Raw water at carb at higher RPM

RenkenII

Seaman
Joined
Jun 13, 2011
Messages
53
Mercruiser 470 - 1987 Renken 1750.

Did a head job this summer for a faulty head gasket between 3 and 4, then found a crack in the head. Found a used head, machine shop worked it up, I installed it and it ran great! Ran great until on plane, and about 5 minutes it would begin to bog down with raw water being drawn into the carb via the crankcase vent. Riser and exhaust manifold are good. Could I have too long pushrods, not allowing the exhaust valve to close, thus drawing raw water in from the riser? Looking for other options if any has had this.

I know I may left things out that may be needed for this forum, but hoping I gave enough.

Thank you.
 

achris

More fish than mountain goat
Joined
May 19, 2004
Messages
27,468
Raw water as opposed to coolant can only come from either the exhaust system, and your pushrod length theory might be right, or a leak between coolant and raw water, with the raw water displacing the coolant then finding its way to the crankcase.

Pressure test the block/head/manifold the way you would a car radiator. That will rule out some possibilities.

“Once you eliminate the impossible, whatever remains, no matter how improbable, must be the truth.” - Sherlock Homes”

Chris......
 

Scott06

Admiral
Joined
Apr 20, 2014
Messages
6,844
So you are filling your crankcase with raw water? i cant see pulling this in from the exhaust, it would fill a cylinder and hydro lock. You would have to have both a leak from raw water side into af side of heat exchanger, and leak from cooling jacket into crank case…
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
Staff member
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Jul 23, 2011
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50,487
Pressure test the cooling water jacket

Acerone test the manifolds
 

achris

More fish than mountain goat
Joined
May 19, 2004
Messages
27,468
It's a 470... Closed cooling is standard on those. The only place the raw water meets the engine is the exhaust elbow...
 

QBhoy

Fleet Admiral
Joined
Mar 10, 2016
Messages
8,348
Hate to be mr obvious…no chance the water in the carb is from the fuel ? Be bad news if it’s from the crank case ! If it’s a 3.7…I don’t think there is any interface that would make this even possible, raw water wise.
 

RenkenII

Seaman
Joined
Jun 13, 2011
Messages
53
The crank case is not filling, no foam or inconsistent oil levels.

Water is not in the fuel. It’s obviously coming in the breather hose.

Thank you for all the ideas. My exhaust elbow looks to be in good shape, but maybe I’ll change as it has slight crack I can’t see or detect.
 
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