low compression

[fastscottie]

hi,

i recently replaced new pistons on my 787. the attached picture is that of the old one. the compression with the new pistons were only 130 and 140, which is lower than my other engine that was not worked on. i also noticed when i ran it on the lake, the rpm fluctuates when it's above 4000. there was also metalic residues noted on one of the spark plugs.

could someone please give me some insights on what could be going on. i am think whatever caused the first piston to fail is the underlying cause of all this. however, i do not know where to begin. please help.

thank you.

 

[Dr Honda]

OK... You said you put in new pistons. Did you have the cylinders bored and honed to the next size? If not... then the jug that had the melted piston is probably way out of spec, and the new piston is eating it's self. (that would account for the metallic residue)

Also... did you remove the crank, and rebuild it? If not... you could have a crank leak, and be causing a lean condition in the one jug.

On that note... did you figure out why the original piston melted? That kind of damage doesn't just randomly happen, and what ever made it melt down, is going to cook the new pistons.

Please answer the questions, and we will get back to you on it.

 

[fastscottie]

the cylinders were honed to the new piston specs. i did not remove the crank, but did rinse it out to get rid of pieces of the broken rings. how can u tell if the crank has a leak? would that explain low compression in both lugs? thanks.

 

[Dr Honda]

Nope... a crank leak won't give you low compression. The only way to get low compression is to have rings that aren't sealing. The only reason the rings wouldn't seal is if the cylinders were not prepped properly.


With that said... you still didn't answer the question about... "Did you figure out why the original piston failed?"

That is a big question.

To check crank seals... you would need to do a leak down test on the engine.

OK... since you said you now have a metallic residue on the spark plugs... I'm going to assume that you have run the engine since the "Rebuild." Now... this is just a guess... so...

The metallic residue is probably from a piston that is starting to melt down. That melted aluminum has now coated the rings, keeping them form sealing, and that is causing your low compression.


At this point... you know something isn't right, and I would recommend popping the head off, and take a look inside.

Also... answer the question above.

 

[fastscottie]

i don't know what caused it to fail in the first place. when i bought the boat there was no compression in the one cylinder. that's when i replaced the pistons.

does it sound like i have a problem with the bottom end?

 

[seadooya]

you need to go thru the carbs, but i say something let loose at the bottom...washers on each side of rod, at crank end....need to pull the motor, bud.

Being that its a 787, was it honed or bored,..either way, did you check clearance of the RAVES prior to install?

 

[fastscottie]

seadooya, the cylinders were bored by fullboreonline.com. i did sand the rave valves down.

 

[seadooya]

sanded down the raves?...might have not take'n enough off?