Top piston erosion

[hydeb]

I have a late 70"s 70 horsepower Merc. triple that splash fouls the top spark plug with aluminum.<br />I have replaced the piston, rechecked the ignition timing, and fuel pressure but it still continues at @4800 rpm. The upper carb was jetted to one size larger than standard.<br />The motor was throughly checked by an experienced Merc. technician. <br />I am using 87 octane gasoline.<br />Has any one seen a similar problem?

 

[kev_79]

Re: Top piston erosion

Has it ever been rebuilt?<br /><br />k

 

[hydeb]

Re: Top piston erosion

3 years ago (@100 hours) below 4500 rpm, at that time I replaced the water pump and it looked perfect, It however happened again within 15 minutes at 5000 rpm. I plan to pull it out again to see if there is any degredation.I am a professor teaching automotive engine technology and I think I have visited all the basics. Please continue correspondance.

 

[kev_79]

Re: Top piston erosion

So when you say erosion, I assume you mean that the piston is melting down?<br /><br />I'm not sure I am totally understanding you, but to me, it sounds like one of the cylinders has leaned out and melted down the piston. If this is the case, you will definately need a rebuild, but it doesn't really make sense, b/c I doubt you could get to 4800 rpms with only 2 cylinders. (You stated it was 3, right)<br /><br />If it's aluminum on the plug, then you definatly have a piston problem, b/c that is the only thing in there made of aluminum.<br /><br />Let us know what you find when you get in there.<br /><br />k

 

[rbruce63]

Re: Top piston erosion

If the motor was rebuilt and the sleeves and crankcase were not thoroughly cleaned, then you are destroying your work with the rebuilt shop grit but it could show on all the cylinders.<br /><br />Also please check the ignition timing which is crucial in two stroke motors and could lead to piston crown melting!<br /><br />Why was the top carburetor jetted differently? It should be jetted according to the altitude where you do your boating and then synchronized with the other carburettor or the other three and then synchronized with the ignition timing.

 

[rodbolt]

Re: Top piston erosion

hello<br /> its time for another rebuild. merc never ever admitted it but we solved the piston melt problem by increaseing the jetting 3 sizes and moving the tell tale to the top of the block. seems steam pockets were building up and scorching the top of the piston/cylinder. your pump is overdue. forget everything you know about automobiles while playing with this high performance 2 stroke and life will be better. its not a car at all.heck I drove an old 64 chevy wagon with a 283 frome pascagoula MS to kitty hawk NC and back. could not go over 50 or the leaky radiator would puke. could not put the radiator cap on it. used 42 qts of oil on the trip. would run until the oil lamp flickered and add 4 qts. never checked it . a two stroke would not have made it out of town much less back. your 2 stroke will run great until something degrades,does not have to fail, just degrade. how many car engines have you trouble shot fuel or ign problems above 4500 RPM's<br /> not busting chops just tryng to make a point<br /> good luck with the tear down.