melted piston top

[swordfish25]

I have a 1984 mariner 115 hp inline 6. It has been running fine. Today I was running it half throttle and it started to make a little noise. It then started skipping. When I pulled the plugs I could see alumn.(number 2 cyl). on it. I looked in the cyl. and I could see a little damage on the dome of the piston. I pulled the carb. and it is as clean as a new one.Still has 110 lbs comp. What could have caused it to burn the piston. Stock plugs, and carb appears clean Run it every weekend about 8 miles. Don`t know if it is worth rebuilding,if so I don`t it to happen again.

 

[Faztbullet]

Re: melted piston top

Its kinda rare to see #2 on a inline take a dump but most likely cause is bad switch box or bias circuit in switchboxes..

 

[swordfish25]

Re: melted piston top

I looked into it a little closer, and I see num.1 cyl is verry dry. Some how it must have leaned out. There are some other cyl. that look like they have started to get burned at somt time. They all have 135 comp. except num.2, which is 110. Don`t know if it is worth fixing, I can do the work ok.

 

[Chris1956]

Re: melted piston top

The compression specs are OK, as they are. I would pull the top transfer cover and inspect the cylinder and rings. If that look OK, clean the top carb (probably smart to do 'em all), link and synch, rebuild the waterpump and run the old girl!

 

[CharlieB]

Re: melted piston top

Check fuel flow, fuel pump output.

The upper carb is the first to show any signs of a reduced fuel supply.

A loose clamp before the pump can allow air to be sucked into the fuel system, causing leaning of the upper carb, old hose, rotted insides, can restrict fuel supply and lean out the upper carb.

Check all fuel lines, fittings, replace any suspect hose,clamps, rebuild the fuel pump.

 

[swordfish25]

Re: melted piston top

I took the top carb off and I could not believe how clean it was. Perfect as a new one. I checked the screen on the fuel pomp and it was clean. The top crank seal seems dry, but for some reason the top 2 cyls. seem to have run lean. 135 compression on all 5 cyl. except number 2 which is 110. No hole in the piston, except the top edge of the dome behind the plug is starting to burn off. It made a ratteling noise before it skipped. It skipped because the alumn. shorted out the plug. I had to run it for at least two miles after that. Ran good on 4 cylinders. No noise.

 

[verado7]

Re: melted piston top

it sounds like the idle mixture is lean for that carb - you say it happens at half throttle - the idle mix is responsible for power up to between 3 and 4000 rpm - if is lean at idle it will get worse until you pass the point the main jet takes over - also its not really agood idea to run at half throttle for very long on 2 strokes - less fuel flow /less lubrication ,

 

[Chris1956]

Re: melted piston top

Hey Sword, Do you have backdrag carbs on that motor? If so, there is a small fuel screen under the brass fittings in the carb covers. It can get clogged reducing fuel flow into the carbs.

I do not believe that an incorrect idle mixture setting will cause any piston damage. If the mains are clogged, that is a different story. 1/2 throttle running is perfectly fine, as is any other throttle setting. The Prev poster was incorrect on that issue.

 

[Faztbullet]

Re: melted piston top

If so, there is a small fuel screen under the brass fittings in the carb covers.

Where???

the idle mix is responsible for power up to between 3 and 4000 rpm

Nope..the boost venturi does this. Idle circuit good up to about 1800rpm.

 

[swordfish25]

Re: melted piston top

I have a I/o fuel filter on and cut it open, nothing in it. Seems to have gray neoaprine hoses. Haven`t checked pump pressure, what should it be. This motor has run great up till now. Never took it on any long runs, just short 3 mile trips to the island.

 

[Chris1956]

Re: melted piston top

Hey Fazt, The backdrag carbs have a small stainless steel mesh under the brass elbows(top and bottom carbs) and tee's(middle carb) that screw into the backdrag carb covers. at least, my '77 Merc 1500 had those screens. I found them all gunked up one time.....