Mercruiser 3.7 distributor & fuel pump questions

[spareparts]

its normal for those distributors have some end play, pull it out and check it, if it needs it, shim it up, you can probably repair the one you have faster than finding a replacement. Moroso or Mr Gasket have distributor shims for Delco distributors. I've taken old distributors apart to get the shims. Does it have an aftermarket electronic conversion, if it does, you are supposed to shorten the rotor on some of them. Fuel pumps are available form Sierra and Carter, look at ebasicpower.com for parts

[gw204f]

Thanks. I'll check those shims out. No electronic ignition on this one. It's still running the points setup.

On a side note, which oil would you recommend for this old thing? Merc's 25W-40, or something like Shell Rotella T1 SAE30 or 40?

[spareparts]

Rotella SAE 40. The oil pressure will drop a bunch when its hot, those engines don't use cam bearings, the cam floats in the aluminum block, once it gets hot, the clearances open up

[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.