1975 Johnson 70 pushing a 66 Boston Whaler.....

[Jet Wrench]

First off, the outboard runs great! The old girl has a top speed of 38 MPH at full throttle (5600 RPM), no complaints! I am concerned about the charge voltage (15.5 indicated on the voltmeter installed in the center console). It seems a bit high and is causing issues with my radio (too high a voltage). Possibly the rectifier? Thanks in advance.

Jet Wrench

 

[Ridiculous]

Re: 1975 Johnson 70 pushing a 66 Boston Whaler.....

that's a cool boat...love old boats with old outboards!

 

[jtexas]

Re: 1975 Johnson 70 pushing a 66 Boston Whaler.....

yup, that's an unregulated alternator, so 15 1/2 is about right.
If you aren't already, try champion QL77JC4 spark plugs, the "Q" being the RFI suppressor plug, see if that takes care of the problem. If not, you can always add a regulator.

caught a but-load of white bass out there on Lewisville last Wednesday...........

if you ever see an old blue kingfisher with an old rude '70, with a big yella hound dog on board, stop & say hello.

 

[joey nathan]

Re: 1975 Johnson 70 pushing a 66 Boston Whaler.....

I too have the '75 70HP 'Rude, but must be different model 70573 B as my cowl has different paint scheme. Runs very well, although my boat must be heavier. I can get 32 MPH (TomTom GPS) at WOT, which on mine is around 5200 RPM. I put a voltmeter on mine as well and have noticed the wide swings in voltage also. Inherent to an unregulated charging system, I guess.

No radio so I can't comment on that but had to mention what a great ol' motor this is...

 

[Jet Wrench]

Re: 1975 Johnson 70 pushing a 66 Boston Whaler.....

Thanks for the info gents! This engine is a combination of a 75 lower unit and a 78 powerhead. I managed to throw a rod on the original powerhead, got a little over zealous running it in the driveway with no load! Won't do that again! I will probably rebuild it this winter. I got both engines for $200.00. These things are built like tanks! You won't see a new one built this tough! Very easy to work on. You definitely need a manual.

JT, I am running NGK's in it now, I will try the Champions you recommended to see if the voltage will drop some. Not too worried about it now, the battery is keeping a good charge! Hope to see you on Lake Dallas soon!

Jet Wrench

 

[Jet Wrench]

Re: 1975 Johnson 70 pushing a 66 Boston Whaler.....

Apple,

Thank you as well, I appreciate the input! If you need a part for your engine, I am acquiring quite a collection! Let me know if I can help!

Jet Wrench

 

[jtexas]

Re: 1975 Johnson 70 pushing a 66 Boston Whaler.....

the Champions won't affect the voltage -- but should suppress the radio frequency interference you are getting from the spark plugs (if in fact that's what's happening).

I ran NGKs in my motor once...the center electrode wore down I mean fast!! The champs last a good 2-3 years. I think maybe the heat range isn't an exact match on the cross reference.

btw apple, the paint scheme you mentioned is the only difference between Johnson and Evinrude....................