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Mercury By-Pass and Water PSI???

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Old 08-11-2010, 10:43 AM
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Default Mercury By-Pass and Water PSI???

Ok, I have a 93 35' Fountain, I have Xr's w 2" Imco shorties, Stelling boxes w the drive mounted in the top position of the box. Over all 3.5" above the stock X dimension, I have the Mercury ByPass Valves for the water psi also. All these parts and motors are 100% new prob 6-8 hrs on total package. This weekend at the Kingston Run I noticed at 4000rpms my water psi was running at 35 psi on the gauge, I back down to 3000rpms and it went to 18 or so and at 3500 it would hit 25psi then drop to 20 psi as they should since the merc bypass works at 25 psi......correct???? Any ideas or insight is appreciated, Thanks
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Old 08-11-2010, 10:52 AM
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Are your water pickups on the front lower part of the drive? It is known that Shorties when trimmed positive will lose water pressure because of the Negative pressure of the water flowing over the front of the bullet with the bullet pionted down. I believe you have to dremel out the sharp edges of the water passage to get water to flow into the hole in this situation.

also it seems like your bypass's are working fine. some Merc's reqiure 50ish psi. thats where the sceniero above showed it's Head..
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Old 08-11-2010, 12:38 PM
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Originally Posted by fastdonzi View Post
Are your water pickups on the front lower part of the drive? It is known that Shorties when trimmed positive will lose water pressure because of the Negative pressure of the water flowing over the front of the bullet with the bullet pionted down. I believe you have to dremel out the sharp edges of the water passage to get water to flow into the hole in this situation.

also it seems like your bypass's are working fine. some Merc's reqiure 50ish psi. thats where the sceniero above showed it's Head..
He has too much water pressure not a lack of it.
The Merc bypass valves should keep water pressure right around 25psi.
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Old 08-11-2010, 02:32 PM
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I have 2" imco short drives on my boat also. I had the same problem when i installed them . Too much water pressure with the merc. bypass valve. I ordered some new springs for the bypass valve from MASTER-CARR PART# 1986 K18. They are somewhat weaker than the merc. spring. After some testing i still took about a 1/4 inch off the new spring. I cruse about 14 lbs. and 25lbs wide open.
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Old 08-11-2010, 06:07 PM
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What does this bypassbsetup look like. Any drawings. I have too much pressure on my 525 efibsportmadter setup
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Old 08-11-2010, 08:40 PM
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Originally Posted by fbc25el View Post
I have 2" imco short drives on my boat also. I had the same problem when i installed them . Too much water pressure with the merc. bypass valve. I ordered some new springs for the bypass valve from MASTER-CARR PART# 1986 K18. They are somewhat weaker than the merc. spring. After some testing i still took about a 1/4 inch off the new spring. I cruse about 14 lbs. and 25lbs wide open.
THANK YOU, what is MASTER-CARR ???? Can you order online or dealer only??

THANKS again
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Old 08-11-2010, 08:48 PM
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what is MASTER-CARR ????

THANKS again
Try McMASTER-CARR
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Old 08-11-2010, 09:14 PM
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What does this bypassbsetup look like. Any drawings. I have too much pressure on my 525 efibsportmadter setup
525s like more raw water pressure. Mine runs 35-45 most of the time at high speed. As mentioned, the pressure drops when the shorty is raised. My pressure gauge does double duty as a trim gauge . I got rid of my 25 psi pop-off and dump hose. It is closed cooling and likes more pressure. Also one less thing to go wrong.
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Old 08-12-2010, 01:41 AM
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525s like more raw water pressure. Mine runs 35-45 most of the time at high speed. As mentioned, the pressure drops when the shorty is raised. My pressure gauge does double duty as a trim gauge . I got rid of my 25 psi pop-off and dump hose. It is closed cooling and likes more pressure. Also one less thing to go wrong.


That's a lot of pressure for a motor. Remember, most cars only run 17-19psi. I wouldn't want much more than 25psi.


Darrell.
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Old 08-12-2010, 06:22 AM
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That's a lot of pressure for a motor. Remember, most cars only run 17-19psi. I wouldn't want much more than 25psi.


Darrell.
Thirdchildhoods boat has closed cooling, His Raw water pressure is what he's talking about. The Closed cooled part is governed by a Cap just like a car. The block never sees the high pressure, just the heat exchanger. His is the boat I was refering too in my first post.
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