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Tired 502 Options

N

Nick616

Active member
Picked up a 253 Convincor with 502 MPI this spring, was slow coming out of the hole on the test drive, but figured it was just propped wrong.

On our first run at home, I blew and oil line, had no oil pressure and shut off the engine - got that fixed and have not used any oil since - oil pressure is good.

I replaced the rotor and distrubutor cap ( they were nasty ), test ran it on the hose it started great and idled ok, but ran rough - the engine vibrates pretty badly - from about 1200 - 2000 rpm.

I had a local shop check it out and the compression on the port cylinders ( 1-3-5-7 ) looked good - 131 - 142 psi. Starboard were another matter #2 - 112psi;#4 - 85 psi; #6 - 110 psi; #8 - 128 psi. They tell me the engine is "tired" - I should either just run it and enjoy it as it is ( it does run mostly ok - so we are enjoying the boat ) or look at a rebuild. They do not have the tools to connect to the engine to check for any codes - so I don't know if they really know what they are doing and if their advise is good or not . . .

It had a 23P prop, I got a 21P and it now comes up on plane good, but top end RPM did not increase any - still around 4400 RPM.

We've had it out several times since then, I think the rough running / vibration is till there sometimes when on the water - but its hard to tell with the engine cover down . .

So I'm looking for advice on the best "process" to get the engine checked and rebuilt / replaced. I plan to keep the boat for at least a couple of years and want a solid reliable engine - not looking for extra speed, but wouldn't mind a little :) I boat on Lake Michigan and really do not want to take a chance of the motor dying 2 miles offshore in rough water . . .

Right now I'm leaning towards just running it this season and getting something done over winter ( maybe I will get a break on cost during the off-season . . .)

Any advice would be appreciated.
 
Could the vibration be the engine slightly out of alignment causing a drive line vibration?
 
The motor should easily Rev to 500 rpm with a 21 or the 24 pitch props. Definately have something holding it back. Any idea of the hours on the motor. I'd enjoy it for the season with those compression numbers and good oil pressure.
 
I had the alignment checked it was ok - I did run it on the hose with the outdrive up - don't know if that caused the vibration or not.

Bottom has around 500 hours, upper was recently rebuilt.

I just don't have a lot of faith in the mechanic that I had look at the engine, don't know if it's worth taking it some place else for a second opinion, or if the first mechanic really knows what he is talking about . . .

Could bad injectors be affecting the compression numbers?

I don't want to spend $ on band aids if it needs major surgery ;-)
 
The starboard head could have an issue since all of those holes have low numbers. Could be warped after the "rebuild". Those cylinders are making less HP. I would imagine this is the cause of low RPM and roughness upon takeoff.
 
Id get a second opinion on it.. 500 hours isn't hatefull, odd for an entire bank to be down on compression. Whens last time risers and manifolds were replaced ?? and were they all done at the same time ?? Sadly enough I have seen people only do one side.. Might be getting some water in the head, enough from keeping the valves from sealing up..

John
 
Did they pull the drive off,, The only way to check alignment is to pull the drive. bad u-joints or gimble bearing will make it vibrate.. That 502 should spin a 24 no problem.. If you got good oil psi when warm the bearings are ok. Wore out bearings cause low oil psi.. If your rings are shot you will be burning oil.. I would let a good marine wrench take a look and advise.. You might just need a head job.. You go jacking with the timming an not sure what your doing you will blow your engine
 
Thanks for all the advise - will have to find a good marine ENGINE mechanic in the area. Place I took it to originally has a good reputation for boat repair / maintenance / storage - but I'm less than impressed with their engine abilities.
 
Got it scheduled into a well recommended marine repair shop - in 3-1/2 weeks:shakehead: . . . will just have to enjoy it as it is for now, keep an eye on things and not push it too hard :( .
 
Check plugs on the bank with low compression.. if they look rusted then water is getting in there some how.. Just food for thought and piece of mind untill you can get it to the shop..


John
 
So my impatience and anxiety got the best of me. Can't find a Merc mechanic available for a month so got a leakage tester to check things out. Is there a good way to rotate the motor (still in the boat?). I assume I should pull all the plugs to avoid working against compression. Any other tips tricks advice?

Also hoping to borrow a camera to take a peek inside the worse cylinder. .

Plan to pull all the injectors and get them cleaned and flow checked. Hopefully will get to the point that I'm comfortable running it this season even if it needs major work over the winter.
 
Pull the spark plugs and use a 3/4" socket with large ratchet/breaker bar on the center balancer nut on front of crank. Turn in clockwise rotation. Will spin fairly easily.

I will have my spare set of Keihin injectors already waiting for you at Smitty's shop to fill in for any faulty ones you may have.
 
Got the leakage test done today - yet another learning experience - 2-6-8 were around 60%, 4 was around 65%. Didn't check all 4 of the 1-3-5-7 bank, ( got tired of hanging upside down . .) only checked 3 - it was around 40%.
All were leaking past the rings - air was coming out the breather tube.

Plugs on 2-6-8 all looked "clean and damp blue" - probably a more technical way to describe them - 4 looked dry and almost rusty - I'm hoping to find that #4 injector was having serious issues - will feel much better if that is the case.

All 8 injectors are out and off for a good cleaning and flow check.

Now just have to wait to get it back together and see how it runs.

Will see if I can get some pics of the plugs and post them later.
 
Spark Plug Pics

SparkPlugBoth_zpsj8yzqpkr.jpg

Plug on left is from bad cylinder #4, right is better cylinder #2

SparkPlug4_zps9eto1zcj.jpg

Bad Cylinder #4 Plug

SparkPlug2_zpsvv2jxvdv.jpg

Better Cylinder #2 Plug
 
I'm far from an engine expert but with that much leak down I would think there would be oily sparkplugs. Time to tear it down before things get real bad.
 
I'm hoping to get through the summer and decide what to do with it this winter - maybe time for more power :).
 
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