at a loss, dont know where to go next. 140 merc

[Tmacular]

Ok gents heres the story,

got a 140 merc 1984ish been running well since i had it rebuilt up 3 years ago. Yesterday on the water ran fine all day but then later in the day i started hearing a metal clanking sound (almost like something was bouncing around inside) and anything over about 2200rpm would set this noise off. I cruised back to dock at 2k rpm no issues ran great.

Fast forward to today, pulled the valve cover today and round cyl 4 lifter was crooked sideways, push rod and seat were fine. reseated the lifter and its now straight. Then I moved on to compression test.

cyl 4 100ps (bent lifter cyl)
cyl 3 60psi
cyl 2 50-60 psi
cyl 1 50-60 psi

I dont get why 3 cylinders are showing such low compression while 4 is fine and showed signs of a issue. Are my rings shot? this was a dry test. I didnt move on to wet test due to the low numbers to start.

Anything else I could be missing thats easier then a full tear down?

The oil looked amazing, no build ups or anything funny with pasages and valves. I thought maybe my compression tester was acting up but the first one we tested came out normal. Just not sure what i should look into next

appreciate the time guys and thanks

 

[Scott Danforth]

Time to do a leak down test to see if its rings or valves

 

[Tmacular]

Ok sounds good. What is the process exactly I should follow whhen doing the test. Id like to add i just fixed the lifter put everything back together put it on the hose and its running great. Doesnt seem like it should be running as smooth as it is given the PSI of the cyl. but 2500-3000 no more metal clanking (think it was the lifter) but doesnt tell me the others.

 

[Scott Danforth]

the low compression screams the motor shouldnt be running. so you will need to verify your readings

here is an article on leak down testing. there is one in the stickies, howevever this one was handy. http://www.hotrod.com/articles/cylinder-leakdown-tester/

as for the clanging, it could be the remains of your exhaust shutters clanking. that would mean you had a serious over-heat. which would also explain the low compression.

 

[kenny nunez]

Have you been adding a zink additive to your engine oil? If not the camshaft could be worn which will also cause low compression and be the reason for the rocker arm and push rod to disengage.

 

[nola mike]

A worn camshaft should still allow the valves to seat fully; where would you be losing compression?

 

[Tmacular]

Ok will look into those things tomorrow morning. As far as I'm aware it's never over heated. My temp gauge seems to always stay right about the 140 degree mark highest I've ever see it go was about 160. As far as I'm aware it's working correctly.

as for the zinc additive no I havent. I've been using 10w30 regular oil and it's been about 60 hours since last change. I was approximately a 1/2 quart low at the time of issues. Should I add zinc? What do you reccomend for brand?

I am using a cheap harbor freight model compression tester but it read cylinder 4 at 100 do I dont think it's wrong. I cranked the engine to allow at least 3 fires on each cylinder to try and get accurate readings. Are there any really good additives I can put in to clean things up in the cylinder and valves? Like a oil add I can run then change?

thanks for the help guys. I really want to be back on the water asap! The walleye are biting

 

[porscheguy]

Your low compression could be due to a dry test. If it were mine, I’d throw it back in the water, run it, and see what happens. If it still runs poorly, then you know you’ve got trouble. If it runs good, then you’re good to go.

 

[Alumarine]

Cylinder 4 at 100 psi is not a good number either.

 

[Tmacular]

I was under the impression that on these motors 100-150 is acceptable with no more then 15-20 diff per cyl

 

[Tmacular]

Maybe my compression tester is acting up. I'll try and find a second one to use today. The engine is running way too smooth to actually have those numbers so I dont know. She also starts right up instantly. Little bit shaky at idle 600rpm +/- but we did the timing adjustment by hand and I'm thinking its very slightly off. Dodnt have a timing light at the time.

 

[kenny nunez]

As far as a zink additive just go the the local parts store and it will be on the shelf. They are all about the same. I also believe that the Shell brand of oil Rotella has the zinc in it.
A worn cam lobe will show a lower compression because the valve is not opening enough.
With the rocker arms off and the spark plugs out turn the engine slowly by hand and check the travel of each pushrod

 

[AnthraxBird]

Always have the throttle wide open when doing compression checks. Put a bit of oil in the cylinders and give it 3 seconds of cranking while doing the test. Your cylinders should be around 100 - 140psi, around 10% difference between cylinders.

 

[Scott Danforth]

I was under the impression that on these motors 100-150 is acceptable with no more then 15-20 diff per cyl

its 150 psi new, 120 psi worn and no more than 5-10% difference total low to high. that is 6-15 psi range.

 

[nola mike]

A worn cam lobe will show a lower compression because the valve is not opening enough.

How would that affect compression? The compression readings are with the valves closed.

 

[kenny nunez]

With a worn intake lobe depending on how much it is worn the valve is not moving enough, I have seen a cam that was worn round and no compression.

 

[nola mike]

Yeah, I guess if the valve isn't opening enough to let air in. You'd pretty much have to wipe the lobe though. Can't imagine that the engine would run in that case