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Mercruiser 420 reversion issues

Old 01-06-2019, 11:47 AM
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Default Mercruiser 420 reversion issues

Im experiencing some reversion issues with my freshly rebuilt mercruiser 420/454. It is built to original 420 specs and bored .30 over. I am running the crane 132561 flat tappet cam, the specs are 114 lsa with 228 intake and 236 exhaust duaration at 0.50” lift. For exhaust I have gil manifolds with standard bravo wet tailpipes. I pressure tested the manifolds using a garden hose and they do not leak. However i am getting moisture on my plugs and getting milky oil after idling. The engine idles at 750 rpm in neutral and a little lower than that in gear. My engine builder is suggesting I go full dry tailpipes but ive seen other people run this cam with wet exhaust and not have issues has anyone else experienced reversion with this cam? I feel its a pretty common cam for budget build 454’s, just want to get some additional input or experiences with others that have ran a similar setup.
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Old 01-06-2019, 12:55 PM
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Are you plugging the ends of pipes when you run the hose through them for testing? I think most Bravo systems run up to 30# of pressure. Most garden hoses are 50# or more so if they’re plugged and not free flowing that should be a decent test. But that doesn’t account for heat and some cracks only leak when hot.

An exhaust leak will cause reversion so make sure the gaskets are good. Also if the nose of the boat is ripped down while running on the hose that will cause reversion.
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Old 01-06-2019, 01:45 PM
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Water reversion in the exhaust wouldn't make your engine oil go milky, did you use the original heads and inlet manifold.
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Old 01-06-2019, 03:37 PM
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First I would up your idle speed to 800 or a bit more so you have 750-800 in gear. With that exhaust you really should not have much of a issue. I ran a 134561 in a 454 mag with Imco exhaust and had an issue until I increased idle speed. That helped clear it up. It was however to much cam for the application but it sure did sound sweet.

Mark
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Old 01-06-2019, 05:42 PM
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Originally Posted by Baja Rooster View Post
Are you plugging the ends of pipes when you run the hose through them for testing? I think most Bravo systems run up to 30# of pressure. Most garden hoses are 50# or more so if they’re plugged and not free flowing that should be a decent test. But that doesn’t account for heat and some cracks only leak when hot.

An exhaust leak will cause reversion so make sure the gaskets are good. Also if the nose of the boat is ripped down while running on the hose that will cause reversion.
yes I plugged the ends of pipes and water hoses on manifolds when I pressure tested, did not see any water leaking inside the manifolds. No signs of an exhaust leak all gaskets were new and good quality oem gaskets. As for the trailer thing I usually run it nose up on the hose but this also happens when the boat is in the water.
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Old 01-06-2019, 05:51 PM
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Originally Posted by Camalot View Post
Water reversion in the exhaust wouldn't make your engine oil go milky, did you use the original heads and inlet manifold.
when the engine was originally torn down one head had a hairline crack, that head was replaced with a good casting. The other head was fine, both heads were gone thru and completely rebuilt by machine shop. Intake manifold was replaced with a good weiand single plane. The oil is not a straight milkshake but it is showing signs of some water. I am going to a leakdown to make sure everything is in order, I do have the engine pulled to inspect the bearings was gonna look over everything with my builder.

Last edited by gageski101; 01-06-2019 at 06:14 PM.
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Old 01-06-2019, 05:56 PM
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Originally Posted by GETTINBYE View Post
First I would up your idle speed to 800 or a bit more so you have 750-800 in gear. With that exhaust you really should not have much of a issue. I ran a 134561 in a 454 mag with Imco exhaust and had an issue until I increased idle speed. That helped clear it up. It was however to much cam for the application but it sure did sound sweet.

Mark
I have the 132561 cam which has a little less duration than that cam, but before I spend the money on dry pipes I am going to try turning the idle up and see how it reacts, thanks for the response!
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Old 01-06-2019, 07:19 PM
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A common culprit on new engines is an intake gasket leak, but are you getting a full milkshake in the oil filter? If not then it’s just condensation for not getting up to heat. A common occurrence especially with cold water.
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Old 01-07-2019, 12:42 PM
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No reason you should have any reversion with that cam and Gil exhaust.
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Old 01-08-2019, 10:44 PM
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Originally Posted by Griff View Post
No reason you should have any reversion with that cam and Gil exhaust.
I second that. I'm running much worse with just 496 aluminum Merc exhaust and not having an issue. If not the intake I would almost think the valves are not sealing.
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