Purpose for water by-pass tubing?

[AMD Rules]

I've been trying to determine why the spray out the upper exhaust seemed <br />excessive. I've since removed the powerhead and exhaust plate. The new <br />shaft seals and power head gaskets are already installed, ready to re-mount <br />powerhead to solve my mysterious oily mess (I hope). Thats another <br />topic...lol<br /><br />I still have a desire to understand how the water system works. In the <br />attached image, I've shown the rubber by-pass hose that leads from the water <br />inlet cavity to the rear upper exhaust. When I blow in the bottom of the <br />water tube, I get air out the normal water outlet, out the exhaust hole <br />shown, and out the tell-tale of course.<br /><br />What is the reason for the water by-pass hose on the external portion of the <br />exhaust plate?<br /><br />Clearly that it the reason that water will inevitably spray out the back, <br />and it was designed that way. I'm just curious what it is for.<br /><br />The picture is the exhaust plate from above, for my 88 60HP 3cyl Merc.<br /><br /> <br /><br />/AMD

 

[heybaylor]

Re: Purpose for water by-pass tubing?

cooling the exhaust...

 

[AMD Rules]

Re: Purpose for water by-pass tubing?

I appreciate the input, but due to the brief nature of your reply I'm not clear what you meant. The water goes around the outisde of the exhaust plate (which would be cooling nothing), then enters the exhaust plate and immediately goes down through a 1/4" hole, travels about 2" and spits out the back of the midsection at the upper rectangular exhaust ports. I don't see where it would be cooling the exhuast at all. If anything it only touches a small portion of the exhaust tube for a moment, then mixes briefly with some residual exhaust trapped in the upper portion of the mid section. Most exhaust cooling would be done on the side exhaust baffles, rather than this lower section. <br /><br />There must be another reason.

 

[heybaylor]

Re: Purpose for water by-pass tubing?

MY "guess" is that at idle speed ..all the exhaust goes out the upper "relief ports"<br />without the "extra cooling water on these holes might get hot enough to burn a person ,and/or paint off. <br /><br />water out of the "outlet" At idle rpm,s would drop down due to gravity/low volume, sit in the midsection..and not make it out the relief ports.<br />only hot exhaust gas make it to the ports.<br />remember at idle the midsection and prop are full of lake water<br /><br />Once rpm is enough to force exhaust and water out of the prop then the volume/speed of the exhaust and water blows it out of both the relief ports and prop..cools the midsection and most important the waterpump body (outside)<br /><br />first thing to look at with a waterpump failure is the waterpump housing/casing for burn damage from the hot exhaust

 

[AMD Rules]

Re: Purpose for water by-pass tubing?

Thanks again for the thoughts. Sounds reasonable. My only concern is I was wondering if I had a restriction in the powerhead, that made the upper exhaust ports the path of least resistence. Kinda like a relief valve. <br /><br />Since re-installing the powerhead, I've run the motor on muffs and in a can and in both instances the upper exhaust ports give me a shower. New gaskets and seals from the powerhead down, including exhaust plate, water pump, and all water tube seals.<br /><br />Mercury designed it to have that water bypass, so I will ignore it and go boating.

 

[AMD Rules]

Re: Purpose for water by-pass tubing?

First run with this boat (ever) seems to run ok.<br /> <br />35mph @ 5500 rpm WOT<br /><br /> <br /><br />But...<br />Compression reading on top cyclinder has gone way low. I'm looking the other way at the moment....hoping it will fix itself. It was the highest cylinder when last checked. Odd.

 

[KCLOST]

Re: Purpose for water by-pass tubing?

AMD, If the poppet valve )pressure relief) is stuck open, it will dump water out of the block at lower rpms and it will exit the upper exhaust...<br /><br />It is designed to only open at rpm levels above 3000 or so to ensure sufficient water flow through the water jackets.. It's worth a check dude! <br /><br />If it is stuck open all the time, you could see overheating issues at lower rpms when it should be closed, as you may not get fully circulated water jacket flow. And the first cylinders to overheat will be the top ones!!!!<br /><br />Is your overheat sensor working?

 

[KCLOST]

Re: Purpose for water by-pass tubing?

 

[KCLOST]

Re: Purpose for water by-pass tubing?

Hold up on my part... <br /><br />I guess we should confirm that you even have a poppet valve on that motor first...

 

[AMD Rules]

Re: Purpose for water by-pass tubing?

Hey Kclost.... thanks for the advice. I don't believe my motor has a poppit valve (not sure to be honest). It is wierd why the water prefers to exit out the mid-section though. The tell tale comes directly off the top of the motor, and seems to have good flow, and a new impeller. The temperature sensor beeps when grounded. Any ideas are always welcome. I'm very discouraged with the pressure at the upper cylinder dropping from 125 to 90 psi after an hour run on the lake. I tested the pressure at least 10 times to be sure. Motor ran like a champ the whole time, with no pecualiar behavior. I did check the temp of the tell tale stream several times while we were out, and at no time did it seem so hot that I could not hold my hand beneath it. Viibration and power seemed normal.

 

[AMD Rules]

Re: Purpose for water by-pass tubing?

Uh-oh...this doesn't look good.<br /> <br />Uneven compression - image 57.6kb

 

[Clams Canino]

Re: Purpose for water by-pass tubing?

Water always sprays out the upper exhaust part of the leg at idle and otherwise. They mix in the leg.<br /><br />-W

 

[AMD Rules]

Re: Purpose for water by-pass tubing?

Thanks Clams... I've accepted the fact that the water spray is ok. Unfortunately the lack of cylinder compression is now my primary focus.

 

[AMD Rules]

Re: Purpose for water by-pass tubing?

Tore into the motor this weekend. My poor compression readings were accurate.<br /><br />(Click on thumbnails for fullsize images) <br /><br /> <br /><br /> <br /><br /> <br /><br /> <br /><br /> <br /><br /> <br /><br /> <br /><br /><br />#3 piston has scoring on the skirt, rings were stuck in grooves and very <br />hard to remove. Scoring on both side of the piston, and in cylinder <br />accordingly.<br /><br />#2 piston looks factory fresh. Rings had lots of spring, and piston and <br />cylinder looked and felt like new.<br /><br />#1 piston had fractured rings, one in two pieces, one in three pieces, and <br />the other stuck but intact.<br /><br />What's the best approach from this point?<br /><br />How to determine if I need to re-bore oversize, and if the pistons require replacement?<br /><br />Disappointing, but I expected to see bad news.<br /><br />Thoughts & advice appreciated.