Piston hitting spark plug

[Rrosenbloom]

Hi all. Bought a project boat and while trying to get the motor started, I pulled plugs and found the gap completely closed. Regapped, and cranked again, gap closed again. I can see a mark on the piston where it is hitting the plug. The motor is a Johnson 20RL71S. Both cylinders have have low 90s compression. Any help on next steps is appreciated!

 

[racerone]

Both plugs ?-----What is the number on the plug ?-----Sounds like the wrong plugs.-----If that is not it , you have major $ issues.-----The correct plug is a short reach J4C.

 

[Rrosenbloom]

Well it’s only one plug, the top cylinder. The plugs are champion L86C. For $5 I’ll pick up the ones you mention and see how that goes. Thanks for the reply, and fingers crossed!

 

[GA_Boater]

Both plugs are Champion L86C and only one is getting a crushed electrode.

New plugs won't help.

 

[racerone]

You do not want an-----L plug------In that motor.------Perhaps this motor has different piston in it than original too.-----Or major internal problems

 

[Rrosenbloom]

Replaced plugs and she fired right up. Shut it down and checked plugs....all looks good. Still need to check the impeller before the water test, but I’m at least more optimistic now. Thanks for the info.

 

[Oldnbold]

When my 1973 115 Evinrude with 28 years and who knows how many hours started closing the gap on one spark plug in one cylinder, it turned out to be the wrist pin egg shaped the piston pin bore.

 

[juno pierrat]

soooo, what plug did you put in?

 

[Rrosenbloom]

I used the J4C

 

[tramsdell]

When my 1973 115 Evinrude with 28 years and who knows how many hours started closing the gap on one spark plug in one cylinder, it turned out to be the wrist pin egg shaped the piston pin bore.

Op is probly headed that way. J4c are just a hair shorter plug so it works for now...

 

[racerone]

J4C is the correct plug and a full 1/8" shorter the the plug that was getting hit.----The bearings in a 71 model 20 HP are quite durable !!.

 

[F_R]

True, a durable engine design. Also, as previously mentioned, it may have serious internal damage. For instance it may have broken internal parts, such as a piston ring, and a broken piece got between the piston and spark plug, and is gone now. However, before worrying about stuff like that, go with the correct plugs and see what happens. I do suggest a compression test to look for internal damage before starting it though.

 

[tramsdell]

Sounds like op is all fixed up then with the j4c !

 

[Rrosenbloom]

Thank you guys for the responses. I let the engine run with the new plugs, and it seems to run great. Getting ready to post some lower unit questions now! (BTW, I did a compression test and all was good.

 

[Tjy7510]

Hi all. Bought a project boat and while trying to get the motor started, I pulled plugs and found the gap completely closed. Regapped, and cranked again, gap closed again. I can see a mark on the piston where it is hitting the plug. The motor is a Johnson 20RL71S. Both cylinders have have low 90s compression. Any help on next steps is appreciated!

This is an old post but being the same subject thought I would share my issue. Just found the same thing on the NKG plugs that were in my motor pushed down. But had to resurface the head cover that was warped. I will try J4C's but if that doesn't work will just add a washer to each of them. The height on J4C says .375 but not sure from where to where? .375 would work great if that was from the crush washer to the end of plug. If it's 375 thread height only including the end would be .500 which is what I have now that is hitting.