Blowin Piston Again (Help)

[paul tracy]

ok this is my problem, iv got a 1979 115 mercury, just bought it this summer,had it out on the river a few time, and it;s one thing after the other. So i had it out the other day was running great we where pulling the tube behind i hit full throttle and it just lost power don't no what happend it still turned over

Get it home take it apart find out the rings melted in second ciylinder and theres a little bit of shevings on the spark plug fine put on a new used piston rings take her out the next day for just a test 1/2 hour she ran fine the RPM was at 6000 insted of 5500 where she was befor so i thought Right On

well guess what take the f..kin thing out toady she ran good all the way up the river for about 10- 15 miles and all the back. push her up to full throttle and she lost power again started shaking, i guess thats because it was off balance so this time we make it back thank god, but to find out i blew the third piston same ****in thing as befor little shavings in the spark plug the hole to where the spark plug sits is full of them

i don;t no what im doing wrong i thought it was my oil mix im mixing it 1/2 alittler two 5 gal of gas could it be wal-mart oil or what

 

[j_martin]

Re: Blowin Piston Again (Help)

The mistake you made was giving it heck the same way the first one failed, without finding out why the first one failed.

Likely, a little short of fuel in that cylinder, leaning it out and burning down the piston.

hope it helps
John

 

[paul tracy]

Re: Blowin Piston Again (Help)

So are you saying it could be my carbs? if there dirty i guess so or what do you think (FUEL STARVATION)

 

[j_martin]

Re: Blowin Piston Again (Help)

So are you saying it could be my carbs? if there dirty i guess so or what do you think (FUEL STARVATION)

Yup.

I wouldn't make that repair without checking out the related carb. Then when I fired it back up, I'd be reading plugs till I was sure it was right. Intake air leaks can do it also, but a carb could idle perfectly, and starve at speed and burn down the related piston.

hope it helps
john

 

[cannonford57]

Re: Blowin Piston Again (Help)

top,s are sure to flake metal on plugs from heat.surely ya replaced the impeller a?

 

[Faztbullet]

Re: Blowin Piston Again (Help)

Check your timing ,sounds like trigger in distributer is going bad. Usually it takes out cylinders 1,3 or 5 but have seen #2 fail...

 

[Chinewalker]

Re: Blowin Piston Again (Help)

I concur with the above - unless you find the source of the meltdown you are doomed to repeat it... Check carbs, check timing, check cooling...

 

[arsenalpsu]

Re: Blowin Piston Again (Help)

All above is good to check, also what is your timing at WOT? I'd have to check my OEM book but should be no more than 21 btdc, any more with today's crappy fuel you'll melt the pistons. Does she hesitate when you punch if for a hole shot? If so you're running lean, i don't think it's just that one carb is dirty as it was 2 then 3, not just 1/2 or 3/4. Do a link and sync, check timing and make sure she runs dirty. OH and i'd replace all 6 pistons with the newer 2 ring systems. The ring pins on the older pistons can push into the piston over time and let that top ring move around causing a whole world of problems.

 

[paul tracy]

Re: Blowin Piston Again (Help)

So could anyone explain how to time it? I figer the number 1 piston should be all the up. and the fly wheel tdc but distributor is way off

 

[paul tracy]

Re: Blowin Piston Again (Help)

well got it apart again last night to find out this time it is the same but worse the ring meted right in to the piston and the connecting rod snapped off
i'm ready to throw it out please help

 

[daveswaves]

Re: Blowin Piston Again (Help)

well got it apart again last night to find out this time it is the same but worse the ring meted right in to the piston and the connecting rod snapped off
i'm ready to throw it out please help

Paul, sorry you are having so much trouble with your tower of power, these old girls need a little TLC but are solid engines. I read all of your posts and from what I can gather you are on your third piston in the same cyl and now have a broken rod. I did not see any mention of what you did to clean up the cyl after the damage . There is no way that #2 hole is in good enough shape to put in yet another piston. At this point you are looking at re boring that hole to the next oversize and starting over.

Your oil mix is ok at 40:1, nothing wrong with walmart oil, the timing should be max at 19 degrees for the type of service you are using the boat and motor for (tubing). There are lots of things to consider when building this engine to run hard. If you are anywhere near my neck of the woods I would be happy to look at the damage for you and give you some direction.

 

[paul tracy]

Re: Blowin Piston Again (Help)

i'v never really worked on a outboard like this befor, so there are a few things i don,t understand like my pistons two of them have only two rings 2-5 i think never seen this befor (strange)

 

[daveswaves]

Re: Blowin Piston Again (Help)

Many replacement pistons are only 2 ring, perfectly normal. It just means they have been changed before. A few times apparently.

 

[paul tracy]

Re: Blowin Piston Again (Help)

Many replacement pistons are only 2 ring, perfectly normal. It just means they have been changed before. A few times apparently.

So i could put on another piston with three rings it wouldn't matter?

 

[daveswaves]

Re: Blowin Piston Again (Help)

Paul, yes you can put a 3 ring piston in. As I previously stated though, you need to do some work on #2 cyl before you throw more pistons at it.

 

[paul tracy]

Re: Blowin Piston Again (Help)

I have another motor laying around, im going to ues the block out of it, but am running out of rings, the pistons that im putting on now are all three ring,anything else i should know befor moving on.

 

[daveswaves]

Re: Blowin Piston Again (Help)

I have another motor laying around, im going to ues the block out of it, but am running out of rings, the pistons that im putting on now are all three ring,anything else i should know befor moving on.

You can use a used ring on the lower slot, preferably the ring that was on there in the first place. Check the condition of the cyl in your replacement block as well. If there are melted pistons there you need to pay attention to the cyl walls. There will be melted aluminum on them. Pay particular attention to the ring end gap. Many times the rings will cease due to insufficient end gap. If memory serves me you need about .015 . Check the ring locating pins in the piston, make sure they are there.
If you are going to proceed with the rebuild post pictures of anything you are in doubt about or concerned about.