1960 Johnson 75hp. Backfired; stripped head threads for plug

[61mysteryboat]

I just entirely rebuilt a 1960 Johnson 75 horsepower super seahorse. I tried starting it today. It started almost immediately but wouldn't continue to run. I primed the one bulb one more time and when I turned the engine over it backfired and strip the threads in the head where the one of the spark plugs inserts. I can purchase a new head off of eBay reasonably but I'm wondering what may have caused this and what other damage may have been caused. The engine was just completely rebuilt with new Pistons, rings, carburetor rebuild, and all gaskets. Thanks for any input

 

[F_R]

Never heard of a backfire blowing a plug out. The threads must have been damaged before this happened.

 

[F_R]

Of course the backfire may have been caused by a timing issue. You are sure you got the belt timed correctly, right? And flywheel nut properly torqued?

 

[Chris1956]

Likely the second set of points was not set correctly. My fat fifty did that same thing. I was able to remove the crush washer and there was enough threads to hold the spark plug in.

A solid steel spark plug insert is another option. You would simply need a new head gasket and the insert.

 

[61mysteryboat]

Of course the backfire may have been caused by a timing issue. You are sure you got the belt timed correctly, right? And flywheel nut properly torqued?

After further inspection I noticed that the boss marks on the distributor bracket and magneto pulley weren't exactly lined up. I had indexed the bosses by looking at 90° to the distributor bracket arm. I believe I needed to look at 90° to the distributor itself. When I changed my point of view and aligned the bosses the belt shifted one cog (there are only about 30 some cogs total). They are now in line. Am I right in thinking that this was enough to cause my problems?

 

[racerone]

You need to line up timing marks on flywheel and block.-----Then line up pulley on the magneto before the belt is installed.

 

[61mysteryboat]

You need to line up timing marks on flywheel and block.-----Then line up pulley on the magneto before the belt is installed.

I did do that prior. They were lined up first. Then the distributor. Sorry I didn't add that previously.

 

[Chris1956]

You can use an ohmmeter to verity the points open at the right time. Remove the point wire from the coil. Set the ohmmeter to read continuity, and connect it to the point wire and ground. turn the motor over and see that the points open (continuity is zero) when the timing marks on the distributor pully base aligns with the timing mark on the pully.

 

[F_R]

Correct belt timing.

 

[oldboat1]

Might have been an insert that wasn't properly installed or gave way. Smooth bore in the plug hole would indicate that, although likely the threaded portion would have remained on the plug. Compression could have popped it. Make sure you are using the Champion plugs recommended by the factory.