Carb 502 mercruser low compression
 

Hi I have a 1986 welcraft novaiii with a 1994 502 last time out I was at wot and noticed I was loosing rpm when I came off plane I noticed the boat had a slight shake to it and didn't have the power to get back up on plane idled back to the trailer and did a compression test the next day low compression on front cyl on the left side and front two on the right between 60 and 90 psi on them noticed some rust/ coolant on a few plugs (has full closed loop cooling system) and was slightly low on coolant at the time but when I finally looked at the temp when I was back at idle it was not hot but for some reason the oil pressure was like 45 at idle normally its 25 I know I know need to do a leak down test and will in a week or so when I get home from work and up date, my biggest question right now is what could cause the high oil pressure reading and what are the chances of loosing compression on both sides of the engine at the same time or is it likely that the side with one low has been low for a while and didn't notice till I lost the 2 on the other side?

possibly head gaskets if it got hot, check the dip stick for water / coolant

Not to derail the thread but my first boat was a Nova 23xl with a single small block. I beat that thing like a redheaded stepchild. Loved it.

this what im hoping but think I would be awfully lucky if both fails at the same time I pulled the plugs out of my risers and they were pretty pluged up I would imagine that would cause high exhaust Temps. But what I find most strange is the 45psi of oil pressure on a hot engine at idle right after this happened

If water mixes with the oil it will be very thick

or really interesting if any thing figured it would be thiner

Yea, "water and oil don"t mix" but if it is forded through the oil pump and rest of the system it will turn into a thick gue.

So finally got a little further in to the engine and turns out it's actually a mark iv 454 upon removing the valve cover I first noticed one rocker popped off on cyl 1, than performed leak down test and lots of leakage throug most exhaust valves and some intake strangely enough cyl 1 with poped rocker performed best in the leak down test and cyl 5 was leaking to the middle coolant port on the head so the head is prob ruined so any way gonna get the heads looked at. So when I pulled the heads all cyl looked good except one that has a score in it perfectly straight up and down not very deep just bearly ably to catch It with my finger nail

So my biggest questions are what could cause the rocker to pop off and could i get away with honing the cyl with the score and re ringing the pistons?
thanks for any help tried to post pics but says I need 10 points what ever that means

10 posts. Once you post more than 10 times you can post pics. So, just post nonsense or whatever to get there :)

You need 10 POSTS to be able to post pics. Post 6 more times and just post 6, 5, 4..etc in each post

He could be gun shy because his cubic inches keep getting lower each time he posts.
(Sorry OP, trying to crack a smile)

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https://cimg0.ibsrv.net/gimg/www.off...ee87f32d73.jpg
Poped rocker
https://cimg4.ibsrv.net/gimg/www.off...542470c01a.jpg
Exhaust valvs look rusty
https://cimg8.ibsrv.net/gimg/www.off...16b49300f1.jpg
This is the worst cyl
https://cimg7.ibsrv.net/gimg/www.off...684aee36a8.jpg
Most cyl look this good
https://cimg5.ibsrv.net/gimg/www.off...6addc6d6ed.jpg

So heads are at an engine shop geting looked at right now pulled the oil pan and found some mystery pieces in it there prob aluminum as there not magnetic any one know what they could be
https://cimg9.ibsrv.net/gimg/www.off...08a22ac4bb.jpg

Those are the tabs off the bottom of the valve covers. You need to remove them to clear roller rockers. Looks like got self clearanced

Yup, good call :thumbs those are from OE cast aluminum valve covers. Those tabs are called oil drippers by some. They where designed to collect oil spray(squiring) and drip it back onto the stamped steel rockers.


https://cimg5.ibsrv.net/gimg/www.off...d1b962395b.png

yup your right noticed the tabes were missing when I pulled the valve covers just figured someone trimed them never put the two together till you mentioned it haha

Also anyone have any advice on what I can do with this cyl as this it my biggest concern as of now and also I'm back to its a 502 bored out as my cyl measure out to be 4.500
https://cimg5.ibsrv.net/gimg/www.off...53cae7dcfd.jpg

Anyone?

post 18 picture 4
the top of piston seem to have suffered from heat / detonation of some kind, is'nt it ?
i would tear it down and make sure to have a look at all the component...

I'm not an engine builder but I will answer your request for "anyone".
I have a couple questions out of curiosity. Do you know if all 502's came with roller rockers? Do you also have roller lifters?
As for the pushrod question, if you have roller lifters, could the water in the oil possibly have gummed up the roller and perhaps rounded off the cam lobe till the pushrod came loose? I suppose you could be lucky and rocker could have just come loose.
If the vertical scratch is as slight as you say, then perhaps a simple hone will take care of it.
Good luck with your project.

Blue 502’s came with roller rockers.
Black 502’s did not.

You ever read this book ?
sorry man (mate)
https://cimg3.ibsrv.net/gimg/www.off...a6e761794.jpeg

nope it doesn't have roller lifters

I agree they old things gettin clapped out but just trying to get a few more season out of it think I'm just gonna get the heads hone the cylinders new rings and hope for the best

Well this post just saved me, I am also refreshing my 502Mag MPI and adding roller rockers. I will be knocking those tabs off ASAP! LOL

My boat would not pull past 4,600Rpm with a 22p 4blade prop and would run 59mph and just started dropping from there. Decided to pull it and found teo different head gaskets. The original one is toast between the cylinders along with exhaust seats very pitted due to reversion.

Yes my kid is looking for a ball he threw in the bushes lol
https://cimg1.ibsrv.net/gimg/www.off...7c0e3b8ff3.jpg
https://cimg2.ibsrv.net/gimg/www.off...1fcfeb45bd.jpg
https://cimg4.ibsrv.net/gimg/www.off...3b1ee42237.jpg
https://cimg5.ibsrv.net/gimg/www.off...9ca6edbf72.jpg

Unfortunately, common for the OE headgaskets.

Do yourself a favor, since a part, and get those injectors to ArticFriends. :thumbs

Are there better head gasket recommendation for the 509? It seems to me that these engines blow head gaskets a lot because the cylinders are so close together? No real-estate to speak of to maintain the seal. Or is it surface prep? Combination? Just curious.

Just the OE gaskets are an issue. The felpros are fine. Some go mls but imho not really needed unless yuge power.

The entire MPI setup is not going back on, I am going with a Holley EFI setup consisting of Holley HP ecu, Holley 4150 style intake with throttle body and multi-port injectors, Holley distributer ignition box and distributer.

100% putting a set of MLS gaskets back on none of that crap paper material stuff. You can see apparently my engine blew one already as I found the stock and a Felpro on it.

Ok so the so turns out my heads are cracked I found a good set of ls6 heads it had 990s but the engine shop here is telling me if I put the heads on it will prob increase my compression ratio to much and it won't last at all any one know if I can run em?