Well, that's disappointing
 

I removed a 502 mag EFI and replaced it with a used HP 500 carb engine in my Cobalt 252.

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A lot of you read about the number of issues I had to iron out after the install. Finally got everything squared away with the help I received from this forum. (Thanks again) Was actually enjoying the boat and managed to get about 30-40 hours on it this season.

The stock prop was a Mirage plus 23p that turned 5200 RPM at 63 MPH with the 502 mag. The new to me engine turned that prop at 5200 RPM at 63 MPH with a full load of adults. Got up on plane quite easily and has a nice midrange "hit"

Threw on a Bravo I 24p prop and turned 5200 RPM right around 62 MPH and planes pretty fast. Is also pretty responsive to throttle control. I like the Bravo I.

Looking for more top speed I installed a new Mirage plus 25p. Only turned about 4800 RPM at 62 MPH. Plane was not as good as the 4 blade, and not quite as responsive.

Hoping to gain the plane and responsiveness back I tried a Bravo I 26p. Only turned around 4500 RPM at 60 MPH. Still planes out pretty good and responsive.

After spending the $$ to buy this engine, then iron out all the wrinkles of the swap, I'm not even able to turn a larger prop than the engine I replaced. Pretty frustrating.

Hoping there might be something wrong, I did a compression test and found 2 cyls low. 1 on each bank.


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Ended up taking the boat out after doing the compression test and replacing the spark plugs. It ran fine for that outing.

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Loaded it up on the trailer and the engine didn't sound right when pulling it up on the trailer. Even the GF noticed it sounded weird.

Bought a leak down tester today and attempted to test the 2 low cyls. I was getting almost 80% leakage and could hear air coming out the carb. Figured I did something wrong with the test. Not at TDC or something like that. Figured screw it, fire it up and see what happens.

Ran for about 4 seconds and made such a clatter. Clicking, tapping, knocking sounds from the top end.

So.... With this being a used engine with no real history on it. It being a HP 500 carb with known valve spring issues. And a cam that is notorious for sucking water at low RPM.

Do I yank the engine out? Going to be difficult this time of year as my mechanic will most likely be busy with winterizing.

Or do I remove the heads while in the boat and see what I'm dealing with here?

There's quite a bit of room to work inside the engine bay, so top end wouldn't be too difficult to do. If I were to address the cam, it would definitely need to come out.

Not quite sure what to do? Any insight would be appreciated.

I’ve done a top end refresh with the engine in the boat and it sucked. The next time I did one, I pulled the engine. Both times were on single engine boats with quite a bit of room. I would never do it in the boat again.

I would pull the motor out and give the lower end a once over, check or replace the bearings , look at the rings, maybe a light hone if needed and a peak at the oil pump. The piece of mind will be priceless next summer.

The thing with performance builds… you get that added punch and the ability to swing a bigger prop (but not always… sometimes same prop but more RPM)), more speed, etc.

But there’s always a trade-off… support and maintenance.

Merc’s blue engines need more maintenance than the typical black engine. Where it used to be standard maintenance items on the black engines (oil and filter, tune-ups), the performance engines require you to go into the engines to make sure the valvetrain and heads are holding up at scheduled intervals (under 300 hours). And it’s a gamble with proper maintenance and used engines.

Take the engine out and have the heads redone by someone familiar with setting up the proper valve stem clearances for a performance marine engine. As stated above, it’s also a good idea to go through the bottom end to make sure all is good.

Another benefit of a refresh will be freshly machined surfaces for the head gaskets to seal against (502s like to weep coolant down the sides of the block when the deck surfaces aren’t perfect).

Don’t do the heads in the boat. You’ll hate it. Your mechanic will hate it. And you’ll probably have to redo them again sooner than later.

Pull the heads in the boat. The bottom end is probably fine and the top end is easy to take apart in the boat. Pulling the engine is just a lot of extra work.

Having done a head gasket in a boat, unless your a contortionist and enjoy being scrunched into a little ball with a chiropractor on speed dial, its much easier out of the boat...

In a single with room Id attempt to do it in the boat, probably not in a twin app, are those batteries scrunched up next to the headers ?

Well...hell. You know I've been following this build thread for a long time as my 'someday intent' to repower, exchanging my 454MPI with a 502 Mag or (dream engine!) HP500EFI is going to get a lot of scrutiny for just this reason. With the stock engine, my 252 Islander will run 54 - 55 max, and I would certainly expect 62 - 65 with the 502 Mag and (dream-o-meter?) 70 mph with the HP500EFI. The idea of going through all the trials and tribulations as you have, only to not really gain the expected results would be very frustrating to me.
It really seems as if, according to objective evidence, your engine is way down on power?

From a 454MPI to a HP500EFI you'd lucky to 63 maybe 64 if the moons align. 1MPH/20-25HP
https://www.go-fast.com/Knowledge-ba...eed-calculator

how many batteries does this boat need?

Good catch. Lol.
Must be a party boat. :thumbs

I should get a better pic for you guys of the engine bay. I really do have a lot of room inside there. Getting to the bottom end is a ***** because of the stringers ad the motor mounts, but the top end is really not a problem .I can stand or kneel on both sides of the engine with no problem.


I was wondering if I should remove the top end and inspect before removing the engine. If piston damage is observed, then it's obvious it needs to come out. Funny how most say no and a few say go for it. I guess that's normal for the internet.


I'm going to call the mechanic tomorrow and see what his schedule looks like. If he can't get to me until winterizing boats is done, it may take a while. But in all honesty, I'm not going to need the boat for at least 6-7 months. (dam you year-round boaters have the life) I'm also waiting to speak with the engine builder and see what input he has, and what his schedule looks like.


Deep down, I know the bottom end needs to be inspected. Just for piece of mind as someone stated. But I'm not looking forward to the additional costs.


It really does suck to know it cost a lot of $$ to get this engine and get it running properly, and now it crapped out in just 30-40 hours on me. :(


I knew the down side of the additional maintenance with this, but was told the engine had around 200 hours and the springs were already replaced. Pretty sure that was full of ****. I'll be finding out real soon.


Thanks for the advice gents.


Has anyone determined exactly what the difference in HP is between the HP 500 carb (470 hp) and the 502 MAG EFI (415 hp) is? Are both those figures taken at the prop? In all honesty, I was hoping I would be able to turn a slightly bigger prop and gain 3-5 MPH over the 502 Mag. What I was told, was this particular engine made 529 hp on the dyno with the MSD distributor, QFT 850 carb and the CMI E top headers. (beginning to doubt that claim too)


F2, my battery boxes are about 1/4" away from the exhaust. Not touching, but darn close.

2 for the engine and 2 for the stereo

I can always add 1 for the engine into the stereo bank if need be.

I call it the party barge.

(4) amplifiers
(8) 6.5" speakers
(4) 8.8" speakers
(2) 12" subwoofers .

Blue LED lights in the speakers and throughout.

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The HP500 is advertised making 500 at the flywheel and 470 at the prop through a Bravo drive. The 502 MPI makes 415 or so at the flywheel and about 380-385 at the prop. Engine and drive condition, humidity and temperature, and load all affect the numbers.

Nice sound system!

I have a nice sound system in my Mustang convertible. It's amazing how much current those amps draw while running on only 12 volts. Sometimes I wonder if a power inverter and 120v amps would work better. Good luck getting your problem sorted out. Once the batteries and exhaust are out of the way you will have plenty of room to work on the top end.

OP where are you located ? did you buy the new engine from a member here

As always, different strokes for different folks.

Cannon plug, drive, front and rear mount bolts, exhaust couplers, fuel hose, battery lead to starter, and a few ancillary wires here and there.
Then it's pulled. You or your mechanic can work on it in a Lazy Boy next to the toolbox.

I'LL take that any day versus climbing in and out of the boat 50 times, lying on your head, bent 50 ways and whatever else.
Not to mention no major grease and oil mess in the bilge.

I'm all for pulling the heads in the boat if you have the room. You say the leak down test had air coming out the carb. Seems more valve related than piston anyways. You would probably not be able to see a bad piston or leaky ring and probably just leaky valves. I say pull the heads in the boat. Use the extra labor cost to pull the engine to dump into porting the heads, adding better valves, or rocker arms...you get the idea.

How many hours are on the engine? Top end is the weak link, bottom end is good. The valve springs break and then the valves hit the pistons. Hopefully its just a head problem.

Thank you. So approx 100 HP difference, Should be able to turn a bigger prop and about the 3-5 MPH I was expecting. Thanks for that.

Cleveland. Yes, it was listed in the classifieds.

The leak down test was almost 80%. It was after that I started it up and it ticked and clattered immediately, so I shut here down. Hoping I didn't damage the pistons or heads by them making contact.


This is what I was sent. The motor has approx 180 hrs. Underwent a full rebuild done by Chief before he bought it. (He has receipts, I never saw or got them) New heads, springs, cam, carb etc...

So with hind sight and reading between the lines, I do not know how many hours. Because I had to replace the carb and when I questioned him about it, he said it was not the stock carb, so that is what he meant by new. The engine came without an exhaust and I thought there seemed to be quite a bit of rust/scale build up inside the exhaust ports for having new heads. He listed a cam, but sent the specs of a stock HP 500 cam. Why would you install a new stock cam that has reversion issues instead of one with more lobe separation? So I'm highly suspect of any internal work that I cannot verify.

I do wish it was just the top end and I could get a set of performance heads to remedy this. I could leave the engine in the boat and would prob get away fairly cheap compared to a complete rebuild. But I have a feeling I should have the entire engine looked over.

Didn't speak to the mechanic that would remove the engine, but his wife did tell me they are in full swing with winterizing now. I will most likely need to wait until winterizing has died down.

Still need to talk to engine builder and see what his schedule looks like as well.

So trying to coordinate yanking the engine, getting it serviced, engine put back in, and getting the boat put up for winter and into storage. Sheesh..

I would not do a topend in the boat. That's a pain in the ass. 1-2 hours and the motor is sitting on the garage floor. So much easier to install heads, manifold, dist, etc.... Plus you can take the opportunity to look at the bottom end of needed.

Time for a poll? lol

My vote is for pulling the motor BUT it could be an age contingent thing. The last time I pulled the exhaust diverters to replace the O-rings it = about 20 motor R&R's in my 30's!

You can't really go by me, I take crate engines apart and go through them to make sure they are good!
I have everything needed to pull a motor, as Seafordguy said, in a couple of hours, but it wasn't always that way! I have pulled many a head and swapped many a cam IN the boat. It's messy, and requires 20 trips in and out of the boat to get crap you need-but forgot! LOL!
If you don't have, or have access to, a motor hoist or gantry, drive alignment tool or friends with beer to hand you tools, here's my thought:

In real life, as a Single engine boat with 180 hours (if true???) I say there was a better than average chance the bottom end is ok (good oil pressure, no noises, no leaks, no milkshakes?). Seems you have room to work(remove the back deck cushion, take out those white dividers, they just unscrew -my small boat is the same) and pull the heads IN the boat. You can set stuff off to the side still connected, etc. If all is ok, heads to the shop, check your valvetrain and cam lobes, and put it back together. If you're real rambunctious, you might even warm up the cam, but I digress. IF you see something in the holes, you can still pull the motor then. (at some point, you had to pull the heads anyway) No harm, no foul. It's a gamble, but if it's all in the heads, could save some time and aggravation.

That is spot on what I've been thinking also.

I do not have the tools to remove the engine myself. I did speak to my mechanic, and he says it will be no problem removing it for me. Just need a couple days adavnce notice and he says about an hour or less and it will be out.

Still waiting for the engine builder to get back to me though. I need to speak to him and see what he has to say about the problem, and what his schedule looks like.

I am contemplating removing the top end for inspection. As you said, no prob removing the engine anyway. However.... The cam in this engine (Crane Cams HR 284-2S-10 .576/.598 222 230 110 LSA) has always had me concerned about low RPM and reversion. It would be a good time to upgrade/replace the cam to something with 112* lobe separation.

It would be nice to bump up the HP a little, but I don't want to destroy my Bravo 1 in doing so.

Thanks again for the replies guys

Dont worry about a cam change hurting the drive...........look for one with a 114 split like the 525 cam

Thank you. Will discuss that option with the builder.

I know I'm starting to approach the threshold of HP the Bravo I is able to handle reliably.

you can add rpm and wind it up higher as that does not hurt the drive its the torque down low thats hard on them

Merc 500hp is the crane 721 which has 222/230 at .050" with 110lsa
Crane 731 is 226/234 at .050" 112lsa
Merc 500 EFI is Crane 230/236 at .050 " with 114LSA
Crane 741 is 236/244 at .050" with 112lsa
Merc 525EFI is Crane 236/244 at .050" with 114LSA

Work on the engine in the boat !?. Hard pass! So much easier to work on it sitting in a shop as mentioned.
Can`t be that hard to pull the thing out .. I pul mine to change the oil :whistle:

Doesn't that add more stress on the valve train though? I thought big blocks didn't like a lot of RPM? What's generally a safe RPM to spin a BBC? Merc recommendation is usually around 5200.

That is some good info right there. Thank you

If I had the tools and space, I'm sure you're right. I however, need to arrange someone else to remove, another to rebuild, and the first guy to re-install. I truly envy some of you guys.

merc 1075's go to 6200,I spun my old 454's to 6800,and held it there for 25miles.:D,would have gone further but started to run out of gas,,in the tank.:D
need to get the valve springs right and they'll spin up.

Exactly.

Old sixties NASCAR big blocks turned well over 6k plus for 500 miles with parts that were much older technology and heavy. Spinning a more modern-built Gen VI 502 to 6k for short sprints (with good oiling and cooling) isn’t going to hurt it all.

Don’t be afraid to spin it up!

Just disconnect everything except the engine mounts and have your mechanic pull it and set it in your truck.
Do you have a garage?? Does it have a steel beam or wood ceiling joists that you double up to strengthen??
If so all you need is chain hoist and an engine stand which will cost about $300 total.

That would be an awesome video.

First option will most likely be the way to go.

My boat won't fit in the garage. :(

If you have a decent amount of room to sit and pull apart the motor in the boat is do it in the boat. Just have to keep the internals clean when you start pulling it apart and keep all the parts coming off organized. I understand not working on a side by side in the boat, no f’n way, just a terrible setup for any kind of maintenance period, but a single or staggered it’s not that big of a deal. Even with a hatch that only opens to maybe 30 degrees I didn’t have much of an issue pulling a head on my blown 525s. Having someone to hand the heavy parts as they’re coming off really helps. If you can remove your hatch that makes a world of difference too.


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I would absolutely agree, the HP increase to get from 54 to 70 would be probably around 250

Talked to the engine guy today.

He wants the whole engine. (Actually just the long block. says I can save a few bucks by removing all the accessories)

Hope it's just the top end. Otherwise this thing just became a REALLY expensive Hp 500. Coulda bought a rebuilt 525 for what I'm looking at now. (including purchase price of the used engine)

That sucks to hear, but based on what you described, it’s probably just up in the heads. Doesn’t hurt to get a sampling on the rest of its condition while out though.

Looks like you have through hull exhaust, so I wouldn't worry about the cam. To get water ingestion you need back pressure and you'll get very little with through hull exhaust.


I'm surprised the shop said an hour to remove the engine. It comes out quick, but not really that quick two guys can have it out in an hour, but that's two hours shop time.


I agree with most everyone, I want to see the bottom end and make sure it's solid. However, sounds a like top end problem. Did you hear noise from the engine when you launched? The poor leak down could be from bad valve adjustments, bent push rod, leaky valve, or any number of other things. But it does sound like a top end problem. Poorly aligned piston rings during assembly can also result in a bad leak down test, but this is such a rookie move I doubt that's the issue. If the motor was new and wasn't fully broke in, i.e. rings seated, that could also do it.


Based on the thread, pull the motor, have it torn down, inspected, then reassembled. One problem though; I fully balance all my motors - it makes a HUGE difference. So, if it were MY engine, I would have the whole thing re-balanced to be sure.