Need help with overheated Johnson 15

[krivanj]

1975 Johnson with 3 month old impeller was running fine, no wake zone, then full throttle for five minutes then no wake again. After two minutes in no wake it stalled and lots of smoking. (Would not pull.) Becuase this is a 1975 it doesn't always show water outthe back hole.
After cooling for long while, it pulled but would not start.
Coils had gotten hot enough to be melted but when I took plugs out they both showed good spark when grounded and cord pulled. I checked compression to be 30 psi in each cylinder so I took off head.and will have to buy a new head gasket tomorrow. (Head did not look warped)
I took apart the lower end and impeller looked fine, no vanes broken, etc. Mystery to me why it overheated to this extent after running fine for 1.5 hours.
Sorry so long- any ideas/help out there?

 

[CFronzek]

Re: Need help with overheated Johnson 15

You have one of two problems. For your sake I hope it is the second problem.
First, I don't think a motor with 30 lb on acylinder has enough compression to start. If the reading was accurate you will have to figure out what has gone wrong.
Second, It is rather unusual for two cylinders to have the exact same reading. Especially a very low reading. What may have occured is that the valve core in the compression gage broke and someone replaced it with a valve core from a tire. They look identical but, are not. For a buck, or so, you can get a new compression gage valve core from an auto parts store and do the test again.
If you still get 30 lb it's time for a tear-down to determine what went wrong.