1975 70 hp johnson

[palmrose2]

My wife dragged home a boat. A Duo tri-hull in need of some TLC. No problem, boats are cool and I'm handy with tools. I've been lurking since it showed up 2 wks ago. I got to the point where I stole a battery from one of my other gas powered do-dads. Reality check.

Mod# 70ESL75E Ser#J4295740

Cranked her up on muffs. No spark. Not cool but let's move on. Had plugs removed so did a compression test. Top cyl 120. mid cyl 115. bottom cyl 160 and plenty O' water. Even for a newbie this looked to be not good. Without a manual I started tearing it down. All cyls were gorgeous, as were the pistons as observed from the exhaust ports. Top two pistons were blackened with carbon and the lowest one was clean. I assume that the water in the combustion chamber of #3 is responsible for it's cleanliness.

I'm thinking that the person who had it before me had problems with the motor and failed to mention it. The head gasket was NEW. Other gaskets were old but had been sealed with what appears to be gold high temp silicone. It looks like someone had water in #3 cyl and tried to fix it, failed, and sold it to my beautiful wife.

Are there water passages in the crank case? (doesn't look like it)

It looks like mechanic # 1 couldn't fix the engine. I'm mechanic #2 and don't want to repeat.

Any insight is appreciated.

 

[palmrose2]

Re: 1975 70 hp johnson

How many ways can you get water in the combustion chamber?

 

[mikesea]

Re: 1975 70 hp johnson

Check the head for flatness,if its ok ,then ,THOSE engines are prone to have leaks in the exhust ,the large triangular cover on the starter side of the engine with all the 3/8$head bolts is where to start.Take the covers off ,should be 3,look for burn throughs ,blown gaskets etc.

 

[palmrose2]

Re: 1975 70 hp johnson

Thanks mike.

I took covers off and found a small spot down low next to the lowest cyl. exhaust port where the gasket was damaged.

 

[palmrose2]

Re: 1975 70 hp johnson

Anybody know how much play there should be at the propeller? With the engine in gear I can rotate the prop back and forth about 80 degrees. Seems like a lot to me but I'm a newby.

 

[jmendoza]

Re: 1975 70 hp johnson

Normal play for the prop, the slots in the gears the shifter dogs go into allow this much movement.

Gasket leaks you described quite normal. Lap the head w/ some fine grit on a flat surface as well, replace head gasket too. Retorque the power head bolts after the engine has been test run and reached operating temperature, then cooled completely.

Nice how water in the cylinder shocks the carbon right off the piston crowns, eh? You will also want to run some marine motor decarb solution like Valv Tect into the cylinders with the piston at TDC with a hot engine. Let each cylinder soak for about 10 minutes, with the cylinders somewhat vertical(tilt the engine) to allow this stuff to soak past the rings and loosen them up. Put in the plugs and fire it off, expect an oil slick of black carbon worse than BP to appear in your test tank! Run for about 15-20 minutes, or better yet, take out the boat for a 30 minute cuise on the water. Compression should then be real good. BTW, those compression readings will be off as you described if the carbs were not wide open when cranking for the compression readings.

I saw a 1975 Johnson 70hp cruise by on a 14 foot Glastron fiberglass runabout this weekend on Big Bear Lake while out fishing. We both stared at each other as our engine cowls and paint were exactly the same, so then we both waved....nice engine!

 

[F_R]

Re: 1975 70 hp johnson

Check those exhaust cover parts for flatness in the port area. Warpage will make them leak and squirt water in to the exhaust ports.

 

[palmrose2]

Re: 1975 70 hp johnson

Thanks everyone for the tips. This project may take a while. I just got back from a 13 hr work day and my bud still wants me to weld him a trailer type dock ASAP. That's a bigger project and probably more important this June. Wife says it's OK if the boat doesn't see water till next summer. I really want to see this thing run though.

I've discovered through research that my lower unit is the "large" type. A good thing? Beefier? Last forever? Hope so.

 

[palmrose2]

Re: 1975 70 hp johnson

Finally got the power head back together. Now it's time to tackle the lack of spark. I'm currently borrowing a Clymer manual. The diagnostics for the ignition system call for the use of an OMC neon test light. I don't have one nor do I know where to get one. It seems to me that there should be some way for me to do the checks with a multimeter. I have a digital and a pretty nice analogue multimeter.

I've already disconnected the red engine elec. connector to rule out the ignition switch itself. Any help is appreciated.

All knowledge has value.