70 johnson blow up

cklages · Apr 4, 2005

Well I was out in south Dakota this last weekend and the middle cylinder siezed so it is time for a rebuild I took the power head apart and took the block and head to get machined. so what could have caused this to happen so I can fix it so it doesn't happen again? It seemed it was getting oil when I pulled the plugs there was oil on them. Any tips you can give is appreciated. Thanks Chad

rayjay · Apr 4, 2005

Re: 70 johnson blow up

Blockage in a jet could do it. What did the pistons look like?

cklages · Apr 4, 2005

Re: 70 johnson blow up

The tops looked good with little to no carbon built up the middle cylinder that blew looked like the rings melted down on one side Chad

McGR · Apr 4, 2005

Re: 70 johnson blow up

It seems to me I've read posts on this site regarding middle cylinder problems with the OMC 3cyl. motors. Something about less efficient cooling to the middle cylinder. Search the site under Johnson/Evinrude 60,65,70,75 and you should come up with some insight into your problem.

Dhadley · Apr 5, 2005

Re: 70 johnson blow up

Before we head down the wrong path, lets get the model number so we can "see" what you have for sure. There are a couple of 70 hp designs and although they look similar they each have their own quirks.

cklages · Apr 5, 2005

Re: 70 johnson blow up

The model number is J70TLCEM 1989 with the VRO still hooked up. I have only owned it for 3 years so i don't know the history to much. Thanks Chad

Dhadley · Apr 6, 2005

Re: 70 johnson blow up

OK, thats the 56 cid model. They do have a tendency to run lean in the mid range. It probably scored the side of the piston. Probably the exhaust side worse. There are a few things to look at but lets take it one step at a time. What grade fuel are you running? Was it fresh? What spark plugs were you running?

seahorse5 · Apr 6, 2005

Re: 70 johnson blow up

There were a number of service bulletins for those years of 3-cylinder outboards. Since they were published over 10 years ago, a dealer should have made the changes when the motor was in for service. The factory water pump kit is special for the 3 bangers and includes fittings to reposition the "pisser" hose to the top of the block. There was a bulletin about changing out the temp sender and one about the changes in the exhaust plate. There was one about a timing change and re-jetting certain model years. Find a knowledgable dealer and have him check the bulletins so they are done if you have the motor rebuilt. Recently someone posted overheat problems with a 70 and had a dealer find the bulletins and fix the problem. Do a search.

cklages · Apr 6, 2005

Re: 70 johnson blow up

the gas was fresh and it was 89 octane NGK R BZ7HS 10. Chad

Dhadley · Apr 6, 2005

Re: 70 johnson blow up

Oops! We need to get those plugs out and get some cooler Champions in there. And go back with 87 octane. Once we get to cooler plugs and fuel things should be better. We still need to review the set up also. Top rpm etc.

cklages · Apr 6, 2005

Re: 70 johnson blow up

I am running a 13.75x15 alum. prop prop which runs at 6100 so I throttle back to about 5900 At the time it blew I had two other people in the boat and was running at 5700. The motor is in the top hole and is mounted on a 1990 Sylvan Profisherman. Thanks Chad

Dhadley · Apr 6, 2005

Re: 70 johnson blow up

Great set up. Good job. I'd suggest a ss prop if your looking for more performance but that will come. It may just have been the hot plugs that got it. Its not unusual though to richen up the mid range jets by about 3 numbers, especially on the middle cylinder.

cklages · Apr 6, 2005

Re: 70 johnson blow up

How can I tell what number jet I have and what is suppose to be in there and can you adjust the jet or just screw it all the way in and that is it and where is the mid range jet i have a parts book is it the one on top or the one on the bottom? Thanks Chad

Dhadley · Apr 6, 2005

Re: 70 johnson blow up

The fixed jets are stamped. Just remember that the mid range jet is an air bleed jet on some models. If it meters air, you go smaller to go richer. If it meters fuel, go bigger to go richer.

ledgefinder · Apr 6, 2005

Re: 70 johnson blow up

South Dakota this time of year - was it cold & sunny? If the air was real dense (cold & high barometric pressure), that might leaned out the mixture some, cutting into the margin of error.

70 johnson blow up

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