#1 Piston gone what next?

[jhester]

Ok everyone, I have looked on here for a while to try and find some kind of answer to my problem. Took the boat out on the river the other day and it just lost power and died. Paddled over to the bank to let it rest for a little while and after an hour or so it started back up but was sluggish and had a cough to it but it got me to my fishing hole. On the way back it died again so after a 2hr drift down the river finally got it on the trailer. Here is what I have done so far:
1. changed all the gas out with fresh to eliminate the possibility of bad gas.
2. checked the fuel pump that I just rebuilt not long ago and working fine.
3. rebuilt the carbs.
4. have spark at all cylinders (3)
The engine will start if I idle it up but it still coughs and the water coming out gets warm very quickly. I will do a compression test tomorrow to check that but I am a little stumped about this problem. I have ran this boat for a while and never had any trouble out of it. Any help on where to look next would be great.

 

["G"]

Re: 1978 Merc 700 Problem Running Please Help Me!!!

Re: 1978 Merc 700 Problem Running Please Help Me!!!

Jhester-
The compression test may provide you with the problem. If you overheated to the point of sticking a ring or scoring a piston the top cylinder will have a lower compression. It sounds like the issue gets worse with running. Make sure you are pumping water. If it did overheat, the reed locating pins on that engine were plastic so be aware there may be future trouble there. That it still runs is a good thing.

 

[jhester]

Re: 1978 Merc 700 Problem Running Please Help Me!!!

Re: 1978 Merc 700 Problem Running Please Help Me!!!

thanks for the info, I hope it will stop raining this morning so i can go and check the compression. It does still pump water very well and it is not scalding hot just a lot warmer sooner than before. if it is a stuck ring how bad of a job is that to fix? I am pretty good with the tools so that is not a problem just didn't know if there is anything I should be aware of.

 

[jhester]

Re: 1978 Merc 700 Problem Running Please Help Me!!!

Re: 1978 Merc 700 Problem Running Please Help Me!!!

ok found the problem. the #1 cyl. was shot only 45-50psi with #2 &#3 being at 145-150psi. My question is a local mechanic said that year had a problem with overheating the top cyl. due to the water passage not letting enough get to it. Now my question is should I just part out this thing or go ahead and bore it out and install a new piston. If I can save the motor is there anything I can do to help with the water around the #1 cly. I am not definiate about this being the cause but I would really like some options on what some of you guys with experience with this stuff.

 

[chrishayes]

Re: 1978 Merc 700 Problem Running Please Help Me!!!

Re: 1978 Merc 700 Problem Running Please Help Me!!!

I assure you that fixing this motor will cost you less than buying a new one. And to the future reliability, you are dealing with a 1978 motor, that should tell you how durable they are, regardless of having flow problems. You know what I mean?

 

[jhester]

Re: 1978 Merc 700 Problem Running Please Help Me!!!

Re: 1978 Merc 700 Problem Running Please Help Me!!!

Thanks, that is what i was thinking but I didn't know if anyone else has any issues with this same problem. Going to try and find a good machine shop to bore it out and put her back together. Does anyone have a different idea about running or rerouting the water flow to help keep this from happening again?

 

[Robj]

Re: #1 Piston gone what next?

I would take it apart and inspect it first. See what the damage is and go from there. You may only need a hone and new rings.

Have a great day,

Rob.

 

[jhester]

Re: #1 Piston gone what next?

I already have it apart and am going to have the #1 cly. bored and a new piston installed. Has anyone had any problems after having work done on the powerhead especially just the #1 cyl. fixed. Apparently the guy at the machine shop said this was a common repair on the inline motors.

 

[Robj]

Re: #1 Piston gone what next?

While I have not done that myself, I have read many times that it is common and acceptable to just have one cylinder bored and the piston replaced. You should be OK.

Have a great day,

Rob.

 

[CharlieB]

Re: #1 Piston gone what next?

How old is your water pump and impeller?

Many make the mistake of thinking that it pumps plenty of water, but you have just learned that what 'appears' to be enough wasn't.

Change that impeller at LEAST every other year, whether you run it or not. Impellers age and deteriorate just from the air. They actually last longer when left in the water, but, many of us insist on keeping our boats on the trailer at home and our impellers dry out, embrittle, age, and die, risking an overheat.

 

[jhester]

Re: #1 Piston gone what next?

I rebuilt the waterpump and replaced the impeller back about 3 or 4 months ago. I have a good stream of water comming out. I am wondering though after reading some of the other posts about adding a tell-tale if the volume of water comming out is to much, what i mean is that the hose comming out of the top of the water jacket is about 1/4" and from what i have seen the others like OMC are much smaller letting them build a little pressure inside. I know the water system only has about 5psi but I was thinking about moving the outlet to the cover so I can see it while I am driving and adding a smaller outlet hole to increase the pressure just a little.

 

[AMD Rules]

Re: #1 Piston gone what next?

Any pics of the damage? You'd be surprised at how well the cylinders can survive these scenarios, and only require cleanup (muriatic acid) to remove built up aluminum. Follow that with a bottle brush hone, check for dimensional tolerances and you're off to the races with only one new std piston and rings.

Might be wise to move the telltale outlet as high as you can to allow the water cavity to vent. Have you checked you maximum timing advance? Other pistons/cylinders look AOK?

If you do bore oversize, no worries. Mercury makes sure the oversize pistons are weighted so as not to throw off the balance.

 

[jhester]

Re: #1 Piston gone what next?

no pics but I know i'm going to need to replace the piston and bore the cyl. The outlet is on the top of the water jacket where the spark plugs are so I am sure it can't go any higher but I am going to move where it comes so you can see it to the side of the covers and going to make the outlet a little smaller to build a little more pressure to make sure all the air is out.

 

[jhester]

Re: #1 Piston gone what next?

OK got the powerhead at the machine shop and he is going to bore the #1 cylinder and hone the other two. I hope this will keep the motor running for a long time or at least long enough to get a new one. I did find a couple of water passages that had what looked to be old impeller pieces clogged in them so I cleaned everything out and we will see what happens with new rings and piston and complete gasket set.