[thefishingfreak]

my friends boat.
a few weeks ago it felt{to me} like it was low on power and low max rpm arround 4800 max. momentarily, then problem seemed to disapear.
fast forward to the next weekend. my friend took it out over 1.5 hrs on plane to the fishing grounds. 6 hrs trolling. 1/2 way home they stopped to use the head. on power up #4 piston blew up.

this is where I come in as the somewhat mechanicly inclined friend.

I stripped it down, #4 looked like either 1" of the top ring broke? or 1" of the top of the piston broke? didn't look overheated/melted it just looked like a part{piston or ring} failure.

we honed the cyl. installed new piston/rings, deburred the head cover and buttoned it back up. runs super, only problem is it is down 800-1000 rpm max {4200-4400 max} and it sputters slightly on acceleration. with the air box off if you crack the throttles fast it just boggs down.
we ran out of time/daylight today to search for the cause but I suspect whatever it is, is what caused the piston failure.

any tips on what to look for first?
thanks.

[CharlieB]

How much 'coking' or carbon build up was under the rings and did you clean it ALL out?

What color were the piston domes?

How much carbon 'wash' was there down the sides of the pistons?

Did you thoroughly clean the carbs, rebuild the fuel pump and replace the impeller?

These are the tree items that most commonly cause an engine to toast. Not identifying the original cause dooms the engine to failure again, also remember to ADD OIL during the break-in, new rings and a new piston need break-in.

[tashasdaddy]

{to me} like it was low on power and low max rpm arround 4800 max.
Did you thoroughly clean the carbs, rebuild the fuel pump and replace the impeller?

to me a dirty carb, could have been running lean, no providing enough fuel or lub.

[CharlieB]

'Coking' in the ring lands fills the void behind the rings, eventually squeezing/seizing the rings and they then fail to transfer piston heat to the cyl wall.

Substandard oils, excessive idle time, both contribute to early failure.

This is the reason for Deep Creep, top end cleaning

During rebuilds, reusing pistons, ring lands MUST BE SPOTLESS

Coke 'softens if run WOT for a period of time, shutdown without adequate cool down the softened coke solidifies, adhering to BOTH the ring and piston, seizing the ring, next start-up and immediate WOT, BOOM, piston seizure. Which is what happened when they made the short pit stop on the way back from fishing.

[thefishingfreak]

thanks for the help so far.
here's some pics.
I pulled the center carb off and tore it down, but there were no obvious signs of any crud in it ? it was spotless.
I will start with the carbs- take them all apart and clean them out.
can you explain the deep creep top end cleaning?

Thanks again..

[CharlieB]

Look at that ring land very carefully, where is the locating pin for that ring?

Screaming these things can loosen the locating pin, it comes out and makes a small mark on the wall while the ring begins to rotate until an end snags a port, then all DONE.

If that pin came out then I'd have to wonder about the other pistons.

There is a guy (Jay) on the Scream & Fly boards that specializes in top pinning pistons for those guys that have to twist them engines 'tight'.

Hopping it off another's wake and catching a bit of air revs it T. I. T. Tight too. These things catch R's too quick to be fooled with and have to be built a little different if he wants to turn it above the factory rating.

[thefishingfreak]

ohh thats a good point..
the piston is down the boat. I will definitly check this tomorrow.

The locating pin belongs where the red arrow is. From my pics it sure looks like it's missing...