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




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.




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







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?