1981 35HP Johnson boom sound at startup

[lennon2010]

I bought this 1981 35 hp Johnson motor it sat for 2 years. I replaced alot of electrical stuff and now I got it to start for 3 seconds. But usually it wont start it'll just put off a loud ear deafening boom sound. Is that considered a misfire and what would create this problem? It happens practically everytime I try to start it and I haven't gotten it to stay running past 5 seconds. ANy help would be appreciated. Thanks Paul

 

[F_R]

Re: 1981 35HP Johnson boom sound at startup

Sheared flywheel key???

 

[BoatBuoy]

Re: 1981 35HP Johnson boom sound at startup

Plug wires crossed?

 

[Lubosleftboot]

Re: 1981 35HP Johnson boom sound at startup

Plug wires crossed?

Defo correct the HT or LT circuits are wired back to front.Change the coils over so that the top is on the bottom and vice-versa....seen this loads of times on motorbikes as well as outboards...

 

[CATransplant]

Re: 1981 35HP Johnson boom sound at startup

You said you acually had it running for a few seconds? If that's the case then it's not switched plug wires.

It sounds like a sheared key on the crankshaft. Often, that will throw the timing completely out of whack. That's my bet.

You'll have to pull the flywheel and install a new key. When you put it back together, it's critical that the nut that holds the flywheel be torqued to the specified value. That's by far the most common reason for a sheared key.

Have you had the flywheel off?

 

[R.Johnson]

Re: 1981 35HP Johnson boom sound at startup

I"m not certain what year they starting doing this, but, If the key is not sheared, check the center hub of the flywheel, this contains the sensor, or trigger magnet's. See if you can turn this inner hub, as it is held with epoxy.