Burnt/melted pistons on brand new rebuilt engine!! Cause?

[Doug L.]

Hi everyone. Today has been a very sad and horrifying day. Last summer, I was running my 1986 Mercruiser 140 3.0L WOT when it spun a rod bearing. The engine was already in need of a rebuild then so I invested in doing a complete rebuild. I had the crank turned, cylinders bored over, all new bearings, seals, everything. I put the engine back in my 19ft Glastron and it ran like a champ. I set the valve lash following Mercruiser's TM Number 10. All 4 cylinders were around ~140PSI. I also converted the points/condenser to Pertronix Ignitor kit. I was careful to follow the "first 20 hours" break-in rules as they are crucial.

I noticed towards the end of last summer I had a lot of issues getting the engine to idle. It would only run when above 1100 rpms. After numerous attempts of adjusting the air/fuel mixture and timing, I decided it was likely the MerCarb as I had never had it professionally rebuilt (never occurred to me to perform a compression test).

Last week, I had the Carburetor rebuilt by National Carbs and installed it after de-winterizing. I ran it an hour at the lake and still had issues with it idling without being well above idle speed. Today I performed a dry compression test cold. The idle issue was clear now. I found the following readings:

Cyl 1: 150PSI
Cyl 2: 150PSI
Cyl 3: 175PSI
Cyl 4: 55PSI

Cold and wet I got the following readings:

Cyl 1: 175PSI
Cyl 2: 175PSI
Cyl 3: 200PSI
Cyl 4: 60PSI

I adjusted the valves again and got the same exact readings. I pulled the head thinking a valve was stuck in Cyl 4 and was absolutely horrified and heartbroken to find 3 of my 4 pistons are disintegrating on the edges. I also found the Cyl 4 exhaust valve was badly burnt/melted. The engine only has 27 hours on it. I've used only Mercruiser 25W40 motor oil with numerous changes during the break-in period. I've run only NON-ethanol fuel and primarily run 93 octane but have run 87 too when 93 is not available.

What could have caused this to happen or what could I have done wrong?? The engine (at least according to my dash gauge) has never run hot during this time. The head gasket was intact though I could see it was disintegrating on the inner edges too. I'm so saddened and not sure what to do. Should I attempt a rebuild again? I would hate to rebuild only for it to happen again. I would greatly appreciate any input!

Photos of the engine and pistons can be seen in the attached link.

Doug L.

 

[Scott Danforth]

Your timing was most likely off, and you were probably running lean

 

[Bondo]

I noticed towards the end of last summer I had a lot of issues getting the engine to idle. It would only run when above 1100 rpms. After numerous attempts of adjusting the air/fuel mixture and timing, I decided it was likely the MerCarb as I had never had it professionally rebuilt (never occurred to me to perform a compression test).

Ayuh,...... Welcome Aboard,........ Detonation destroyed yer motor,.....

Primary cause was no doubt, a lean condition,......

The new carb might be the fix, but the entire fuel system is suspect,.......

 

[Scott Danforth]

Also, any cylinder above 150 psi will most likely have water or oil in it

Water would be from a leaking exhaust manifold or manifold/riser gasket

Oil can be coming from valve guides

 

[Doug L.]

Thank ya'll for the thoughtful replies. I truly regret not having the carb professionally rebuilt right away. Timing was set to 6 degrees BTDC at 700 rpms. I started thinking that maybe the mechanical advance was not working correctly and possibly affecting timing at higher rpms. I plan to tear the engine back down today and take to my machine shop to have inspected. Hoping con rods and bearings are ok and can just hone and install new pistons. Gonna have the head completely gone through and magnafluxed. I'll look into ensuring valve guides are also checked in case oil is leaking around them.

I'll update.

Doug

 

[Bt Doctur]

depending on the year it could be as far back as 1 deg ATDC

 

[Lou C]

Also to add what was said you should always set the fuel mixture using a tach and pull the plugs to check the color of the center insulator for proper color...and always check base timing and total advance

 

[Doug L.]

depending on the year it could be as far back as 1 deg ATDC

Hi Bt Doctur, What do you mean? My glastron is from 1986 and according to my engine block code, it is from 1977 model 140. The valve cover is serialized for 1986 as well as the MerCarb (likely original). Someone did either a short or long block swap before me (not sure year of head).


Thank you Lou. I've always used a vacuum gauge for setting air/fuel mixture. Is that not right? Also, how would the tach help set it? Aye, really good advice on the plugs and checking advance. This has been a really hard lesson to learn.

 

[Lou C]

The vacuum gauge can be used as well. The 3.0 is known to be sensitive to timing advance and tends to blow head gaskets. On my OMC 4.3 they gave 2 timing specs, 1* Btdc if using regular and 6* btdc if using premium. I set it at 6 and always used premium. Never had issues with preignition or detonation. I think you need some more info on these engines and how to avoid these problems. Total timing advance at manufacturers specs is very important.