'71 Mercruiser inline 4 - loss of power and surging

[nibs75]

Okay, so I am fairly new to boating. Purchased a 1971 Cobalt with inline 4 Merc motor for 1st boat earlier this year. The boat is in good shape for its age. The previous owner primarily used it for fishing, while I use it to have fun with the family. We have had the boat out several times and it runs ok. When I got it there was some water dripping from what appears to be a head gasket leak. No water in oil, and still pulled hard (for an inline 4) and topped out around 30mph with my family and I in it. I had a shop look and was told it's no "big" deal as the water is not in the cylinders or oil, and since I pull lake water for cooling I am not going to run out. Now I know having a head gasket issue is not ideal and can impact power. That being said, and after a quite long winded intro, here is my problem.

Last weekend on the lake we had been pulling tubes, etc. having a great time. The next day I was out running and came out of a corner and felt like I needed to speed up - only I was at full throttle. Now with only me I can only go about 20mph... considerably less pulling power felt, and at any throttle position other then WOT I get this kind of surging. The tach jumps and everything. Almost like I was letting on and off of the throttle.

Looking at the motor, it does look like there may be more water dripping then before, but still nothing in the oil. I could see that I could have blown out more head gasket, and could imagine that may be what is making the boat have less power... BUT, I can't not relate the surging to the head gasket.. Could there be any relation?

The surging makes me think of fuel starvation, or a possible intake leak. Anyone have any ideas or helpful pointers for a boating noob?

I plan on compression testing the cylinders today. If all are the same, I'm guessing the head gasket has not blown into a cylinder. (Is that a valid assumption) I hate to tear into the motor in peak usage season, but if it's broke I gotta fix it!

Sorry for the GIANT post, but I need someone with more inline 4 experience to help point me in the right direction or point out things I have not considered.

THANKS!

 

[oops!]

Re: '71 Mercruiser inline 4 - loss of power and surging

check your points......they are nasty little rats that can cause a multitude of whammys

 

[oops!]

Re: '71 Mercruiser inline 4 - loss of power and surging

and welcome to i boats

 

[arrkerr]

Re: '71 Mercruiser inline 4 - loss of power and surging

Sounds to me like it is missing, for whatever reason. I woulnd't think a head gasket though - you'd have all sorts of issues at that point. Any water shoot ouf the cylinders if you yank the plugs and crank it?

I agree with Opps!, check the ignition system - cap, points, wires, etc. Or better yet, just replace them all .

 

[nibs75]

Re: '71 Mercruiser inline 4 - loss of power and surging

Thanks!

I've checked the compression in each cylinder, and all are between 127-133.. not horrible, and not a problem.

I'll check the ignitions system after lunch.
Thanks!

 

[Bondo]

Re: '71 Mercruiser inline 4 - loss of power and surging

The surging makes me think of fuel starvation, or a possible intake leak. Anyone have any ideas or helpful pointers for a boating noob?

Ayuh,... I think you're right,... Change the Fuel Filters...

 

[bigskiohio]

Re: '71 Mercruiser inline 4 - loss of power and surging

do you mean manifold gasket on the head?

 

[nibs75]

Re: '71 Mercruiser inline 4 - loss of power and surging

There is water dripping from the port side of the engine. I can not see exactly where it is coming from, but it drips down from under the manifold.

 

[bigskiohio]

Re: '71 Mercruiser inline 4 - loss of power and surging

could be rusted out manifold ,leaky plug, a head gasket would cause big problems with runabilty as coolant would be getting in cylinders. there is aplug under your manifold mine drips as it is alittle rusty and time for new manifold soon.makke sure all bolts are tight. your coupler could be on its way out.my coupler went after tubing. had to pull motor fairly easy job and if you have help fairly fast to do. one day job. i dont know if it is you bad head gasket but that is not a big job unless your bolts are all rusted on manifold.
ther is no water at exhaust and intake ports except for what runs thru the manifold and of course your inner block area which is sealed whith head gasket.the guy i bought my boat told me to always carry extra points in boat,
Check your fuel filter ,pump. pull your plugs should be tan color .

 

[nibs75]

Re: '71 Mercruiser inline 4 - loss of power and surging

I pulled the plugs when compression testing the cylinders. They looked ok. Right before I bought the boat the previous owner had a new engine coupler, and impeller installed at one of the local shops.

So if the water is leaking from the manifold, and the leak is getting worse, would that cause the revving of the engine?

 

[bigskiohio]

Re: '71 Mercruiser inline 4 - loss of power and surging

spun prop hub

 

[nibs75]

Re: '71 Mercruiser inline 4 - loss of power and surging

Better yet... pulled the intake manifold and there is a nice 4-5" long horizontal crack just under the intake manifold. That appears to be where the water is coming from. There is a temp gauge, and it never showed an over temp situation. Now the previous owner, I've had this about 5 weeks, could've over heated it and cracked it. I suppose with me running it a lot harder it has gotten worse..?? At any rate not sure how to, or if I should proceed. I think I could see about having it welded.. does that work? is it worth it..? OR just trying to take it and trade it in... hoping to get next to nothing for it.

Any advice?

 

[Bondo]

Re: '71 Mercruiser inline 4 - loss of power and surging

and there is a nice 4-5" long horizontal crack just under the intake manifold. That appears to be where the water is coming from.

Ayuh,... That's because it's been Frozen, complete with Water in the cooling jacket...
Overheating will Not cause that kinda cracking...
Best bet, with it Not dumping water into the oil, is to epoxy it over,..
Forget the Welding,...

I've seen epoxied blocks last for Years,+ Years....

 

[nibs75]

Re: '71 Mercruiser inline 4 - loss of power and surging

thanks for the reply!

I've been reading about epoxying in lots of threads.. so I've already ground the area down with my dremel and cleaned it. I'm going to the store now to get some jb, or other epoxy. We'll see. For no more then it costs, it's worth a try.

I bought the boat as out first and hoped to get 1-2 relatively trouble free years.. if I this holds for the rest of this season, I'll be satisfied and will buy something newer in the offseason.

 

[nibs75]

Re: '71 Mercruiser inline 4 - loss of power and surging

As far as the intake manifold gasket goes, can I get that at an automotive store? If so, do I just ask for the intake manifold gasket for an early 70s inline 4 GM?

 

[bigskiohio]

Re: '71 Mercruiser inline 4 - loss of power and surging

the crack would not cause surging the leak is no big deal, mine leaks at the plug sombody tried to get it out and cracked but mine runs great.

 

[nibs75]

Re: '71 Mercruiser inline 4 - loss of power and surging

Yeah, I kinda figured that.. but it's a long crack and is leaking a lot of water at this point. I've got it all apart, I may as well epoxy it up. Then put it all back together.

I only see 1 inline fuel filter, so I'll change that too. Then I guess I'll hook up the muffs and see how it runs. If it seems ok, a lake test will be in order.

Just need to get the gasket and get it back together.

 

[nibs75]

Re: '71 Mercruiser inline 4 - loss of power and surging

So I got everything put back together today. Runs fine on the muffs. No water seepage, so far anyway. Idles fine and very smooth. Seemed like there was a little more noise then normal from the lower unit. I checked the prop, which is in bad shape anyway. It seemed like it had a bit of play, in and out.. I took it off and looked at everything. Visually everything looks fine. Put it back together.

As I've said, this is my first boat. I don't know if maybe I have a spun prop..? I think I'll mark it in the morning and go get a new one anyway, then head to the lake to try it out. Could a spun prop have caused the surging and loss of power I described?