40 HP Johnson

[RetNav]

I was given this Johnson today and the previous owner said it ran good but then just bogged down and only ran on one cylinder.

The model number is 40E75C.
The serial number is J425571.

Using previously posted info under "what year is my E/J' it is a 75 model.

When I got it home I pulled both plugs and the bottom plug's electrode was bent until it was touching the center on the plug. Plugs that were installed are Champion L86C.

Checking compression shows 110 on both cylinders.

This particular model in not included on the manuals that I have. Would someone please check and let me know what plugs are recommended for this motor and what the gap setting is supposed to be?

 

[ezeke]

Re: 40 HP Johnson

Champion UL81C at .030 is recommended:

http://www.boatsetup.com/SparkPlugChart.pdf

The Champion L82C is also sold as a direct replacement for the original Champion L4J.

Check to be sure there is a crush ring on the spark plug, otherwise it may sit too deep. If a Heli-Coil has been used, the plug may also sit too deep because of that.

 

[F_R]

Re: 40 HP Johnson

You might have a bad bearing, letting the piston come up too high. In fact, that's pretty likely.

Remove both plugs. stick a screwdriver into the suspect hole and against the piston. Turn the flywheel by hand and follow the piston down with the screwdriver, with a light touch. When the piston is about half way down it's stroke, stop turning the flywheel and give a push on the screwdriver. If the piston moves down a bit with a clunk, you have found the reason for the mashed plug.

Another cause could be a shattered piston ring or broken piston. There would be debris in the combustion chamber and spark plug if that's the case.

 

[RetNav]

Re: 40 HP Johnson

Thanks for the responses. I have checked for play in the bottom piston and there is none.

Looking where the plugs bottom out there is a definite difference in how deep the plugs go, with the bottom plug going in further than the top one.

How about surface gap plugs, or maybe an extra washer on the bottom plug?

 

[ezeke]

Re: 40 HP Johnson

I would buy a cylinder head gasket and take the cylinder head off to see why the plug is seating incorrectly. A good machine shop should be able to put the correct heli-coil in. They come in two depths, 1/2 (391443) and 3/8 (391442). If you use the wrong one, you get a mashed plug.

 

[RetNav]

Re: 40 HP Johnson

Thanks, that is probably the best course to take.

I'll post back when I get this resolved.

 

[samo_ott]

Re: 40 HP Johnson

Another free engine... sigh...

 

[RetNav]

Re: 40 HP Johnson

Thanks for the responses. I found another head and replaced the one on the motor. Connected a battery and fuel tank and it fired right up, no knocking or other noises. I will have to get a carb kit and clean it as the float is sticking.

All in all I believe it was a good investment.

BTW, I have traded an extra trailer for a 14' boat and trailer that a friend had setting in his back yard. Total investment 00000.00. Hows that for luck?

 

[tashasdaddy]

Re: 40 HP Johnson

now do the water pump. no need to burn up such a high cost investment.

 

[RetNav]

Re: 40 HP Johnson

Got one on order now and will change it out when it comes in. I agree, with everything invested no need to take chances.