Serious Piston Damage

[fgbutler]

71 Merc 800, 80hp. Pulled spark plugs to check condition, and found #3 covered in metal flakes the rest were a little oily. Pulled out crank and found the fin of the piston being pitted away. What could cause this? Except for carbon, all the other pistons were fine but #3.

 

[PeteMax402]

Re: Serious Piston Damage

I just rebuilt a 1975 402 that had a similar problem... I would suggest you check your carburetor and fuel pump. The most likely cause is that no fuel was getting to the cylinder and that caused it to overheat and the metal to start flaking. <br /><br />How many carbs does this engine have? <br /><br />Where is the piston located is this a v or inline is it near the bottom or top of the block?<br /><br /> In mine the piston at the bottom had plenty of fuel while the top did not. This caused the bottom to fire while the top was overheating and caused it to melt. I would also check your thermostat and water pump and see if those are operating properly. <br /><br />If the cylinder walls are scored you will need to tear the whole thing down and have it bored and new pistons and rings installed.<br /><br />I hope this helps

 

[MercBassmaster]

Re: Serious Piston Damage

It could be a couple of things, one being the carb - that cylinder could have "leaned out" meaning it was getting too much air and not enough fuel - often caused by an air leak in the intake somewhere, incorrect carb settings or a dirty carb, or possibly even worn out reeds. I agree with PeteMax on this - pull the head(s) off and turn the engine over by hand and inspect the cylinder walls, if they're deeply scored/scratched you'll need to bore, if they don't look too bad then the thing to do is get a ball hone and hone out the cylinder - best hones to use are called FlexHone (which you can see at www.brushresearch.com then click on Flex Hones on left side, use around 400 grit or so). It's fairly easy to hone yourself, it just goes on a drill, you use LOTS of oil, and while spinning move up and down cylinder to create a "cross hatch" - if you're unsure you can find out more on how to do it on the web, or get a book. Be sure to wash block COMPLETELY - it's recommended that it's rinsed thoroughly in (believe it or not) hot soapy water! You need to dry it competely and oil it immediatly after washing though or it'll rust and you'll ruin it. <br />The other possible cause of the problem here - if the piston was pitting away on the dome (top) of it near the center - like where the spark plug is? It's possible that you have the incorrect spark plug heat range in there and it's burning too hot and literally melting away at the piston - I've seen this in other 2-stroke engines, but I'm quite sure it can happen in any, including boats. <br />good luck.

 

[greasemonkeyozi]

Re: Serious Piston Damage

I hope my 2 cents in this is worth it.<br /> Did you run a compression test on your top cylinder to see if it is blown? If not, you can purchase one at a local parts store. Just take your spark plug out and stick it in the hole. Have someone try to start it and see what it reads. It should read around 130 psi., just compair it to the other cylinders. If they read the same, its ok.<br /> Reason being is that my motor had the same problem, wasnt running good at all, pulled top spark plug and seen metalic corrosion on it when the bottom ones were fine. All it was was my electrical system was firing on the top cylinder way hotter than the bottom ones causing alot of carbon build up. That will show the same sighns of a sheered piston.<br /> I hope my advise was somewhat helpful. Let us know how it goes.

 

[Gold Bear]

Re: Serious Piston Damage

gb_franz <br /><br />For some reason (unknown to me) the number 3 cylinder is known for running lean on your inline 6 engine. It is an inline 6 isn't it?<br /><br />I've heard of guys installing larger jets on the center carb to help with this problem. <br /><br />After you make repairs, make sure you clean all three carbs, and also make sure you do not exceed 21 degrees timing, regardless of what you manual might say. <br /><br />If I were you I would mix my fuel at 40:1 and set the timing at about 20 degrees just to be sure.<br /><br />Good Luck,<br /><br />Gold Bear

 

[andrewkafp]

Re: Serious Piston Damage

I think all 80hp were 4 cyl 2 carbs..<br /> http://www.marineparts.com/partspages/techhelp/TECH15.HTM <br /><br />All this advice looks helpful, but I don't quite understand the 40:1 suggestion as this would not help with (Fuel/air) mixture.<br />I have read other posts that suggest it would in fact promote more carbon buildup and the extra oil would make the mixture even leaner, even though it would provide more lubrication.

 

[Gold Bear]

Re: Serious Piston Damage

Just a suggestion to increase the amount of oil. If you know your are using more oil then you should de-carb more often. <br /><br />The problem with a lean condition is that the amount oil going inside the engine is not great enough, that is where the bits of metal come from, lack of lubrication.<br /><br />I'll bet gb_franz wishes he had had a bit more lubrication in this instance. If that were the case perhaps we would nave never even seen this post and everyone would be happier.<br /><br />Gold Bear

 

[fgbutler]

Re: Serious Piston Damage

Thanks for all the info.<br />After talking to a few mechanics myself I have a better idea of possible problems.<br />Too much air is a good one. If there was a leak somewhere in three it would cause severe heat.<br />Preignition is a probability that I did not hear. The manual suggests preignition in its troubleshooting guide.<br />I think the spark plug fouled (for whatever reason) or some preignition occered causing little flakes. After those welding themselves to the spark plug the plug arced to the piston (acting as a ground) allowing damage to happen at a much faster rate. How would you agree?