1973 65hp, new lower unit, now what?

[Porto]

Hello, I have a 1973 65hp 3 cylinder motor that had a bad lower unit. I replaced it with the lower unit off a 1970's (serial # tag is unreadable) 50 hp. I understand these lower units had different gear ratio's, is this right? If so, what size/pitch prop would be right for this motor so I'm not over-revving it but still getting full power? Also what is the max rpm of this motor? Thanks......

 

[Laddies]

Re: 1973 65hp, new lower unit, now what?

5300 RPM, The 500 was 2 to 1 gear ratio but I can't seem to find the 650 3 cyl.

 

[Porto]

Re: 1973 65hp, new lower unit, now what?

Thanks for the response. I just read in another post that the gear ratio for the 1973 650 was 1.64 to 1, does that sound right?

 

[Porto]

Re: 1973 65hp, new lower unit, now what?

I checked the rotation of the the driveshaft versus the rotation of the prop shaft on the lower unit that came off the 650 and 1.64 to 1 seemed accurate. Does this mean I want a higher pitched, smaller prop then what was on the 650 or a bigger, lower pitched one. The prop that was on the 650 had a 13 pitch and is stightly bigger and less pitched then the one on the 50hp. The boat this motor is going on is a 1975 16' baja fiberglass ski boat. I don't plan on skiing with it, I don't even care about going 'fast', I just don't want to ruin the motor. Thanks again......

 

[Porto]

Re: 1973 65hp, new lower unit, now what?

I have another question while I'm waiting for info on my prop dilema. The starter on this motor doesn't work every time I turn the key. It will engage the flywheel and spin but it won't turn the flywheel. After several tries it will turn the flywheel. Do I need a new starter? Thank you.......

 

[GA Paramedic]

Re: 1973 65hp, new lower unit, now what?

I had this happen to me as well... I used some wd 40 on it and made sure nothing was prohibiting it from spinning up. Also it still does it to me occasionally but only when i try to crank it without letting it totally stop before I try it again. Just impatience on my part I guess. Hope that helps. But i don't think it would be the starter. just something keeping it from raising when it spins.

 

[Porto]

Re: 1973 65hp, new lower unit, now what?

A little wd40 did the trick, thank you.<br />Now I have another problem. I plugged the tachometer into the front of the control box and started the motor. When I turned the key off the motor kept running. I killed the motor with the choke and unplugged the tach and restarted the motor. This time when I turned the key off the motor shut off. Do I have a wiring problem or am I missing something? Thanks.......

 

[Texasmark]

Re: 1973 65hp, new lower unit, now what?

1.64 sounds right. That engine turns a small diameter prop (10"?) at high rpm's with the small gear box. Not a lot of thrust as compared to the same thing on the larger gear box turning a 13" wheel (other 65's built over the years), but if you are operating in shoals (shallow water) it helps you stay outta da mud.<br /><br />Mark

 

[Porto]

Re: 1973 65hp, new lower unit, now what?

Thanks Texasmark. Since I have changed to a 2to1 lower unit, what size and pitch prop would you recommend? Can I damage the motor by running the wrong prop? Thanks again.......

 

[Texasmark]

Re: 1973 65hp, new lower unit, now what?

It is a screw thing. Engine rpms divided by gear ratio is prop revs. Only difference in props and screw threads is fancy blade geometry and prop slippage in the water.<br /><br />You can run the numbers of what you have vs 2:1 and come out about the same.<br /><br />Ran back thru your initial thread and I think I saw a 10" dia by 13 pitch. The 2:1 new gearbox may allow you to run a larger diameter (have no idea) but let's say it doesn't.<br /><br />Then the ratio of 1.64 to 2 is 1/1.22. So if you keep the same diameter and raise the pitch to 1.22 what it was, you should be doing the same work. So a 13P x 1.22 = 16. Your new lower unit should provide approximately the same performance with a 10x16p propeller.<br /><br />HTH.<br /><br />Mark

 

[Porto]

Re: 1973 65hp, new lower unit, now what?

Thanks Mark, I'll give that a try......