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Author Topic:   Blown Motor -Why?
lakelanier posted 10-18-2003 07:28 PM ET (US)   Profile for lakelanier  
After having all four carbs rebuilt on my 50 Mercury 4 stroke it failed catastrophically within an hour. Both sides of the crank case blown out. Was running at about 4800RPM at the time. No warning--just clank! First thing I did was check oil-it was full. This is a 96 Motor but with very little time on it.
Is there any way this could be traceable back to the carb rebuild job-which incidently I paid a small fortune for?
Dr T posted 10-18-2003 08:04 PM ET (US)     Profile for Dr T  Send Email to Dr T     
Here is one possiblity (and worth what you are paying for it).

Is it possible that the oil sump was contaminated by gasoline or some other volitile solvent?

If it were, and the solvent turned into vapor, when the engine got hot enough, it would start to burn. Burning rapidly, it would give off gas as a product of combustion. This would expand in a closed space. When the pressure got high enough, something would have to give.

Glad you are not picking pieces of hot metal out of your back.

Dr T

kglinz posted 10-18-2003 08:33 PM ET (US)     Profile for kglinz  Send Email to kglinz     
That's a very good possibility. I've seen oil pans blown off car engines driving down the street. I would think that, if this is the case the crank and rods would be OK and not show any signs of scoring or overheating.
lakelanier posted 10-18-2003 08:42 PM ET (US)     Profile for lakelanier    
Could such contamination have occurred during the carb rebuild? Now that I think about it I was probably exaggerating a little when I said failure was within an hour. Lets see- the shop water tested it and then I had it out for two runs before the fatal run. It seams like it would have gotten as hot as it was going to get on the previous runs.
kglinz posted 10-18-2003 08:52 PM ET (US)     Profile for kglinz  Send Email to kglinz     
I'm not sure on that motor. On a car engine it's normally a fuel pump diaphram with a hole in it. I would sure look for a problem in the carbs that could cause it.
lakelanier posted 10-19-2003 09:13 AM ET (US)     Profile for lakelanier    
Was reading a thread titled "Four Strokes making oil" The consensus seemed to be that gas in the oil would just boil off at higher temperatures? Problems were solved by replacing the thermostat so engine would run hotter. No references to crank case explosions.
jimh posted 10-19-2003 10:39 AM ET (US)     Profile for jimh  Send Email to jimh     
The failure of the engine is probably related to the way in which it was stored. Rust may have formed in critical areas.

Below I append a hyperlink to a good source of information regarding this type of storage and failure relationship:

kglinz posted 10-19-2003 11:40 AM ET (US)     Profile for kglinz  Send Email to kglinz     
I'm sure small amounts of fuel do boil off. Actually it would be the boiling off that could give you an explosive mixture. I don't know what you relationship is with your dealer but you need to dis-assemble the engine to eliminate things like rust or oil starvation.
lakelanier posted 10-19-2003 12:27 PM ET (US)     Profile for lakelanier    
Did a further inspection this morning. viewing through the newely created inspection port I can see that the rod is indeed broken. Interestingly the crank case is a little overfull. I do not think it was before. In addition there was a oil loss in the water last evening and when I tilted motor up for the drive home.
Barney posted 10-19-2003 02:50 PM ET (US)     Profile for Barney  Send Email to Barney     
Broken water jacket or head gasket leaking water into the cylinder?
triblet posted 10-19-2003 05:45 PM ET (US)     Profile for triblet  Send Email to triblet     
Engine failure after a carb rebuild? Maybe carbs running


lakelanier posted 10-19-2003 06:35 PM ET (US)     Profile for lakelanier    
Running lean would be a problem on a two stroke but don't think it would be a problem on a 4 stroke.
lhg posted 10-19-2003 11:19 PM ET (US)     Profile for lhg    
I'd say that who ever re-built the carbs owes you a new engine! Let them deal with it.
lakelanier posted 10-20-2003 08:14 AM ET (US)     Profile for lakelanier    
That is what I am hoping for but without coming up with some sort of causal relationship between the carb job and the engine failure my expectations are not real high.
In the way of additional information, it was the number two cylinder that broke the rod. The number two carb was the one that was the most fouled and dwas probably the cylinder that was cutting out prior to the rebuild. Could damage have occurred at that point?

Bigshot. What are your thoughts on this? Would appreciate your incite.

tarbaby posted 10-20-2003 08:52 AM ET (US)     Profile for tarbaby  Send Email to tarbaby     
Why blame the guys that rebuilt the carbs? After reading the other post on this engine it is obvious to me that there were engine problems when you bought it. It did sit up for a year and only the previous owner knows what happened to the engine prior to you owning it. I have been down the same road as you and it is not fun. I had to repower after 4 hours of my use on an engine that had less than 100 hours on it. Sorry for your troubles but I don't think the guys that did your carbs are to blame. Good luck with your winter project. Shay
lakelanier posted 10-20-2003 09:26 AM ET (US)     Profile for lakelanier    
Not sure they are to blame- probably they are not, but you sure have to ask the question.
kglinz posted 10-20-2003 09:50 AM ET (US)     Profile for kglinz  Send Email to kglinz     
I would say that the carbs did not cause the engine failure, however rebuilding the carbs may have been unnecessary. If the rebuilder is a engine dealer you could have some leverage to get a new engine at a good price.
Bigshot posted 10-20-2003 11:13 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
I gave you my thoughts BEFORE you had them rebuilt....and you did not listen.

If crank case is overfull with gas or you can find that the oil has a high amount of gas in it, then you have a case for them replacing the engine. I would NOT bring the oil to them, I would use a different source. Just about any autoparts store would know where to get an oil analysis. If it is filled with gas the LEAST you should get is your million dollars back that you spent on the rebuilt carbs.

lakelanier posted 10-20-2003 11:46 AM ET (US)     Profile for lakelanier    
Actually I did listen. The Techron didn't do the job. Had no choice but to move foreward and take off the carbs.
Bigshot posted 10-20-2003 12:25 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Not that part.....the one where I said get another mechanic. Techron takes a few days to work, not an afternoon.
lakelanier posted 10-20-2003 01:30 PM ET (US)     Profile for lakelanier    
Not trying to argue with you--I do appreciate your advice. By the time I got the bill for the rebuild the toothpaste was out of the tube - hard to get it back in; the carb job was done. (Still negotiating that by the way)

Would have liked to give the Techron more time, but the motor was just running too poorly to continue operating it.

For the couple of hours the motor did run after having them rebuilt it was perfect. May have cost too much but there is no question that it solved the symptoms I was experiencing.

Again, Thanks for your advice.

Bigshot posted 10-20-2003 01:43 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Just pulling your leg. Not much else you can do in your situation.
lakelanier posted 10-20-2003 02:06 PM ET (US)     Profile for lakelanier    
I understand.
Funny thing is I have drug home any number of old motors from the 40's & 50's, slaped them on a boat and they have all run like scalded dogs. This is the first used "modern" motor I have bought. I am beginning to think that my number was just up.
There is a bright side--I am sarting to visualise a brand new Suzuki/Johnson 70 hp 4 stroke in my future!!
lakelanier posted 10-30-2003 09:26 PM ET (US)     Profile for lakelanier    
Well, I attended the “autopsy” and we cracked the head open. Rod was broken just about in the center. There was no evidence of why the rod had broken. The piston was not frozen in the cylinder and there was no scoring on the cylinder wall and no damage to the piston near the rings. Only damage to the piston was the piston skirt which probably occurred when the rod broke. I guess this will remain a mystery. Thanks for all your help.
dgp posted 10-31-2003 08:28 AM ET (US)     Profile for dgp  Send Email to dgp     
What about the condition of the main and rod bearings?
It's very rare that a con rod will break in the middle I-beam section but it can happen.
lakelanier posted 10-31-2003 07:44 PM ET (US)     Profile for lakelanier    
Have not torn it down that far yet.
Steve Leone posted 11-08-2003 01:00 AM ET (US)     Profile for Steve Leone  Send Email to Steve Leone     
Were the cylinders dry or was there oil present? Did the mechanic re-adjust the carbs using the proper Mercury 4-stroke tool? It is critical that you use this tool. It lets you know if the carbs are running rich or lean. Yes, it is very possible to run a 4-stroke too lean. There are plugs on the sides of the carbs on those 4-strokes. These plugs hide the air/fuel mixture screws. The plugs are there to keep the ignorant out. There is a method to remove these plugs and adjust and sync the carbs. The Merc tool MUST be hooked up to the carbs when performing this proceedure. It operates by vacumn. It cannot be done by ear. Also, we have had pretty good luck with the Merc 4-strokes. The only one that blew up had a bent and pinched oil pick-up hose, restricting the oil flow. That is why I asked you if the clylinders were lacking oil or lubrication. If there was little or no oil the cylinder walls will be dry and heat scored, thus causing the mechanical failure. Steve

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