Mercruiser 3.7 distributor & fuel pump questions

[gw204f]

Thanks again. Mind if I run something else by you?

Checked compression this evening. Cylinders 1 thru 3 were 150 give or take a few. Cylinder 4 only pumped up to 42. I checked it like 6 different time with 2 different gauges.

So, I added a tsp of motor oil to the cyl, rechecked and there was no change. This was the first wet compression test I had ever attempted so I wasn't sure how much oil to use. Been doing some reading since and I don't think I used enough.

Anyway, I ran it again for probably 10 minutes and let it get good and hot. 1, 2 and 3 each came up about 5 psi. 4 came up 50 to 92.

Still not quite sure what all this means. Oil doesn't appear to be contaminated w/ coolant. I did note what looked like some rust on the number 4 plug though.

The longer I let it run, the smoother it got. She definitely loses power when you pull the plug wire to that cyl, so it IS contributing.

Took a few videos of it running.





The oil pressure guage was jumping between 40 and 60.

Any thoughts?

Thanks again for your help.

[step up here]

Where did you get that wide 3.7 and skinny outdrive ?
was that a cell phone camera?

[spareparts]

No improvement from the oil indicates the rings are OK, you've got valve seat issues. You've probably got a leaking riser gasket allowing coolant to leak into cylinder # 4. Look at the gasket between the riser and the manifold, one side should be scalloped between the bolts, the other side should be straight, if its scallop on both sides, you have the old style "bad "gasket. Figure a head job, Not too difficult job, cylinder head will have to go to the machine shop

[reelapeelin]

Originally Posted by step up here

Where did you get that wide 3.7 and skinny outdrive ?
was that a cell phone camera?

Click on the wide 3.7...

[gw204f]

Took those vids w/ my iphone.

Thanks for the info spareparts!

I am going to try and take another set of dry/wet compression readings on cyl 4 tonight simply because I'm not convinced I did my first wet test properly. This time I'll use a tablespoon of oil and give it a little more time to flow down into the rings.

I'll take a look at the riser gasket as well. Sounds like it's a common issue. If that does turn out to be bad, could I just replace it for now and wait until the offseason to have the valves redone?

Thanks again.

[spareparts]

better off to fix it now than wait, its not that hard of a job, biggest wait will be with the machine shop

[gw204f]

The good news... It looks like I don't have the old syle "bad" riser gasket...

Port side



Starboard side




The bad news...

I redid my compression tests on cylinder 4. One cold and one cold/wet. The plain old cold test blew 92. After that, I added about 1 tbsp of oil, let it sit for a few minutes so the oil could work it's way into the rings and ran the test again. 106. Looks like my compression problem isn't in the top end after all. :-( That being said, it was encouraging to see the cold compression at 92 instead of 42 like the first time...

Prior to buying the boat, the seller did tell me that he sucked up some seaweed and I believe overheated the motor. His insurance paid for a bunch of stuff to be replaced, but he didn't recall what. I didn't know how sensitive this 470 was to overheating at the time, so I didn't think much of it. I am going to try and talk to him tonight to get some more information about what was done.

Sigh...

[spareparts]

you got valve seat issues, go ahead and pull the head. Look real good at the riser to make sure its not burnt out and leaking water back into the manifold

[gw204f]

Thanks.

For the distributor shims, would these be correct?

http://www.summitracing.com/parts/MOR-26140/


I've also found three different part numbers for the riser gasket on Merc's parts express. 18272 and 97542, which is superceded by 856705. Do you know which one is correct?

I am assuming it's 18272 as it has only has one scalloped side.

18272


97542/856705



Thanks for helping me figure this stuff out!