1965 Johnson 40hp no spark - HELP!!!

brian0938

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Jul 20, 2012
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Long story long...

I can take anything apart and put it back together, but I cannot diagnose anything and I have a limited sense of parts ID and vocabulary...

I bought an old Starcraft runabout recently with a 40hp Johnson (1965). It ran "ok" the first time out, but would only go about 20 mph. The next time out, I could only get up to about 12 mph. I brought it back, rebuilt the carb and cranked it in the driveway...I did have water in a barrel so no overheating. It soulded a little better, but still not like I though it should. I pulled the top spark plug and the engine kept running exactly the same. I pulled the bottom plug and the engine died. Repeated the sequence and figured I had a bad coil. I removed the flywheel, cleaned everything very well, sanded the magneto, replaced the coils, plugs, points, wires and condensers. Put it all back together and it immediately cranked, ran for about a minute and I tried to cut the key off and it just kept running and then died on its own about 30 seconds later. I disassembled everything again and reassembled and it will not start. I am getting good fuel to the carb, but there appears to be no fire. It is turning over well. I rechecked my gaps and used a spark tester and no spark. What am I missing?
 

Chris1956

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Mar 25, 2004
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28,577
Re: 1965 Johnson 40hp no spark - HELP!!!

First think. Check to see if the woodruff key is sheared. If you do not torque the flywheel nut enough, they sheer every time. Torque was something like 90 Ft-lbs. PM F-R and ask him.

Make sure the killer wires are not rubbing against the flywheel. If the short you will not have spark. What do you mean by "sanded the magneto"?
 

brian0938

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Re: 1965 Johnson 40hp no spark - HELP!!!

First think. Check to see if the woodruff key is sheared. If you do not torque the flywheel nut enough, they sheer every time. Torque was something like 90 Ft-lbs. PM F-R and ask him.

Make sure the killer wires are not rubbing against the flywheel. If the short you will not have spark. What do you mean by "sanded the magneto"?


The woodruff key did shear. Wish I would have noticed that!!! All wires are tucked away and clear from rubbing. I used 800 grit sandpaper to clean off the magneto contacts. Even with the nut tightened, are you thinking that without the key that the flywheel is just spinning and not turning the motor?
 

tx1961whaler

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May 31, 2008
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5,197
Re: 1965 Johnson 40hp no spark - HELP!!!

That flywheel gets torqued down to 105 ft-lbs. It is important that it is torqued to that value with a torque wrench. If the key shears too many times the motor turns into an anchor.
You will not get spark if the key is not in place.
 

F_R

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Jul 7, 2006
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28,226
Re: 1965 Johnson 40hp no spark - HELP!!!

Flywheel and shaft must be in perfect condition, not battered from keys shearing.

Tapers must be clean and dry

You MUST use a torque wrench to tighten the nut to 100-105 ft lbs.

Somebody said that continued shearing will turn it into a very cumbersome boat anchor. I agree.
 

brian0938

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Re: 1965 Johnson 40hp no spark - HELP!!!

okay...just so I am clear on my vocabulary, the key we are takling about is about an inch long and secures the flywheel to the shaft...correct? It does not go below the taper on the shaft down into anything below that...correct?

Are these leys something I can pick up at a parts store? The NAPA here has a bin full of different size keys so if I knew the measurements I could probably find one.
 

lindy46

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Nov 27, 2008
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3,886
Re: 1965 Johnson 40hp no spark - HELP!!!

Use only factory key from a Johnnyrude dealer.
 

tx1961whaler

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May 31, 2008
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5,197
Re: 1965 Johnson 40hp no spark - HELP!!!

+2 on using the factory key only. Ones that "almost fit" will almost certainly shear.
The key PN is 307480
 
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