Mercury 800 - metal shavings on plug - Likely Extent of the damage??

[Phil303]

My Merc 800 - serial 3052172 - i think it's a '71?

I've been fixing this engine up for a while, previously spent 10 years in storage.

Finally got the boat on the water, it's cooling fine, worked great for about 30 mins, after that i went into a tight ish turn at 1/2 to 3/4 throttle and then something went crunch i guess?
I lost power, 1/2 power at full throttle and then it started making a knocking noise, i pootled back to the shore immediately and switched it off.

On inspection i pulled the plugs and there are some metal shavings stuck to the spark plug on cylinder number 1 (top).

Compression test on all cylinders is 140.

I put cylinder 1 to TDC and tried to see if there was any play in the piston by moving it with screwdriver, it seemed ok although i don't specifically know what level of play i'm looking for?

What i did notice was the other 3 piston tops are all brown and dirty looking whereas number 1 is shiny and looks like fresh metal, can't really tell if it's chipped/damaged?

What do i do now?

What would you suggest as the next avenue to go down?

how can i isolate the root of the problem?

cheers guys, help much appreciated!

 

[aussieflash]

Re: Mercury 800 - metal shavings on plug - Likely Extent of the damage??

Your still getting 140 on #1 ?This shiny clean piston is probably due to steam cleaning(water intrusion).You also may have chewed a ring.

 

[Phil303]

Re: Mercury 800 - metal shavings on plug - Likely Extent of the damage??

to get at the heads and rings do i need to strip everything back? i can get the dizzy, carbs and elecs off easy enough, do i need to pull the fly wheel to get at the heads? i guess i'll need a special tool for it?

 

[aussieflash]

Re: Mercury 800 - metal shavings on plug - Likely Extent of the damage??

If your going to start pulling the motor apart,your first buy should be a service manual.

 

[Faztbullet]

Re: Mercury 800 - metal shavings on plug - Likely Extent of the damage??

That is preignition damage and aluminum throw off from the piston eyebrow. Several things can cause this: (1)Weak cooling (my guess) (2) The max timing is either set incorrectly or the trigger has a problem.

 

[Phil303]

Re: Mercury 800 - metal shavings on plug - Likely Extent of the damage??

That is preignition damage and aluminum throw off from the piston eyebrow. Several things can cause this: (1)Weak cooling (my guess) (2) The max timing is either set incorrectly or the trigger has a problem.

Thanks Fatzbullet, it seemed it was cooling well, i put a new impeller, checked the gaskets on the pump, and it has a strong pee? although it is old and i guess it could be leaking cooling elsewhere?

I forgot to mention, when i removed the number 1 plug i noticed the plug was cracked, would that have contributed to the pre-ignition?

If i were to clean it out of any metal filings, check the timing, replace the plug, would it be good to go again? or do you think i need to pull it apart anyway to check for further damage?

would the pre ignition and possible overheat explain the knocking?

thanks
phil

 

[Phil303]

Re: Mercury 800 - metal shavings on plug - Likely Extent of the damage??

is there a way to clear out the metal shavings from within the cylinder without having to strip the whole powerhead? can i flush it out with something?