cause of piston meltdown?

[bezell1]

I am starting to attemp to overhaul my 79' 70hp omc. I had lost compression in top cylinder, so with nothing to do this winter I decided to try and fix it myself. I have broken the power head down to find that the top piston some how fused to the cylinder head. I guess once restarted it broke free leaveing the cylinder head damaged and also the top of the piston. I can not see any damage to the cylinder but will take it to someone to examine. My question is why did this happen to begin with. The motor was pissing fine. I had just replaced the gaskets and reed value in the carburators. I don't want to fix the powerhead just to have this happen again. any thoughts? thanks

 

[MURX]

Re: cause of piston meltdown?

Perhaps a vacum leak caused a lean condition and the heat from running lean killed it

 

[bezell1]

Re: cause of piston meltdown?

were would the vacum leak came from? Gaskets or fuel line?

 

[Trent]

Re: cause of piston meltdown?

Very possible a lean carb caused the problem.. Why did you change the reeds??

 

[clanton]

Re: cause of piston meltdown?

After you get engine running, it would wise to install the latest waterpump kit, it has 3 upgrades in the kit. the inpeller, pressure relief spring, and fitting with instruction for drilling hole in top of block to prevent air blockage in top of block. Insure overheat alarm working.

 

[trevorcday]

Re: cause of piston meltdown?

I have several bad experiences rebuilding these @#!^#%^@$#%%^@^ 3 cylinder mercs that have melted pistons. At our shop we have now decided to retire and part out all blown up 3 cylinders. Unless you can find a concrete reason it melted the piston I wouldn't waste my money. A lean condition would be the first thing to look for such as a blocked main jet on the top carb. From my past experience it will more likely be some mysterious cause that you can't put your finger on. My current theory is that the ignition timing goes screwy on one cylinder and causes the combustion temp's to rise and melts the piston. We had at least one motor where a bad trigger caused this to happen.<br /><br />Sorry for the negative attitude towards your engine. Just the same my advice would be to find a working one and keep your old one for parts (or sell them on ebay).<br /><br />Good luck,

 

[Dhadley]

Re: cause of piston meltdown?

The 49 CID 3 cyl is a great motor. To determone what happened we need a little more info. Obviously the reeds are not in the carbs so we need to know exactly what you replaced and why. What spark plugs? What grade fuel and how old? Was anything else changed, like timing? <br /><br />Good luck!

 

[hondon]

Re: cause of piston meltdown?

I'm going with Dhadley on this one.These motors ,of that vintage were very hardy.How are you propped?All other ?s apply as well. Cooling problems on these engines did not start until redesign in the late 80s.This engine really hates to be lugged.

 

[bezell1]

Re: cause of piston meltdown?

thanks everyone for replying. I place carb kits on the carburators because I was having problems getting the engine started but had good fire in spark plugs and assumed I had fuel problems. I was useing new champion spark plugs. Fuel and oil were new in new portable tank. Engine had been running fine. I ran the engine at least three times a week before she quit starting and once carb clean and restalled engine ran once again with no problems. Nothing else on engine was changed. Lost compression 1st time out on water(1hr after lauched) after carbs reinstalled . Motor would run intil put in gear than died. Took to shop and had no compression in top cylinder. wondering what would have cause lean fuel? I think that might be more of the problem than cooling.

 

[Trent]

Re: cause of piston meltdown?

Stopped up or possible restriction to the main jet.

 

[bezell1]

Re: cause of piston meltdown?

just so I understand correctly. When we say we have a lean mixture means that some how the top carburator was not shooting fuel into the top cylinder thus not lubercating the cylinder causeing it to overheat?

 

[12Footer]

Re: cause of piston meltdown?

Yes, Bezell. 2cycle,no fuel-no lube. Remove the exaust port, and look for any blockages too. It may be, that the the holy cylinder got no coolant circulation,and acvtually DID melt down.

 

[dick]

Re: cause of piston meltdown?

Two more things , they droped the max timing on those engines from 19' to 17'BTDC and change they high speed jets on all three carbs to 53D's Theres a kit that comes with the jets and a new timing decal. The jet change was to allow for more fuel at higher rpms , and the timing change was to help reduce detonation at is common on 60/70. Not to say there bad engines there good engines . One more thing , prop it out to the high end .

 

[trevorcday]

Re: cause of piston meltdown?

Three cylinder merc's are good motors except they melt pistons more than any other motor. Other than that they are good I guess.