76 Mercury 1500 low compression

[ssanderson69]

I have just picked up a 76 16.5' ski boat with a 76 or 77 mercury 1500. it had not been run in 3 years. the fuel was drained from the tank and I was told it was run dry before it was put away. I foamed the cylinders with sea-foam and turned the engine over by hand before starting it. It will not idle at this time but does seam to run good once you get it past a flat spot mid throttle. I did a compression test and all cylinders 1,2,3,4 and 6 are between 110 and 115 but number 5 cylinder is way down at about 60. Any ideas and were do I go from here? is this motor worth a rebuild? what should the compression be? Any help would be great thanks: SSA

 

[emckelvy]

Re: 76 Mercury 1500 low compression

At least #5 still has some compression. You might find that the cylinder isn't scored yet and be able to replace the piston and rings without having to bore the cyl to fit an oversized piston.

Definitely a good time to tear it down, rather than risking further damage by continuing to run it.

These are great old motors with excellent performance on smaller hulls, you should easily get 50+ with a healthy 150 on your ski boat.

Compression readings are gonna vary with the gage used, cranking speed, and technique for checking. But what you're really looking for is even readings between the cylinders. On an Inline I start worrying if the compression values vary more than 5 psi. Sounds like your other cyl's may not be hurt.

Powerhead removal and teardown is pretty straightforward on these, you'll need just normal hand tools and a flywheel puller. This one on eBay seems like a decent deal:

http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=310233894291

Here's one with a lifting ring, which you may find handy if you have a hoist, it'll screw into the flywheel and you can easily lift the powerhead:

http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=370415944058

At any rate, this is a true Classic design and well worth fixing, as long as it's not a nasty ol' Salty Dog. Parts are readily available and there's a ton of Inline info on this site to help. Recommend you pick up a good service manual, you're gonna need the details and specs within.

HTH & Happy Wrenching..........ed

 

[ssanderson69]

Re: 76 Mercury 1500 low compression

Thanks for the info. Would you re-ring all cylinders while you have it apart?
this makes sense to me. The boat is in great shape the original gel coat is still in good shape and I like the classics. I have read that soaking the cylinder with the same oil used in the fuel sometimes brings back the rings is this true or is it a waste of time?

Thanks:

 

[emckelvy]

Re: 76 Mercury 1500 low compression

Thanks for the info. Would you re-ring all cylinders while you have it apart?
this makes sense to me. The boat is in great shape the original gel coat is still in good shape and I like the classics. I have read that soaking the cylinder with the same oil used in the fuel sometimes brings back the rings is this true or is it a waste of time?

Thanks:

Yes, renew the rings on all pistons while you're in there. Plan on replacing the crankshaft seals, (1) at the top and (2) at the bottom. The lower crankshaft ball bearing is cheap and you should plan on replacing it as well. The top bearing is a double-row of balls and more durable but of course should be checked out too. Any auto parts store or bearing house will have or can get these commonly used bearings (6206 for the bottom and 5206 for the top).

If you pick up a wedge-type bearing puller it'll be easy to remove the bearings which are pressed onto the crankshaft. To reinstall heat the ball bearing up to 180 deg F in a toster oven then quickly slide down the shaft. It'll go on slick as a hot knife thru butter.

You'll need of course various gaskets, soft goods, and sealer to complete the job. I use Loctite/Permatex 518 anaerobic sealer on the crankcase halves, a thin coating of 518 and new cork sealing strips will keep the crankcase leak-free.

If you can get the exhaust manifold cover bolts out without breaking them, it's always a good idea to renew the cover gaskets. A good time to check the baffle for perforation. But don't open up the inner exhaust cover (the one around the exhaust ports) unless there are signs of water leakage or damage to the cover. The bolts are sealed in with special sealer and it's a Bugger to get apart. If it's not leaking it's generally best to leave it alone.

If you can get the bolts out of the back cover (cylinder block water jacket cover) over the spark plugs, it's an easy gasket replacement and will save you grief later on if the original gasket were to start weeping around the spark plugs (quite common).

And it's very unlikely you'll bring a cyl with 60 psi compression back to normal. Guess it can't hurt to try a Seafoam decarbonizing treatment, you're only out the cost of the can. But don't get your hopes up, I've never seen one that low come back.

Here's good info on decarbonizing:

http://forums.iboats.com/showthread.php?t=158076

BTW if you want to look up parts, here's a few sites that have diagrams to go with:

www.boats.net www.crowleymarine.com www.mercruiserparts.com

HTH........ed

 

[Chris1956]

Re: 76 Mercury 1500 low compression

I would recommend you remove the transfer cover and have a look at the offending cylinder. Perhaps the rings are just stuck, and can be loosened with some penetrating oil.

Look for score marks on the cylinder wall. This damage would cause the low compression, however, it is unusual for #5 cylinder (on an IL6) to get damaged first.