1974 70HP lost compression in middle cylinder...

[John90290]

Part 1 - #1 and #3 are good but #2 is down to 60 psi and sounds like hell when it runs. I'm thinkging valves or rings - in your experience with these motors, what do you think is most likely?


Part 2 - I have a 76 70hp on a stand that has not run in years in any capacity. I did wire it up and start it and it ran great. Compression good and being a lake motor, it looks brand new. Question: I might trade out the motors and am going to do the following to get the motor ready for duty, please let me know if you see anthything I'm missing:

• Impeller
• Lower unit oil
• Plugs and coils

 

[ezeke]

Re: 1974 70HP lost compression in middle cylinder...

There are no valves, but rings are a possibility. Before you go too far, check the cylinder head gasket; it could just be blown, especially if the engine has overheated at any time.

For part 2: http://forums.iboats.com/showthread.php?t=158086

 

[HybridMX6]

Re: 1974 70HP lost compression in middle cylinder...

If the '76 hasn't run in years, I think it would be wise to rebuild the carbs before any extended use, just to make sure there isn't anything blocking them and keeping fuel, but more importantly oil/lubrication, from getting into the cylinders.
Also, a good decarb wouldn't hurt it once you get it running on the boat.
All that being said, I'd also pull the head and check out the '74. It'll tell ya all you need to know. If it sounds horrible, I'm also going to go with rings, or the wristpin clips may have popped out and the pin is loose, and now tearing up the cylinder. Could also be bearings, but I can't hear it run, so it's hard to know.

 

[JB]

Re: 1974 70HP lost compression in middle cylinder...

What is "good" about #1 and #3?? 60psi sounds too good to be a dead cylinder. A good decarb might well fix it up.

Do a decarb first, redo the compression and, if #2 is still low then pull the head to inspect the gasket.

Let us know what you find.

 

[R.Johnson]

Re: 1974 70HP lost compression in middle cylinder...

60 lbs. is not good on that 3 cylinder engine. Pull the head, and take a look.

 

[John90290]

Re: 1974 70HP lost compression in middle cylinder...

A lot of great info. Ill pull the heads this weekend and get back to the forum next week. I hope those head bolts feel like coming loose and not breaking...30 years is a long time..

Thanks again.

-John

 

[JB]

Re: 1974 70HP lost compression in middle cylinder...

What is the compression in #1 and #3?

Decarb and new test can be done without pulling the head.

60psi is not good, depending on what the other 2 are, but fixing what ain't broke is worse.

 

[R.Johnson]

Re: 1974 70HP lost compression in middle cylinder...

If you can gain another 60# with a can of erasol, go for it.My best luck has been with a wrench

 

[John90290]

Re: 1974 70HP lost compression in middle cylinder...

Done and done. Two days and some sore muscles and the new 70hp is in. Changed impeller, the original was installed incorrectly and was fried. Swaped the carbs from the old 1977, hoisted it into place and off she went.

only issue is that while both 70's sound like they are on the same rpm when im running, one reads about a grand higher on the rpm and i know there is now way that could be possible based on the sound and speed we are travelling. Any tips would be helpful.


Thanks for all the help guys, could not have done it without your help.

 

[R.Johnson]

Re: 1974 70HP lost compression in middle cylinder...

You have a tip, 60 lbs. compression on # 2 cylinder.

 

[JB]

Re: 1974 70HP lost compression in middle cylinder...

Is the compression on #1 and #3 some sort of secret??

 

[John90290]

Re: 1974 70HP lost compression in middle cylinder...

Guess there is some confusion.

I have a 18 foot boston whaler outrage that had two 1977 70hp evinrudes. One developed loss of power and 60lbs in #2 cylinder. #1 and #3 were both around 118lbs.

I had purchased a 1976 clean looking 70hp evinrude from a craigslist ad for $280 bucks. The motor looked like it lived it?s entire life on a lake, and the compression was 120lbs on each. I stored that motor for a rainy day and then the 1977 crapped out so I went to the back up 1976.

Took the 1977 off the boat, put the carbs from that motor on the 1976 and installed the 1976 on the boat. (also changed lower unit oil- it was clean and no water ? and changed the impeller ? it was trashed)

The 1976 rpms per the gauge did not match the still functioning 1977 that remained on the boat. After my post I banged on the rpm gauge and it dropped 700rpm and now matches the 1977 so I guess there is a wiring or gauge issue.

Thanks for all the help. Attached find a picture of the 1976 being lowered by a hoist.