WOT rpm & overheat horn '77 Johnson 140 hp

[87Aggie]

What should my WOT rpm be on this motor? I'm not sure what size prop I have - is there a way to measure it somehow? The motor is mounted on an 18 ft. Glastron trihull skiboat. Also, I grounded the tan wire on my overheat sensor and made the horn blow. Is this verification enough to know that the overheat alarm system is working correctly? When I turn my key is doesn't sound, not sure if it is even supposed to.<br />Thanks for any help ya'll can provide!<br /><br />Don

 

[ob]

Re: WOT rpm & overheat horn '77 Johnson 140 hp

The older systems did not sound a test horn when the key is turned on.The test you performed only confirms that if the temp sensor detects an overheat and closes circuit that the alarm will sound.It doesn't test the sensor itself.I wouldn't settle for any less than 5500 rpm on that engine and 5800 wot would be better.Look on the center hub area of the prop and see if the dia and pitch is embossed on the hub.Also look on the prop barrel.

 

[87Aggie]

Re: WOT rpm & overheat horn '77 Johnson 140 hp

I don't have my manual handy, but I think I remember something about testing the sensor being in it. I have an OEM manual. Do you know if there are instructions for testing in the manual, if not, how do you go about doing it?

 

[ob]

Re: WOT rpm & overheat horn '77 Johnson 140 hp

The temp sensor needs to be removed to test.Not sure what temp your particular one is designed to alarm at but I believe it is on or near the 163 degree mark.You would need an ohm meter and an accurate thermo to test .Submerse the sensor in water that is temp regulated and connect the meter leads and watch for a continuity change.Might even be easier once removed to just replace with a new one to gain some peace of mind.

 

[87Aggie]

Re: WOT rpm & overheat horn '77 Johnson 140 hp

That sounds like a good idea ob. There's one sensor for each set of heads, right? Also, I'll need a copy of new head gaskets (assuming you have to remove the head covers to access the sensors)

 

[87Aggie]

Re: WOT rpm & overheat horn '77 Johnson 140 hp

That's supposed to be a couple not copy of head gaskets.

 

[ob]

Re: WOT rpm & overheat horn '77 Johnson 140 hp

Yes sir.Two sensors in all.Not sure on your model but I believe you are correct in that the head covers will have to be removed to access them.Good luck and keep us posted on your findings.

 

[87Aggie]

Re: WOT rpm & overheat horn '77 Johnson 140 hp

Thanks for all your help!!