1977 Mercury 85 HP bad compression

[Old Irving]

I just found out my 1977 Mercury 85 has compression of 90, 90, 110, and 112. It also is not producing any spark. I am using it on a 1977 Starcraft Super Sport 18. Should I bother with the cost of repairing or look for another motor?
Thanks

 

[Barnacle_Bill]

Since that motor is almost 40 years old I would look for another one.

 

[Chris1956]

Compression gauges are often inaccurate. The specs call for all cylinders to be +/- 15%. Since you are close to that spec, I think the motor is good.

Troubleshoot the ign system. You should have an ADI igniiton with 4 coils. They are still in use on many motors today.

 

[Chris1956]

Examine the motor wiring harness. They fail often in motors of that age.

 

[gm280]

Yes the engine is old, but I still think there is some good life in it yet. Get the ignition working and you may surprise yourself how well it runs. Since you stated there is no spark, then check for whatever feeds all four spark coils to see what is wrong. I mean the odds of all four coils being totally shot is questionable in my thinking. JMHO!

 

[Old Irving]

Thanks for the advice. I know next to nothing about engine repair. I am mostly a sailor so there will be a learning curve.

 

[444]

Compression looks fine to me. No way I would be replacing.

I would first check to see if the ignition system is getting proper power when the key is on. Then check the stator. cdielectronics.com has some good troubleshooting info in their tech section. If you plan on fixing, I suggest you pick up a service manual.

 

[Faztbullet]

Try a different gauge and if you end up with number in the same range don't waste your time unless it special to ya. That motor should have a min off 110 psi any less and it wont idle or pull itself out of the hole...

 

[racerone]

????----If the motor has 4 coils then it is driven by the alternator / stator.--Battery voltage is not used to generate the spark for ignition !

 

[Old Irving]

Thanks for all the input. Looks like I am going to get a replacement motor. I don't want to put a lot of money into a 40 year old engine and tilt / trim to find out it needs more work.

Anyone interested in a 85 HP project motor in Chicago?

Thanks again.

Bob

 

[Old Irving]

I wanted to give everyone who responded an update on what I ended up doing. I bought a donor boat (Crestliner 16') with a 1985 Evinrude 115 hp. Very good compression and tilt / trim work great. Ran it with the muffler and sounded good. Once again, thanks for all the advice.