[bwatters]

I bought an 86 boat/motor combination with the motor not working. I took it apart and found number 1 piston and number 2 piston badly melted. I bored it out .030 and installed 3 new piston/rings. The water pump looked fine but I replaced the impellor anyway. I took the carbs apart and all jets looked squeeky clean. Put it all back together and it ran poorly (would not accelerate past half throttle. Third test drive it ran full tilt for 3 min. then quit. When I restarted it it knocked badly. Long story short, I have melted piston number 1 again!! My gas/oil mixture was 25:1. The only thing I can think of is that it is running lean on the top carb - possibly fuel pump problem - I changed the diaphram when I rebuilt it. Has anyone experienced this problem before with the 3 cylinder/2 carb models?

[jebeebe]

thats the one that usualy goes. My 60 merc did it. I don't know what caused it because I didn't own it at the time. I know a lean carb can do it. Excuse my typing, I have my arm in a sling from rotator surgery. I rebuilt mine and it runs good now, but I tore the carbs down and soaked them over night. I figured the water didn't get all the way through but like I said I don't know.

[emckelvy]

What was the maximum spark advance set to? You can easily 'smoke' a piston if the spark advance is set above 21 deg BTDC. And I'm also wondering if there's an ignition problem which is causing detonation.

What kind of fuel are you running? If you have non-alcohol fuel, you might try running premium in it.

You might try, at least during break-in, installing main jets one size larger, this may give you an extra margin of safety and you probably won't burn a piston next time.

Are the jets currently installed appropriate for your elevation? For example, if smaller jets were installed to run at higher elevations, they'd be way lean at sea level.

Probably didn't help to run the poor thing full tilt for 3 minutes straight with it being hardly broke in! That's an awful long time to run full throttle anyway, until the motor's been run for at least 20 hours. You're only supposed to use occasional bursts of full throttle up to around 10 hours and then just short periods between 10 and 20 hours.

Can't recall if this 3-cyl carried a thermostat, I'd definitely check that out if it did. Check the poppet too. Curious how warm it was running during the "burn-in". If the exhaust manifold was very hot, that's way too warm.

If your Wide Open Throttle (W.O.T.) with a lightly-loaded boat is too low (e.g., prop pitch too large), the motor can't spin up to its rated rpm and this puts great strain on the innards to the point of burning up pistons. Hopefully you have a tach and can tell what she's running.

The last suggestion I'd make is to do the mod where you relocate the 'telltale' from the side to the top of the block. This ensures that no 'steam pockets' or hot spots can develop, since the block will be fully flooded. You'll have to remove the cyl block water jacket cover to find the right location for drilling/tapping for a 1/8" NPT fitting.

HTH...........ed