1975 500 Mercury Outboard Low Compression and help for grade 12 project

[hxcfuk]

i believe its a 1975 (serial# 7061413)
its 500 Mercury Outboard 50hp

i am a grade 12 using it as a small engines project.
so far i know that it has low compression, 50 in all cylinders
i am wondering what may be the cause of this, rings, pistons, sleeves?
any help would be appreciated

i am also wondering where id be able to get a service manual for cheap or even free. (scans, diagrams, etc., doesn't matter i just don't want too spend a lot or kill the engine completely)


thanks very much, and ANY help will be appreciated
Mike Gillette

 

[Fuzzytbay]

Re: 1975 500 Mercury Outboard Low Compression and help for grade 12 project

The low compression could be caused by a few things.
Stuck rings, wear and tear (in other words, its been used a lot) scored cylinder walls from being abused etc etc.
Best bet is to pull the plugs, and look inside the cylinder. Check for visable scoring.
For a cure for stuck rings, try spraying some deep creap into the cylinders, let it sit over night, then give it a few pulls over. Re spray. Do the sitting again, pull again, and then re test.
If its more than stuck rings, (I suspect it is, since the cylinders are similar) then your looking at a ring job, and cylinder honing. There are no sleves.
For a manual, you might try the libarary, or E-bay.
This might be the right link for exploded views, if not just go the main page and redo the search.
http://www.mercruiserparts.com/selectDocs.asp?doc_nbr=71732

 

[hxcfuk]

Re: 1975 500 Mercury Outboard Low Compression and help for grade 12 project

thanks for the response.

im gonna get the motor to school either tomorrow or Tuesday and ill start looking at what you said. hopefully theres not to much damage. and thanks for the link, ill keep ya posted. hopefully it works out

mike gillette

 

[emckelvy]

Re: 1975 500 Mercury Outboard Low Compression and help for grade 12 project

Mike, you're in luck 'cause the 44 c.i. Four is one of the longest-production outboards ever made, the same basic design goes all the way back to the 50's. The Little Four is a good runner and very popular as a fishing motor, as it idles very nicely and is pretty thrifty on fuel for such a Stone Age motor.

Most common cause of low compression on these is carbon buildup on the rings and subsequent ring sticking. This can lead to damaged pistons and cylinder walls. Overheating is also Bad Juju, you'd typically see the worst compression reading in #1 cyl since it'll run the hottest and lose water circulation first.

Hopefully yours is just a stuck-ring kinda thing, if the cylinders are not scored you can use an abrasive ball-type "glaze-buster" to clean up the bores & re-ring, replace crankshaft seals, gaskets, etc.

You'll also want to check the end bearings for wear and take a look at the conn rod 'big end' roller bearings & condition of the crankshaft. And definitely install a new impeller. Good insurance against a future meltdown!

The crankshaft bearings are readily available at your local auto parts store or bearing specialty house (or eBay for that matter), at a lot cheaper than Mercury prices.

You can look up parts/prices at www.iboats.com or try www.boatfix.com

Check out this site for parts diagrams, you'll need your serial # to get the correct breakdown:

http://www.mercruiserparts.com/SelectModelType3.asp?class_id=2

Here's some great wiring diagram info from the www.maxrules.com site:

http://www.maxrules.com/wireindex.html

HTH & G'luck with the project..........ed

p.s. there are (2) rectangular-shaped intake port covers on the Stbd side of the powerhead, these are held on with 1/4-20 screws/bolts. If you remove these you can see directly into the cylinders and check out the intake side of the piston/rings. If you position that cylinder's piston to the bottom of its stroke, you'll be able to see the exhaust ports and get a good idea if there's any scoring.

p.s.s. if your school's shop happens to have a bore-scope, that'd be Sweet! Just get a look thru the spark plug hole.

 

[JB]

Re: 1975 500 Mercury Outboard Low Compression and help for grade 12 project

First off, Mike, 50psi is not necessarily low compression if all cylinders are that close. Tell us more about how you got those numbers and what the conditions were.

http://forums.iboats.com/showthread.php?t=313884

Inspect the cylinders as instructed above and let us know what you find.

 

[hxcfuk]

Re: 1975 500 Mercury Outboard Low Compression and help for grade 12 project

k so i finally did my own compression test today and it came out way better then i anticipated


cylinder #1 112psi
cylinder #2 115psi
cylinder #3 117psi
cylinder #4 105psi


i was just wondering what the compression range is supposed too be. i am gonna check the impeller and the water pump later this week then ima try too start it.

 

[Fuzzytbay]

Re: 1975 500 Mercury Outboard Low Compression and help for grade 12 project

While I'd like to see them all above 110, the range is "ok", its not best case, but I'd venture its good enough to get it running. There is a thread here, in the FAQ's, about whats, and hows of waking an outboard. I'd recomend you read it, it might help you take the right steps in the right order.

 

[emckelvy]

Re: 1975 500 Mercury Outboard Low Compression and help for grade 12 project

105 is a bit low, but you should be able to get 'er running, then Seafoam the heck out of it. If that cyl is still quite a bit low, you can either tear it down or run for a while and monitor compression to see if there's a declining trend.

Usually low comp on #4 would make you suspicious of leaking lower crankshaft seals (and resultant water intrusion causing piston/ring sealing issues), so be on the alert for any water droplets or rust on #4.

HTH & Keep us posted..........ed