1984 Johnson 70hp 2 stroke

[Tndsmith33]

I was out on the lake last week, going around 20mph. Went full throttle and it started to lose speed and rpm's. I backed it right off and turned the motor off. I turned around and saw a decent amount of smoke coming from under the cowl/motor cover. I then used the electric trolling motor to get us in.

I took it in to a shop. They took a look and said there was water in the head, that's why it was smoking bad. They weren't sure if it was the head gasket or the exhaust plate gasket. Wouldn't know until they opened it up. They did a compression test, #1: 120, #2: 120, #3: 160. They said it looked like the gaskets were newer, as if someone had changed them out in the last couple years.. They wanted $2000 to fix it.

I bought this boat and motor a few months ago. I ran Seafoam through it as I wasn't sure if the old owner ever did (and he told me he only ever used regular gas). It had been running great up until we went fishing on a shallow, weedy lake. The former owner told me when I bought it that with the 3 blade prop it would chug and stall out when taking off, but the 4 blade prop had no problems shooting the hole.

What causes the compression in one cylinder to be that much higher?
Does the shops diagnosis sound correct?

Thanks in advance.

 

[racerone]

Did you install a new water pump impeller when you bought this boat ?----Did you test the overheat horn ?------The compression on 2 cylinders is MUCH lower.-----Just 22 bolts to take the head off.----That will tell the story.----Nothing else needs to be done at this time.----Take the head off.

 

[Tndsmith33]

I did not install a new water pump impeller. Water flowed strong and consistently out the pee hole so it didn't cross my mind to do so.

Was unaware of an overheat horn. To think about it, I don't recall ever hearing a beep when I start the motor. That is now on my list.

Lower compression in the two cylinders makes more sense, but with it being a 2 stroke isn't 160 too much?

Taking it to a friend's who has room. Will get that head off.

Thanks.

 

[racerone]

??----I see numbers that high on good motors with a good gauge.----Not all control boxed have a horn with a self check ( beep ) feature.

 

[Tndsmith33]

I was able to test the overheat alarm. It buzzed away. I definitely didn't hear that on the water when the motor was smoking badly.