1981 Mercury 70hp 2 stroke blown rings?

[ross_larter]

Don't think this is good news.<br />Did an compression test and here is the reading:<br /><br />top: 25 psi<br />middle: 135 psi<br />bottom: 135 psi<br /><br />If i pore oil in the top cyl the psi goes up to 70 PSI then slowly drops back down.<br /><br />engine number:925314*<br /><br />My question is can I pull the power head and replace these rings without having to take apart the whole engine? If I chose to do it myself what are the things to watch out for and how long should it take or should i just take it to a garage and pay the hugh bill. (I do like working on the engine but I think this may be a too big a job for me.)<br /><br />Cheers<br />Ross

 

[ross_larter]

Re: 1981 Mercury 70hp 2 stroke blown rings?

More info:<br />Just before the problems started the choke got stuck open on the carb, would this cause the cylinder to break the ring or just make it stick.<br /><br />How can i tell if the ring is just stuck or broken? If i look in the spark plug hole there seems to be some carbon build up on top of the piston but that us all i can see.

 

[mellowyellow]

Re: 1981 Mercury 70hp 2 stroke blown rings?

you can try to decarb it first, but sounds like<br />a rebuild is in order. 25psi is way low!<br />good luck,<br />M.Y.

 

[ross_larter]

Re: 1981 Mercury 70hp 2 stroke blown rings?

How long do you think it would take to strip it down and rebuild. and is it worth it should i just start looking for an other engine.

 

[ledgefinder]

Re: 1981 Mercury 70hp 2 stroke blown rings?

I'd agree 25 is real low, and try a decarb. Can even tilt engine horizontal, put #1 at TDC, and put the decarb right through the spark plug hole to give it the best chance of working. <br /><br />Did you mean choke stuck closed, or open? You were saying the boat went 30 mph, then 5mph, then 30, then 5, etc.. That sounds like something other than a broken ring/blown cylinder, or maybe a combination of things.<br /><br />The head doesn't come off on that motor, so you've got to remove the crank to get at the cylinders - pretty much take the motor all apart.

 

[ross_larter]

Re: 1981 Mercury 70hp 2 stroke blown rings?

It think the original problem was the carbs:<br /><br />The guy who looked at the carbs said the choke was stuck, not sure open or closed, it was using a whole lot of fuel, I think because of the carb problem the engine ran too lean and caused the rings to fail, which is going to cost me a whole lot of money.

 

[Chinewalker]

Re: 1981 Mercury 70hp 2 stroke blown rings?

Hi Ross,<br /> A 100+ psi differential is generally a sign of a catastrophic failure, whether it's a broken ring or burned piston. If you're lucky, it didn't take the bore with it and you'll just need to hone the bores, install one new piston and three sets of new rings, plus whatever gaskets and seals necessary. If the compression was only off 20-30% or so, then I'd say try the decarbon agent, but I'm pretty sure you're motor is beyond the "quick fix" stage.<br /> Go through both carbs, check the timing and replace the waterpump while you're at it. Whenever possible, it's good practice to establish the cause of the failure so it doesn't recur...<br />- Scott

 

[ross_larter]

Re: 1981 Mercury 70hp 2 stroke blown rings?

Spoke to a couple of boat yards near me and they are saying it would be better just to replace the whole powerhead with an 2nd hand one. I have no attachment to my old one but if seems a bit drastic for a blown ring, but they say the time it would take them to rebuild it would cost more than swapping in an other powerhead, the saying "better the devil you know" springs to mind, but they may be right i just don't want the new one to fail.

 

[jim dozier]

Re: 1981 Mercury 70hp 2 stroke blown rings?

That's a pretty hefty pressure differential as others have pointed out. But from what you describe the choke was stuck on (which would be choke plate closed) which is consistent with the high fuel consumption mentioned and is a "rich" condition not a lean one. If it were the opposite it might have made it hard to start but would not hurt the engine as you normally run with the choke off (plate open). An excessively rich condition might have really carboned it up, fouling the plug and potentially sticking the rings. You don't have anything to lose by trying a decarb procedure, the best case scenario is that the compression comes up some and you can still use it or it doesn't and you're back to considering a rebuild or a new powerhead.

 

[mellowyellow]

Re: 1981 Mercury 70hp 2 stroke blown rings?

also, take a look at reeds, poss. cause of surging<br />and you may have sucked one into a cylinder.<br />I would check this before you spend any $ or time<br />on the decarb.<br />good luck,<br />M.Y.

 

[ross_larter]

Re: 1981 Mercury 70hp 2 stroke blown rings?

It's a bit hard on these engines to get to the reeds as they are round the crank, i am going to pull apart the powerhead tommorrow, I am hoping there is just some worn rings, but that is being very optimistic.

 

[ross_larter]

Re: 1981 Mercury 70hp 2 stroke blown rings?

Thanks for all the help. BUT<br />I pulled the head the top piston/bore was shot, it was an old engine anyway so I have upgraded to a nice mariner<br />cheers<br />ross