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jrm123180 06-06-2007 07:10 PM

help with my boat please
I need help wih my boat please, you guys are MUCH more helpful than the boat forums.

I have a late 70's merccruiser 120 I/O (basicly a 4cyl chevy engine). When I am in the water under load I can not get over 2000rpms.

Tonight I started to pull the spark plugs while the engine was running to see when happened. When I pulled #1, nothing, ran the same, #2, began to die, #3, began to die also, #4 same as #1, nothing happened.

I checked the compression on all cyls. and #1 @ 70, #2 @ 130, #3 @ 130, #4 @60.

Last year I had trouble but while I was out the last time I advanced the timing while I was out and it took off and got up on a plane. (I got the boat at the end of the season with no stern drive and the boat was free).

What could be the problem and where should I look next? Could it be a valve stuck open (I hope not pistons)?

PLEASE HELP....its boating season.


docvette 06-06-2007 07:15 PM

Doc here, :pimp:

Sounds like a Headgasket to me..

Did it run hot last time you ran it?

Doc :pimp:

Blazin72 06-06-2007 07:17 PM

Do you have a blown head gasket and/or warped head? Did it ever overheat on you? Maybe it overheated when the previous owner had it?

jrm123180 06-06-2007 07:25 PM

I the time before last it ran hot, like 230ish. but the time before that and last night it ran fine temp wise.

The time it was running hot, I was having a tremendous amout of trouble with the carb...kept spitting, hesitating, stalling, hard to start. Had it rebuilt and they said there was dirt in the carb....seemed to be fine after that.

400smb_s10 06-06-2007 07:58 PM

This maybe a shot in the dark! but maybe new plugs and wires? Does it have individual coils per cyl or does it have a dizzy. If so is it the old points style or the newer HEI dizzy. It also might be time for a new cap could be worn out and not getting the proper contact.

Just a shot in the dark as I mentioned

hope it helps chum


jrm123180 06-06-2007 08:01 PM

I did replace the points last year, but they could have gone to crap if all things weren't in check or moister got in there? I also replaced the distributor cap recently, but i bought it last fall and it sat in the boat all winter and it did get wet in the rain over the winter.

I did check and I am getting spark on all four cyls.

RPM 06-06-2007 08:08 PM

Your compression is way off. More than 10% is not good. The 2 end cyls are low. Probably not a gasket. Run a leak down test before you take anything apart. Could just be valves out of adjustment.

400smb_s10 06-06-2007 08:09 PM

Im not a wiz with motors but Im pretty sure thats leaning toward compression I think, please correct me if Im wrong, but it maybe time for a ring job. Maybe that is why it was free. This day of age you dont get nothing for nothing anymore.

But dont get glum yet because Im not 100% sure

If it helps Im glad to help ya. If it dont sorry for getting you confused


jrm123180 06-06-2007 08:13 PM

If it were valves out of adjustment (which is what I am hoping it is) would I hear it or not ness?

curtis73 06-06-2007 09:55 PM

230 is pretty hot for a marine engine, especially considering that the thermostat is probably between 120 and 160. Could be a blown head gasket which caused it to overheat and/or cause rust in the cylinders which has hurt ring seal. Engines that sit for long periods of time (like most boat engines) can have sticky valves. Test for sticky valves with a vacuum gauge. It will show up as wildly changing vacuum numbers if its an intake valve. Pulling the valve cover will also help you diagnose sticky valves.

My guess is that it is a leaking head gasket that has led to other problems. Without the anti-corrosion benefit of anti-freeze, plus the added minerals and things in lake water (or even worse, salt water) can make short work of head gaskets and water passages if not flushed really well before storage.

RPM 06-07-2007 05:48 AM


Originally Posted by jrm123180
If it were valves out of adjustment (which is what I am hoping it is) would I hear it or not ness?

If they are too tight then the valve is not seating all the way. No you probably would not hear it. If the valve was too loose it would tick, but probably would run better than you describe. Run the leak down test, it will point you in the right direction.

boatbob2 06-07-2007 09:33 AM

merc overheating
hi,in the merc manifold,there is a riser with a replacable insert.if rust is around this insert (tube) the engine will overheat.also sounds like a valve,piston or headgasket problem,also you may have to re-pitch the prop,too much prop will not allow the engine to rev.

brian0605 06-07-2007 10:27 AM

you more than likely have a stuck valve.I used to work for a marina in mobile Alabama and we used to see this this time of year. Remove the valve cover (pretty easy on that eng). turn it over by hand and see which valves aren't moving. This is pretty common. Now for the repair, as long as the pistons and valves haven't gotten together and had a party you can use a few different methods to try and free it up. you say it will run right?? Well with the engine running and the breather off of the carb either pour in a quart(in order of my personal preference) 1) marvel mystery oil 2) automatic transmission fluid 3) "Sea foam" fuel additive. Start pouring it in fairly slowly and keep the rpm's around 2000 2500. (It's gonna smoke like a chimney.) When you get about halfway through the bottle start pouring it in faster, The engine may even die. That's O.K. as long as you get most of it down the carb. Now let it sit for a day or two. After a couple of days remove and clean the plugs you might even want to just go ahead and change them. With the plugs out spin the engine over (you may want to lay an old towel or pc of card board over the open spark plug holes to catch the spray of fluid). Now in the past I have just put the new plugs in and cranked it up and run the engine and within a few minutes of running the valves would start to move again and it would smooth out. If it won't free up you can remove the valve cover again and loosen the rocker arm and move it out of the way. You can either use a block of wood or a plastic deadblow hammer and gently tap the stuck valves down further and turn the engine over by hand and work the valves until all are free. reinstall the rockers and valve cover fire it up and see how it does.The reason this usually happens is most people just run some clean water through the foot and engine and pull the tarp over the boat for the winter. Well in a marine engine it is very important to either go out and run the engine for 30 min a month or remove the breather and slowly pour a pint bottle of marvel mystery oil down the carb while it's running and shut it off as soon as all of it is in. Then pull the tarp over it. you might have to remove the plugs at the beginning of the boating season and clean them,but I have found that they will usually crank just fine with out having to pull them. Hope this helps. BTW I have also done this with old cars that have sat up for a very long time too.Marvel Mystery oil ROCKS :thumbup: just ask an old timer!!!! Brian

PatM 06-07-2007 01:26 PM

One other thought . . . . .
Since you have SOME compression in 1 and 4, I don't think it's a stuck valve. Maybe a warped one, but I don't think stuck. It's fairly easy to separate the diagnosis of a ring problem from a valve problem. I suggest pulling the (4) plugs, and applying a little motor oil (or MMO) through the plug hole onto the piston top and cylinder wall. Then, rerun your compression test. If the compression values come pretty even, the oil has sealed up the rings and they are likely the problem. If not, I'd be looking for two or more warped valves (or. . . maybe stuck) at the outer cylinders. If the head gasket is blown, I'd expect the compression to be lower than 60-70, so my guess is either you have worn rings that aren't sealing, or warped valves. If the overheating has happened before, the valves could very well be warpped from that event.

Just one man's thoughts on the matter . . . . .

jrm123180 06-07-2007 06:53 PM

well i removed the valve cover tonight and it appears that a few of the top of the bolts to the rockers have some rust on them....I think this motor sat uncovered at one time. I'm could be wrong, but when I removed the carb to have it rebuilt, there was rust at the base of the carb and I didn't think there should be any was too late to start it tonight but I'm going to try tomrow.

I'll keep u posted.

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