Cause of Piston failure

mustang5

Cadet
Joined
Sep 21, 2010
Messages
27
Hey guys,

Yamaha 200hp 6G6 200BETO C (91 model) has done a couple of pistons on me.

Half way through a 250km trip, the motor started hunting (going up and down from full power), as if it were running low on fuel. As I was 100kms offshore there wasnt really much I could do other than baby it home.

It got us into the harbour where revving at 2500rpm it came to a sudden hault.

Towed home, took her apart, and the top Port piston has snapped at the wrist pin, with a lot of damage around the edges and a lot of debris has been pushed into the head.

The top starboard piston has a lot of damage and debris, but is still connected and in tact. There is a small hairline crack in the sleeve at the end of the bore.

I am looking for a cause, would it be a blocked top carby?? Or the top fuel pump?

I am only looking for a couple more months out of this motor(by no means 250km trips), so would slapping a couple new pistons in and cleaning the carbies be ok for the time being??

Cheers

217042_10150274163201164_549441163_9414932_7078421_n.jpg216439_10150274164001164_549441163_9414939_7296355_n.jpg204373_10150227141990240_584955239_8803834_5176010_o.jpg220385_10150227144905240_584955239_8803853_3617699_o.jpg215222_10150274163831164_549441163_9414937_8379376_n.jpg
 

Silvertip

Supreme Mariner
Joined
Sep 22, 2003
Messages
28,758
Re: Cause of Piston failure

Fuel pumps cannot pick a specific cylinder to destroy so the likely suspect is the carburetor. Any outboard owner should periodically check the plugs and analyze them carefully especially after a long high speed run. Plugs can be read as they are a good indicator of air/fuel mixture.
 
Joined
Oct 28, 2010
Messages
923
Re: Cause of Piston failure

What was the age of the gas you were running, you had preignition at one point and at that point timing for that cylinder is no longer a factor and can be random. Totally could have been carb related. Here is something you need to realize about boats, their is no heavier load you can put on an engine than water, factor in that todays gas starts deteriorate is about 3 weeks. If you want to get an idea of how hard a boat is run, take your vehicle put it in 1st gear and floor it up the steepest hill and you will come close. They make fuel for cars not for boats.
 

sschefer

Rear Admiral
Joined
Nov 13, 2008
Messages
4,530
Re: Cause of Piston failure

I agree with silvertip. It's a lean out. The outter edges are most effected because there is a lack of cooling fuel in the squish band. This causes the flame to spread over the edge of the piston top and in turn causes a rapid carbon buildup in the ring lands. It's all down hill from there as you know.. I'm proud of you! Your quick thinking is what got you home.

Jefferson, two strokes don't pre-ignite, they detonate and the tell tale for that is a dish or hole dead center of the piston. It's almost as if the motor is trying to save itself because if the dish becomes adequate enogh to lower compression, the detonation could actually stop and the motor might almost run normal. Unfortunately because of that, most folks don't know it's happening so the end result is usually a hole instead of a dish.
 
Joined
Oct 28, 2010
Messages
923
Re: Cause of Piston failure

Yeah I am sure their has never been a 2 stroke that has never had carbon build up on it.

pre_ignition.png


Looks like a two stroke piston to me...
 

99yam40

Fleet Admiral
Joined
Sep 7, 2008
Messages
8,851
Re: Cause of Piston failure

Looks like the piston can apart at the wrist pin, was there any discoloring of the crank/rod bearings?
If there is discoloring, maybe lack of oil or oil washed off from too much fuel in the crankcase from hole in fuel pump diaphragm if the top fuel pump runs off of that cylinder crankcase.

Lean fuel will normally get a piston,the ones I have seen melted the edges of the pistons and scuffed the cylinder walls.
lack of oil will get a bearing. Seeing as it came apart at the pin I would think it may have been oil related. Just my 2 cents
 

sschefer

Rear Admiral
Joined
Nov 13, 2008
Messages
4,530
Re: Cause of Piston failure

Jefferson, I know, you see the term pre-ignition used a lot and they are very similar in effect. Theres a book called the the "2 stroke tuners handbook". If you ever come accross it you should grab it and give it a read. It will explain why detonation is the correct term for two strokes.

99yam40, I might agree except it only hit the top two cylinders. It could be heat related from a waterpump failure since the two top cylinders are the last to get water. That does produce a piston top that will look similar but usually the exhaust side of the pistion is badly erroded.

I suspect that when it carboned up the ring lands it hung a ring in the port, lost a wristpin keeper and put the wrist pin into the port. That'll snap a piston in half like that.
 
Joined
Oct 28, 2010
Messages
923
Re: Cause of Piston failure

Could have been a number of things, looking at the top of the piston it looks like you caught a ring on one of the ports. Could have been that the wrist pin snap ring came out and at that point, could have been the ring locator came out (a risk you take when reusing pistons). Here is the deal with shop rebuilds, there are plenty of compressor repairmen out there but when it comes to doing a proper repair, you are not going to get as good a job as a manufacturer rebuild. Plus you will get a warranty that can be covered by any shop. If you go with a shop rebuild their is no way they are going to check every spec listed in the service manual. And their is a lot of specs, including the position of the wrist pin snap ring.
 
Joined
Oct 28, 2010
Messages
923
Re: Cause of Piston failure

Jefferson, I know, you see the term pre-ignition used a lot and they are very similar in effect. Theres a book called the the "2 stroke tuners handbook". If you ever come accross it you should grab it and give it a read. It will explain why detonation is the correct term for two strokes.

That reference is from the 2000 Yamaha tech guide, I don't think you are going to a better reference than that.
 

Silvertip

Supreme Mariner
Joined
Sep 22, 2003
Messages
28,758
Re: Cause of Piston failure

That reference is from the 2000 Yamaha tech guide, I don't think you are going to a better reference than that.

Only one problem with so called technical manuals. It is highly unlikely that an engineer wrote it. It is highly likely that a Technical Writer wrote it. Although the editing process should catch "technical" errors, that doesn't always happen and unfortunately, what appears in one manual tends to get carried over into future documents. Just because the manufacturers document says this is the way it is, doesn't mean it is always true. This is even more true when it comes to terminology. Service Bulletins constantly update errors in manuals or to let dealers know about product changes.

Perhaps this may help describe the differences:

http://dolphinmarineservice.homestead.com/Fail.html
 

Silvertip

Supreme Mariner
Joined
Sep 22, 2003
Messages
28,758
Re: Cause of Piston failure

OK!? (said sarcastically)

No need to feel that way. Sometimes there is a very fine line distinguishing issues. Ever use the word "irregardless" for example? Fact is that irregardless is not a word. Unfortunately, people use it so much (incorrectly I might add) it is being considered for inclusion in the dictionary. Really to bad but it illustrates that if one uses something incorrectly long enough it becomes fact.
 

199675hpforce

Petty Officer 1st Class
Joined
Feb 27, 2004
Messages
299
Re: Cause of Piston failure

What nonsense are you spewing. Back on topic my Yamaha manual states lean out cause hole in piston. Check your fuel pumps! I would replace. Do you know what 360 degree turn of pilot screw is maybe there set half that 180 should be around 1 1/2 check manual.
 
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