1980's 140 mercruiser

[J.Todd]

Hi everyone. Got a problem with my 140, she's only firing on 2 cylinders, 2&3. New plugs points, condenser, cap and rotor, couldn't get points. Compression 120 on 1,2,3, and 90 on 4. Mechanic suggested a bad valve. Could I have one on no. 1 as well or maybe someone might have anoticed idea to try. Thanks for any advice. Jim

 

[GSPLures]

Have you used a spark tester to confirm you are getting spark to all cylinders?

 

[GSPLures]

Also if you do a leak down test you can find the source of your low compression. If you add a squirt of oil down the plug hole then re run a compression test if the number goes up you have an issue with rings if they stay the same it's going to be a valve issue

 

[kenny nunez]

Did this condition start all at once? Was there a over heat event?

 

[Scott Danforth]

below 120 psi is a rebuild. new motors are about 145psi

as Kenny asked, did you go thru an thermal-inclination-engine-damaging event?

 

[Rick Stephens]

Have you used a spark tester to confirm you are getting spark to all cylinders?

I also like the spark tester. They cost next to nothing and tell you a lot.

 

[GSPLures]

A 1980s inline 4 motor with 120 psi across all cylinders is tired and would benefit from a rebuild but should still run decent. I am assuming (I know bad idea) that the motor did not go through a damaging event because the OP did not state that.

With the motor in question though I would definitely consider an overhaul of the heads due to the fact that the #4 cylinder is so low which is more than likely causing the miss on that cylinder. The mechanic he took it to is probably right about the valve since it is only one low cylinder and it is more than likely the exhaust valve. A leak down test will confirm what is the cause.

With the #1 cylinder at 120psi it should still fire. Which is why I would confirm good strong spark to that cylinder before looking elsewhere.

But with all that said without additional information it's all a guessing game.

 

[Scott Danforth]

on the points. did you file them and verify the dwell with a meter? or did you just pull them out of the package?

even new points need to be filed

use a DVM to verify the condenser is good