'76 Mercruiser Inline 6 - Broken Valve Spring

[cnr2003]

So the last time I had the boat out last year, the engine started missing instantly on one cylinder. So I finally did a compression check and found no compression on cylinder 3. Pulled the valve cover and found a broken valve spring. Is this something I can easily change myself? Are there any special tools required? I have a complete spare head from a '75 model that I can take a spring from if needed.

Also, when I did the compression test I wasn't able to warm up the engine (no gas tank) so I did it with the engine cold and these are the numbers I came up with. Note that I didn't put any oil into the cylinders, these were just the initial numbers. They seem low, but are they too low?

1 - 91
2 - 98
3 - 0
4 - 96
5 - 94
6 - 86

 

[Bondo]

Re: '76 Mercruiser Inline 6 - Broken Valve Spring

They seem low, but are they too low?

Ayuh,... That motor is Beat,..... Dead....

 

[fishrdan]

Re: '76 Mercruiser Inline 6 - Broken Valve Spring

You can change the valve spring with the head on the engine (though I have never done it) by connecting an air compressor to the spark plug hole, this holds the valve against the valve seat, and then replace the spring. There are special tools for attaching the air compressor to the spark plug hole and compressing the valve spring.

Those comp #'s look low to me. My 30+ year old 140HP had 110-120# in each cylinder, running the test on a cold engine.

 

[ziggy]

Re: '76 Mercruiser Inline 6 - Broken Valve Spring

i'd agree cnr, them numbers don't look to good.... 100 psi min. range of no more than 20 psi i believe. least your consistent other than #3...
here's the link to the sb i was talking about..

http://www.boatfix.com/merc/Bullet/97/97_25.pdf

short of the broke valve spring, which i assume is on #3...
add some oil and retest. see if compression comes up. if so, rings.
air test per the sb?

worse case scenario. i've got a Aframe i think will work to pull yer engine if that's where it leads too.. tests first though i'd think..

 

[cnr2003]

Re: '76 Mercruiser Inline 6 - Broken Valve Spring

Ayuh,... That motor is Beat,..... Dead....

That's what I figured, but she's been dead for years, but I've been trying to squeeze a little more life out of her.

You can change the valve spring with the head on the engine (though I have never done it) by connecting an air compressor to the spark plug hole, this holds the valve against the valve seat, and then replace the spring. There are special tools for attaching the air compressor to the spark plug hole and compressing the valve spring.

Those comp #'s look low to me. My 30+ year old 140HP had 110-120# in each cylinder, running the test on a cold engine.

Know where I can get one of those adapters? Tomorrow I'm going to try a couple squirts of oil and see if the numbers increase. If they don't, then I'm assuming the rings are ok and I'll just have the spare head rebuilt. That would keep me from having to pull the motor to replace anything.

 

[IVAZ]

Re: '76 Mercruiser Inline 6 - Broken Valve Spring

I have a complete spare head from a '75 model that I can take a spring from if needed.

Is that head in good shape? If it is you could just swap heads. It might be easier that way.

 

[IVAZ]

Re: '76 Mercruiser Inline 6 - Broken Valve Spring

nevermind. I just re-read your last post and it said you were going to rebuild it.

 

[cnr2003]

Re: '76 Mercruiser Inline 6 - Broken Valve Spring

Ok, so I tried to replace the valve spring tonight, but I'm having a hell of a time. I am holding the valves closed with 90 psi of air pressure in the cylinder and using the valve spring compressing tool, I can't get the valve keepers to budge at all. When I push too hard on the tool, the valve pops open even though the air pressure is at the upper end of the recommended range. I have tried tapping on the top of the valve to break loose the keepers but haven't had any luck. What next?

 

[70Silverline]

Re: '76 Mercruiser Inline 6 - Broken Valve Spring

there is a half moon clip at the top of valve did you take those off?

 

[70Silverline]

Re: '76 Mercruiser Inline 6 - Broken Valve Spring

there is a half moon clip at the top of valve did you take those off?

only reason i know i have a dead 250 cubic inch well still runs but its shot also did you test at WOT? i get around 90 in my 250 ci inline 6 if i dont have WOT then its 146 all the way my motors not shot just my carb lol

 

[cnr2003]

Re: '76 Mercruiser Inline 6 - Broken Valve Spring

only reason i know i have a dead 250 cubic inch well still runs but its shot also did you test at WOT? i get around 90 in my 250 ci inline 6 if i dont have WOT then its 146 all the way my motors not shot just my carb lol

I'm pretty sure it was at WOT. The throttle was all the way down anways.

As far as the keepers, that's where the problem is. The valve spring/keepers are not sliding down the valve stem. They seem to be stuck to the stem. I've smacked that sucker with a hammer, but it won't budge. I'm soaking it with PB Blaster overnight and I'm going to have another go at it tomorrow.

 

[ziggy]

Re: '76 Mercruiser Inline 6 - Broken Valve Spring

don't know if this helps but.

The valve spring/keepers are not sliding down the valve stem. They seem to be stuck to the stem

the keepers (keys) don't slide, the cap slides, the cap pushes against the valve spring shield which pushes on the spring. when you slide the cap on the valve stem far enough. the keys fall out. they have male slots on the back side and mate with the female slots on the vavle stem. the cap keeps them together so they'll never spearate till the cap is slid out of the way.. s/m #2, pp 5A6 fig. 9... i reckon ya've already looked at that though..

 

[cnr2003]

Re: '76 Mercruiser Inline 6 - Broken Valve Spring

Well, soaking them didn't help either. It seems no matter what I do, the spring cap will not slide loose from the valve stem.

I may end up just swapping in the spare head I have and testing that out. That way I can still use it this summer while I get the original head rebuilt.

The good news is after squirting a little oil into the cylinders, the compression numbers did not rise, so it appears the piston rings are ok and the majority of the compression loss is due to the head.

 

[mkast]

Re: '76 Mercruiser Inline 6 - Broken Valve Spring

Your first post was the 18th of last month.
Replacing the head would have been quicker.

 

[cnr2003]

Re: '76 Mercruiser Inline 6 - Broken Valve Spring

Your first post was the 18th of last month.
Replacing the head would have been quicker.

Haha, but in that timespan I only spent around 3 hours working on the head. Part of it was waiting for parts/tools and the rest was dedicated to the rest of the restoration.

 

[mkast]

Re: '76 Mercruiser Inline 6 - Broken Valve Spring

Haha, but in that timespan I only spent around 3 hours working on the head. Part of it was waiting for parts/tools and the rest was dedicated to the rest of the restoration.

Three hours is about the time needed to R&R the head!

 

[dan t.]

Re: '76 Mercruiser Inline 6 - Broken Valve Spring

air up the cylnder, compress the spring,then hit the retainer with a small hammer, it should break loose

 

[bigblocksarefun]

Re: '76 Mercruiser Inline 6 - Broken Valve Spring

One method I heard about years ago (and I have personally used this to good effect to change valve seals on a 440 Chrysler) is to take some clothesline, something pliable but that won't fray easily and leave crap inside the cylinder. Rotate the crank to get that piston about halfway through its travel, then feed as much clothesline as you can through the spark plug hole. Once you get enough bunched in there, put that piston as close to TDC as possible, this will give the valve something to brace against. If the hammer method isn't working, try using a scribe (CAREFULLY) to pry the keepers sideways so they will come off. Good luck!

 

[cnr2003]

Re: '76 Mercruiser Inline 6 - Broken Valve Spring

One method I heard about years ago (and I have personally used this to good effect to change valve seals on a 440 Chrysler) is to take some clothesline, something pliable but that won't fray easily and leave crap inside the cylinder. Rotate the crank to get that piston about halfway through its travel, then feed as much clothesline as you can through the spark plug hole. Once you get enough bunched in there, put that piston as close to TDC as possible, this will give the valve something to brace against. If the hammer method isn't working, try using a scribe (CAREFULLY) to pry the keepers sideways so they will come off. Good luck!

Thanks. I heard that method before as well and I think that will be my next step. Since the valve spring is broken, there isn't any pressure holding the cap/keepers in place. It's just stuck there from not moving for 33 years. I think with the rope holding the valve, I should be able to get enough force on the cap to break it loose. We'll see, it will be a few days before I get the chance to try it.

 

[bigblocksarefun]

Re: '76 Mercruiser Inline 6 - Broken Valve Spring

Just make sure you don't hammer so much and so hard on the valve stem that you mushroom it.