1934 Johnson A-70

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10717
Posts: 10
Joined: Mon Feb 20, 2017 3:11 pm

1934 Johnson A-70

Postby 10717 » Thu Aug 24, 2017 9:57 pm

I have been working on cleaning up an A-70 to use on a Thompson car topper. I started it tonight for the first time and it ran very nicely. Alternate firing and underwater exhaust are big improvements over the 1924 A that was my last project! But how do you tell whether the cooling system that relies on the prop is working? It seemed to heat up more quickly than I expected. Thanks.

XR55
Posts: 116
Joined: Mon Mar 09, 2015 3:44 pm

Re: 1934 Johnson A-70

Postby XR55 » Fri Aug 25, 2017 8:18 am

This motor is a vacuum/pressure cooling system. When starting motor on boat you must go to max. engine speed to purge air from the cooling system. The motor will not cool in a tank or barrel. Touching the side of cylinder case on the opposite side from the muffler should be warm but not hot.

10717
Posts: 10
Joined: Mon Feb 20, 2017 3:11 pm

Re: 1934 Johnson A-70

Postby 10717 » Fri Aug 25, 2017 8:34 am

Thanks. I had it in a barrel but was reluctant to put it on the boat in a lake until I knew it was cooling. Guess I had it backwards.

10717
Posts: 10
Joined: Mon Feb 20, 2017 3:11 pm

Re: 1934 Johnson A-70

Postby 10717 » Mon Sep 11, 2017 9:44 am

Had it on a boat yesterday. As you said, the muffler side is hot but the spark plug side stays pretty cool by comparison. I have a short video of it running which I would be happy to share. Thanks again.


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