'87 Johnson 70 Burned Piston

[ddcarter3]

I just bought a Cajun Demon with a Johnson 70 on it. Within the first half hour I lost all power. When I got it home I did a compression test. The bottom cyclinder had 0 compression. I pulled the head and the piston was 1/4 gone, burned. My Dad and I will rebuild it. We weere wondering what would cause just the bottom piston to burn, the other 2 look good. Also the rectifier red wire was burned in half. We don't want rebuild the motor just to have it happen again. Thanks in advance for any help.

 

[rickdb1boat]

Re: '87 Johnson 70 Burned Piston

Odds are it was a clogged carb that did it in. No fuel through the carb. means no oil either. Rebuild all the carbs and clean them out during the rebuild. Also make sure the timing is correct. You must eliminate the cause, or it will surely happen again...When you say 1/4 gone, what exactly does that mean? What part was gone? Any pics you can post?

 

[ddcarter3]

Re: '87 Johnson 70 Burned Piston

Odds are it was a clogged carb that did it in. No fuel through the carb. means no oil either. Rebuild all the carbs and clean them out during the rebuild. Also make sure the timing is correct. You must eliminate the cause, or it will surely happen again...When you say 1/4 gone, what exactly does that mean? What part was gone? Any pics you can post?

1/4 of the piston was melted away. The head was badly pitted on that cylinder also. I will post pix later today if I can figure out how.
Thanks.

 

[ddcarter3]

Re: '87 Johnson 70 Burned Piston

Here are the pix. My Dad has the head. He will take it to a local Johnson dealer.

 

[ddcarter3]

Re: '87 Johnson 70 Burned Piston

Sorry, Itried to post the pix. Error said they were too big. Is there a good image sharing site that allows uploads and links?

 

[Paul Moir]

Re: '87 Johnson 70 Burned Piston

Upload them to tinypic.com. No registration and it'll automatically resize them for you.

I agree with Rick. Running lean (from a partially clogged carburetor) will melt a piston down fast. Next step is to disassemble engine and check the crankshaft & bearings. Get a hold of a manual first though as there are important things to keep track of and know when disassembling an engine. If you don't want to invest money at this stage, you can often borrow a manual from the local library.

 

[Dhadley]

Re: '87 Johnson 70 Burned Piston

Where the piston is burned will tell you a lot. If it's burned in the center of the dome, it most likely is ignition related. If it's burned on the edge by the exhaust port it could be carb related or ignition related. The damage on the sides will give you clues too. Damage only by the exhaust port? Even damage all the way around? Damage all or most of the way around but mostly in 4 places evenly spaced (4 corner seizure).

What spark plugs were in it?

 

[ddcarter3]

Re: '87 Johnson 70 Burned Piston

Here are the pix. I couldn't get Tiny Pics to load.

 

[ddcarter3]

Re: '87 Johnson 70 Burned Piston

Is the Johnson Service Manual the best or is there another?

 

[Silvertip]

Re: '87 Johnson 70 Burned Piston

The OEM manuals (in this case a Johnson manual) is by far the best. Post a picture of the plugs that came out of the engine. Note which one came out of the lower cylinder.

 

[ddcarter3]

Re: '87 Johnson 70 Burned Piston

Mt Dad has the head with plus in it. I hope to see him this week. The plugs looked ok. The head at the lower cylinder was pitted. Dad said it got really hot to do that. I will post pics when I can get them. Dad refuses to go digital. Still on dialup also. The burned area of the piston lines up with ports. I am not sure which one it is. I am sure when I am through with this I will know alot more. I have never worked on a 2-stroke motor. Is there a good site for 2-stroke basics?

 

[ddcarter3]

Re: '87 Johnson 70 Burned Piston

Is the only place to get the OEM manual a Johnson dealer or can it be ordered online?

 

[Dhadley]

Re: '87 Johnson 70 Burned Piston

The damage is by the exhaust port. What spark plugs were you using? I know they looked great -- which ones are they?

 

[ddcarter3]

Re: '87 Johnson 70 Burned Piston

The plugs are: Champion QL77JC4
Dad said the top cylinder at the head shows signs of heat as well. Center cylinder looks good. Top and bottom were oilyer than the center one.
Thanks for the help.

 

[ddcarter3]

Re: '87 Johnson 70 Burned Piston

I also found a late 70's 70 hp Johnson. I wasn't sure if the powerhead would bolt up. The powerhead was rebuilt, the lower unit was bad. The guy only wanted $350. What are your thoughts on this. I ordered the manual today.

 

[Dhadley]

Re: '87 Johnson 70 Burned Piston

The late 70's 70 will be a 49 cid motor. You need a 56 cid 3 cylinder if you're looking for a compatible swap. Rebuilding yours is not a big deal. We'll help you thru it if you like or we can suggest some shops that will do it for you.

 

[rickdb1boat]

Re: '87 Johnson 70 Burned Piston

http://www.kencook.com/evinrudejohnsonmanuals/ ] Factory Manual[/url]

 

[ddcarter3]

Re: '87 Johnson 70 Burned Piston

Thanks guys! I have a service manual on the way. Hope to have it by the weekend. I will drag the boat over to Dad's and tear into it.
Dhadley: Were the plugs right? Champion QL77JC4
Is there a better choice? Was it the carb being cloged that most likely caused this?
Thanks again for your help.
David

 

[Silvertip]

Re: '87 Johnson 70 Burned Piston

Clogged carb, too much ignition advance, water pump not working, stuck thermostat, are all possible causes.

 

[Dhadley]

Re: '87 Johnson 70 Burned Piston

With today's fuel that's a good choice on plugs. Just make sure you use fresh 87 octane fuel and have it set up at 5800.