'69 Johnson 115hp won't start back up once its been run a while. Makes popping sound when stopped.

[smlJohnson115]

'69 Johnson 115hp won't start back up once its been run a while. Makes popping sound when stopped.

Starting it isn't always easy, but it will start. Then it runs nice and fairly smooth. But if I slow down, it sometimes dies on its own. Or if I cut it off, and then try to restart it, it turns over fine but won't catch.

Right after the engine stops, I hear a popping nose from the engine, that slowly decreases in frequency and volume, and then stops. I have to paddle home and let it sit for a while.
I have a service manual, but I don't know what to try fixing. The plugs all look new and the gaps are good. I bought the boat about a year ago.
Thanks for any shared expertise!

 

[smlJohnson115]

The popping noise seems to be the key

 

[emdsapmgr]

Pull the flywheel off and check the flywheel key. If it shears, it won't run right. Have you overhauled the carbs lately?

 

[oldboat1]

^^Agree, and recommend some basic tools: compression tester, open air adjustable spark tester, infrared temp sensor. Look for even compression between cylinders, about 1/2 inch spark, temp no higher than 160F on the tops of the heads. You need clean fuel, mixed 50:1 with TCW-3 outboard oil, no water in the tank. Adding fresh fuel to a tank, particularly a permanent tank, won't cure prior contamination (e.g., water in the bottom, down where the pick up is). Should change out the impeller in the lower unit (water pump) if you haven't done that.

 

[James R]

And while you are doing all this add a water trap filter and Stabil Marine to your fuel. These simple things are invariably overlooked. Had a motor in with the "not restarting when warm" situation. Wrong spark plugs.