within the powerhead that is accessible? This would be my last resort I think. Thanks
 
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itstippy

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Re: 1970 85 hp Johnson - dies after 15 minutes

Someone with thorough knowledge of the V4's will be along with tests to do. Basically force water into inlets and see if it comes out of outlets I suppose. There are guys on this forum who know every single nut and bolt and pin in or on your motor. They have seen it ALL.
 
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Solittle

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Apr 28, 2002
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7,518
Re: 1970 85 hp Johnson - dies after 15 minutes

Is your overheat alarm going off? Have you tested the overheat alarm to confirm that it is working?<br /><br />Is this s salt water motor?<br /><br />Have you ever replaced the deflectors in the block?<br /><br />You might confirm that it is getting too hot with <br />the heat sticks. You also could try running it without the thermostats though I would not leave them out.
 
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PDS

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Re: 1970 85 hp Johnson - dies after 15 minutes

Test your thermostats a little more accuratly. What I do is stick some fishing line through the thermostat(force open spring and insert line, then let it close) then put it in a pot of water with a thermometer in it and heat it up on the stovetop. Note: neather thermostat or thermometer should touch bottom of pan. When thermostat falls off line note temp. That's your thermostats opening temp. P.S. Tye it to a stick or something and set it over the pot(safty first)
 
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chris abela

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Sep 25, 2002
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Re: 1970 85 hp Johnson - dies after 15 minutes

check for leaks in fuel, a lean situation will overheat the engine
 
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