Lets play a game called 'What happened to my piston'!!

[Biggieb400]

Re: Lets play a game called 'What happened to my piston'!!

I have to ask, at this point does it really matter what the root cause was? This is FUBAR'd and may never be able to determine the root cause. If it were a ring how will you tell? It's gone!

It matters because I want to find the root cause so I don't waste $1200 on a rebuild and still have problems... I'm looking for value added feedback from others who may have had similar failures.

 

[Biggieb400]

Re: Lets play a game called 'What happened to my piston'!!

Agree +1....Find the cause so it doesn't happen again.

Thanks for understanding

 

[mercuk]

Re: Lets play a game called 'What happened to my piston'!!

Agree +1....Find the cause so it doesn't happen again.

Agree +2

 

[bonz_d]

Re: Lets play a game called 'What happened to my piston'!!

I do understand your reasoning in that you don't want a repeat. But with the extensive failure that you see do you really think you will be able to determine what caused it to implode. You yourself noted that the top ring is completely gone and if that was the cause how do you prove it even to yourself? It's not like there are many numerous different parts and assemblies to be looked at such as in a 4 stroke engine. You even mention that from the wrist pins down everything looks normal.

So then what do you have? Mechanical failure, ring, piston. Fuel/lubricant failure or an overheat cooling issue. Or a combination of all 3. Which then you'd have to ask which occured first.

I think what's an even more important question than what caused it, because of the destruction found, is it even salvageable.

 

[seahorse5]

Re: Lets play a game called 'What happened to my piston'!!

There were service bulletins about those engines back in its day that included timing changes, jet sizes, cooling mods like repositioning the tell-tale fitting to the top of the block, different temp switches, etc. The changes were for specific years and models so every item may not have applied to your particular motor. They even came up with a heavy duty water pump that included heavier springs for the thermostat pressure relief system.

#2 cylinder runs the hottest and is most prone to detonation when it gets scuffed by overheating and especially if the timing is advanced or the carb runs lean. Overpropping is also a contributing factor. Being over 20 years old, who knows how well it was treated back in its day.

 

[bonz_d]

Re: Lets play a game called 'What happened to my piston'!!

Seahorse5 I was just thinking the same thing as I too recently read a few posting here on just this subject. Mostly because I've just obtained a 1988 70hp 3 cylinder Evinrude and have become courious about the 3 cylinder models.

Also after rereading the original post what or who is not to say that all this damage hadn't occured during the 1st over-heat alarm? Starting with a cracked ring.

I still do not believe he will ever find the "root cause" of this, though he may be able to surmise one.

 

[boobie]

Re: Lets play a game called 'What happened to my piston'!!

Edit

 

[seahorse5]

Re: Lets play a game called 'What happened to my piston'!!

on the older 3 bangers, #2 runs hotter because of the heat from the cylinders above it and below it due to the block design. That is why it is almost always #2 if a piston fails.

 

[boobie]

Re: Lets play a game called 'What happened to my piston'!!

What Seahorse said above is correct. OMC came out with a different block because they had so many probs with this one. The new blocks didn't have bridges in the ports. If you want to use this one, either contact a good BRP dlr and he may have the bulletin updates you can do or contact BRP customer service at 1-262-884-5993. After looking at your model #, I think your prob was just caused by a bad design but just make sure you do ALL the updates especially the water pump. Hope this helps.

 

[Biggieb400]

Re: Lets play a game called 'What happened to my piston'!!

When I replaced the impeller, I installed the new spring that came with the kit, and also read the instructions regarding the update for the engine vent at the top of the exhaust cooling cavity. That update had already been completed prior to me taking possession. It essentially got a replacement impeller and different thermostat spring at the beginning of the season before the first overheat alarm ever triggered.

My research also revealed that there was a 'new' style temp switch released. I read that if the sensor lead has a tan wire with a white tracer (stripe) that this sensor has a higher temp setting. I deliberately kept the old style sensor (solid tan wire) that should set off the alarm at a lower temp and force the speed derate sooner, if an issue is encountered.

I have sent off the block to be resleeved on the center hole and boring out #1 and #3, likely 0.020" over stock. The machinist has a unique technique where he removes a bit more material from the exhaust bridge due to the known heat issue in this area.

I haven't pulled the #1 and #3 carbs apart yet. Does anyone know if the #2 has a larger main jet to account for the increased heat?? This is common practice on Inline 4 cylinder 4 stroke motorcycle engines. This provided extra fuel to the center cylinders since they run hotter than the outside cylinders.

I hope you all don't think I was expecting a full synopsis of the failure. I was looking for plausible explanations based on others knowledge of this engine and the common failures that occur. At this point, I am leaning more and more towards a broken ring.

I would be interesed to hear why piston rings break in 2-stroke engines if anyone has good explanations.

 

[boobie]

Re: Lets play a game called 'What happened to my piston'!!

Excessive heat from lack of proper fuel/oil mix or mtr overheating. Ring gets caught in a port from sharp edges on the port. Dowel pin in piston falls out and ring gap gets caught in a port or carbon build up around the rings. That's a few.

 

[Dhadley]

Re: Lets play a game called 'What happened to my piston'!!

On those 56 cid blocks the motors run pretty lean from the factory (EPA related). The updates are important but don't really hit on the root problem. In the mid range they run on full advance timing but very little fuel. The longer they run (in the mid range) the hotter the EGT gets. EGT temp and engine (cooling water) temps are 2 different things.

While it's important to do the updates and make sure the carbs are clean etc, you might want to richen up the mid range jets. Especially on the center carb. As stated earlier the center cylinder tends to run hotter so thats the most important one to rejet.

Also realize we don't have the same fuel we had in 1989. Those motors were calibrated for the fuel available then, not now. Using only Champion plugs is very important. Using a slightly cooler plug, like the QL77JC4 rather than the QL78YC can help avoid a repeat performace as well. Many time we see someone put the wrong plugs in and see a similar failure in a very short time.

Lugging the motor also has a great deal to do with combustion (EGT) temp.

As for broken rings we find that when the combustion temps rise above design levels the locating pin can - and will - migrate in. This allows the ring to move. Once the gap end gets in the exhaust port it will shear off as it goes in & out of the port. If you find it soon enough, the ring to the right side of the gap will be the one to break first.

 

[Biggieb400]

Re: Lets play a game called 'What happened to my piston'!!

boobie and Dhadley

Thanks for the info. These theories are more in line with what my failed and non-failed parts tell me.

This raises 3 additional questions:
1 - Can you provide the name of a couple retailers that sell jets for this style carb?
2 - How can I enrich the mid range? There is an idle jet and a main jet, and no additional adjustment that I'm aware of...
3 - What is the ideal rpm range to run this engine?

Appreciate all the input so far

 

[boobie]

Re: Lets play a game called 'What happened to my piston'!!

1- Your BRP dealer.

2- None that I know of.

3- 5000-6000 rpm. Best, 5500 rpm.

 

[CaneCutter79]

Re: Lets play a game called 'What happened to my piston'!!

I haven't read all the replies yet so my apologies if I'm repeating but timing being advanced can and could have caused the start of your issues.

A bad ring will also do it. Its like a chain of events after that. Also, do you fog your motor regularly every winter? I've learned all kinds of things since my rebuild. Proper maintenance and factory settings go such a long way.

 

[Dhadley]

Re: Lets play a game called 'What happened to my piston'!!

If I remember right there is a low speed adjustable jet, an air jet (probably like a #32) and a high speed jet in the bowl. If you small on the air jet it will richen up the mix. You can play with that jet and try going 3 to 5 sizes.

Also make sure to not lug that motor, use fresh fuel and Champion plugs only. We try to get as close to 5800 as possible with an average load. Using ethanol fuel may bring your target down some but try to get at least 5600 with the average load.

 

[Biggieb400]

Re: Lets play a game called 'What happened to my piston'!!

I haven't read all the replies yet so my apologies if I'm repeating but timing being advanced can and could have caused the start of your issues.

A bad ring will also do it. Its like a chain of events after that. Also, do you fog your motor regularly every winter? I've learned all kinds of things since my rebuild. Proper maintenance and factory settings go such a long way.

I've only owned it for a year so I have no idea what was done by the original or previous owner(s). I've always fogged the cylinders on my 4 stroke marine engines when storing them...

My machinist mentioned a possible timing issue as a potential problem. I planned to check that once the rebuild is complete. Any suggestions on the procedure?? Timing light and run the engine wide open and check timing?!?!

 

[boobie]

Re: Lets play a game called 'What happened to my piston'!!

You can check your wot timing by cranking it over. Go to the top secret files at the begining of this forum and I think Joe Reeves has a procedure for this.

 

[glenn property of pam]

Re: Lets play a game called 'What happened to my piston'!!

looks like the ring failed and the hot gas went down the side of piston until it caused loss of compression/crank case suction then the piston is just moving with out lubrication until it melts at which point the gasket fails and water got in. must admit it looks impressive