mercury 3.0 exhaust relief.

[chevy355mark]

I've got a pair of 250 efis, CXXL 0g62xxx and cxl(converted to xxl) 0g68xxx. I've been chasing my tail for months on one building rpms faster getting on plain and have pretty much just accepted it as everything checks out. Meanwhile im doing a 30 to 25" conversion at the moment and i have discovered that my "strong" engine(stbd side 0g68xxx) has plugged up exhaust reliefs, the 8 you can view with the lower unit off anyway. Where as my "weak" engine(port side) looks really clean, in many ways the weak motor is becoming more and more obvious that its a new power head or has received a lot more maintenance over the years.

So here's my theory, my good engine, the one that revs up faster probably does so because the plugged reliefs mean that all of the exhaust is forced out of my 4 large vent holes in the prop. it also explains why one outboard has always idled considerably louder then the other. Or am i way off base here? I'm not the expert so please be easy on me, just my relatively uneducated theory here.

So here's the ultimate question.... Can i clean them without pulling the powerhead and exhaust plate. Can i drill a relief hole elsewhere to bandaid the problem?

I have already cleaned the 8 ones you can see with a 3/8" wire brush/hone deal and they're all looking pretty good... but there's what, 16 or 18 of them in total?? I have been reading up and down this forum and information seems sparse on anything 3.0 related when you start talking about exhaust stuff. What i have gathered is that the relief holes allow exhaust to ultimately reach the above water vent and the slot at the end of the lower unit, am i correct here? I've just been studying parts diagrams and eBay listing photos of all the separate components involved. If i put a hole in the baffle on the side aposing the vent wouldn't that ultimately relive the exhaust to the vent the same way the holes do in a much worse and poorly tuned fashion...

Sorry if this is a ridiculous question...

[Merc 2.5]

Unplugged reliefs will rev up faster. The more air introduced to prop on take off cause more slip and let the rpms come up faster so when it grabs it's gone, no vents and it will bog out of hole till it gets in its powerband ( power range) so prop with all plugs in it will take off slower than same prop with em removed , I'm sure this didnt answer everything but it's a start,
Kinda y all surfacing speed props are over hub exhaust, to introduce air into the prop ,gets its rpm up faster and helps turn a bigger blade out of hole.


If u try relive ur exhaust in lower unit or mid section will also hurt ur hole shot if ur already having a hard time it of the hole

[chevy355mark]

I’m referring to the 16 or 18 3/8” vent holes in the exhaust chest/tuner. Not talking about my prop vents. It seems nobody sheds any light on these exhaust reliefs in the tuner on the 3 liters. They get plugged up and need to be cleaned or the motor can not breath until it’s up on plane. I may be off base but unless an expert can confirm this is what I have gathered.

you can clean them from the midsection up by removing the lower unit contrary to anything I can find online. I took a piece of hydraulic brake tubing and ran a coat hanger through the middle of it. The steel brake tube has the backbone to get up in there and the coat hanger pokes through and does the work, just bend the coat hanger into a handle on the operating end and a corkscrew on the cleaning end. Poke it in and twist the corkscrew if that makes sense at all. Worked pretty good so far. I’m up to about 13 clean holes, I few more to find yet

after much thought I have realized that drilling any additional holes below the water line solves nothing. I could drill and tap a relief at the bottom of the tuner/can and add an elbow and a stand pipe to free it up back into the midsection but then it’s likely that it will roast the paint on the midsection wherever it dumps. It obviously makes more sense to just get it all cleaned up and operating as designed

[Chaz]

Chevy 4.030x3.480,

Besides using more material and being heavier, if the honeycomb area was solid it would trap a lot of heat. So that area has nothing at all to do with the idle and the only thing that is relieved is heat away from the gasket.
100% of the exhaust go thru the two center runners. The larger area to the right side is the water bypass dump, and the smaller one to the left is used for the compressor return cooling line on the Opti's. You see they both have a little trench for water to spray onto the honeycomb. Helping to take a little heat away.



On a full-length liner, it's done internally so you really can't see it. I leave the dump ports an inch longer on the cut cans to give the water a head start ..

Cleaning, brushing .. etc. is just a waste of time ...

[chevy355mark]

I really appreciate you clearing up just exactly what every portion of that hunk of aluminum does. I was trying hard to figure out what that slot was for to the left that’s apparently for optis only.

To clarify, what you telling me is that in your opinion it does not matter weather the exhaust can escape up through the honeycomb and eventually out the square vent next to the telltale. With those clogged all the exhaust just comes out the prop so your saying that the exhaust really has no problem just escaping below the waters surface with no vent?? I’m not trying to Prove anything here, I’m really asking because I don’t know any better.

I have seen you post that picture in another thread I have read several times now. When you cut them like that wouldn’t it allow the exhaust to go wherever the hell it wants, out the prop, out the vent, over the lower unit slot, it’s all open at that point much like a lot of other outboards besides the 3.0. Again, confused and looking for clarification.

One last thing. I still have a noisy engine and a quiet one in the water. Sound identical on the hose. If this doesn’t explain it then what would? Sorry to drone on and on but I’m trying hard to understand this.

[Chaz]

The tell-tale is in the water cavity above the exhaust ports. The big hole below the ports is cold water "in". The block and adapter plate have a gasket between them that blocks some of those as the "end of the road". If the tell-tale plugs up it is because sand, rocks, seashells, etc. have gotten in that small cavity and plugged it up.



The exhaust really has no reason (or ability) to backtrack up thru the tuner cooling area, get thru a small "leak" in the pressurized water system and out the tell-tale. Again, the honeycomb (cooling fins) is a way to cool the upper part of the tuner. It's only open to space in the inner can. Not even reaching the gasket on top, it's not connected to any system, water or exhaust. It's sealed off ..



Now if your asking how exhaust gets out of the back of the adapter plate .



Then with a cut inner can or tuner it's a short U-turn away from going out ..



A full length stock one from a 225 or 250 has some slots that "leak" to the area between the inner and outer can and either up and out or down the U shaped channel and out the holes that dump at the cavatation plate.
The 300x cans also had a cats eye slot up high and a hole at the bottom that connected the two area's ... but that's just my opinion ...

Cats eye ...



Connector hole ...

[chevy355mark]

We are losing so much in translation here lol. Your not grasping what Im trying to say and its my fault.

I'm not talking about the telltale, I was trying to use it as a reference to the square hole between the telltale and the flush port which i believe to be an above water exhaust vent. What im trying to say or understand (but failing) is that the square exhaust vent is connected only to the dead space outside of the full length log/tuner/can/whatever. The only way the exhaust that is otherwise sealed to the lower unit prop outlet can make it to that dead space is up through the honeycomb area and out the slots that are at the front most upper portion of the tuner. once it has escaped the tuner and into that dead space in the leg it has 2 options, the slot over the lower unit or that square vent. What have i overlooked, I'm having such a hard time seeing how that honeycomb is simply for cooling.

at the end of the day I'm gonna spray some degreaser in there, clean it up with a gun cleaning brush and re assemble but id sure like to know exactly how it all works. BTW nice welds on that aluminum, spool gun? what series wire do you use

[chevy355mark]

so for clarification, once the exhaust is deadheaded at the prop and has no other path then out the honeycomb it then must come from these slots. i think...



please excuse these ridiculous photos lol its the best i can come up with.

[Chaz]

You may be 100 % correct. Or 50% correct. I'll look next week at one I have to shorten for a 15" deal, as to whether it vents at the long side slot as well.
I normally don't use the bottom half of that can. So I have never followed the "upper path". I have only cut them off, done the cats-eye slot and or the bottom hole and sent it on it's way. Might have taught me something today ..

Castings .. 4043
Plate or billet ... 5356
Both TIGed

[chevy355mark]

If your cutting them off it wouldn’t matter one bit about all of this. From what I can gather almost any other outboard is not setup this way but much more similar to how your cutting them off. I’m just trying to make an educated decision as to where I might want to add a relief hole or if it’s better I don’t.

ultimately if I were to Swiss cheese the bottom half of my tuner it would be the same difference as your cutoff units... but again, in a more poorly tuned and half ass fashion... or maybe I’m wrong, idk. I deal a lot with 2 cycles in many different applications and dabble a bit in porting, pipes, etc and truly love them compared to a 4 stroke but the outboards are the strangest to me, by a long shot. I understand why they don’t/can’t use a chamber but it’s such a strange version of a loop charged 2 cycle, midrange monsters.

[Chaz]

If you swiss cheese the can down low .. it will be underwater at idle. The factory put the cats eye cut on the side just below the honeycomb on the 300x to bypass some of the exhaust before going thru the maze . A single hole each side between 3/4 - 1 inch would be more than enough ...

If you did the large hole at the bottom connecting the "two lower exits" together it will free up the exhaust a bit .. More importantly, not running all the exhaust thru the lower unit takes some of the heat off of the bearing carrier / prop shaft seals ..

The main water supply run's in the middle and then cools the exhaust runners before it makes a 90 and goes up thru the block. I think it get's "pre-heated" enough as is without "picking-up" more heat from the sides along the way.

Putting my neck on the chopping block once again ... LMAO
Have you checked the reed's ... ? I'd be more inclined to think that the rubber on the cages has eroded and / or chipped a reed or two.
When they cold start and sound like a horse sneezing .. they tend to make me cringe / send fire back thru the intake tract ..
Carson makes a reed that fit's de-rubberized blocks if you find yours to be in bad shape ..

[chevy355mark]

So where you have the green drawn up there in the photos your referring to where cooling water dumps? And that’s only on the 300x unit or my fishing motors as well?
edit: the green circle you added is the additional exhaust relief that you have been talking about only on the 300x, water still dumps inside the log rearwards of the main exhaust, correct? So essentially the slot over the lower unit is much more so where all of your cooling water dumps then an exhaust relief?

I just did Carson reeds, I kept the rubber tho. A few blocks had damaged rubber but I replaced the bad ones with good used units and threw it all back together. My week engine does still sneeze even with the tps set to just over 1v at idle, it only does it completely warmed up and less frequently. Before the reeds they sneezed like crazy. Also had the injectors sonic cleaned and all the bleed lines are new oem lines, cleaned all the little check valves too. And yea I use Ql77cc plugs. We’ve been over a lot of this in my previous threads regarding my one engine reving faster then the other.

[Chaz]

Circle on the left picture shows where the factory put a slot on the 300x. All other models don't have this.
I merely suggested replicating the hole.


Right side picture is the main "water in" conduit.
I posted that to show that if you don't drill the side holes. 100% of idle exhaust must pass thru (each side) that area, instead of diverting it out of a secondary location.

[chevy355mark]

So I ended up just cleaning the 14 holes that I could with my gun cleaning kit and degreaser. I drilled a 3/8 hole in the divider between the main cavity and the rear water dump slot at the casting line. This should be above the water line to offer some relief or it will likely do absolutely nothing at all but hey, after all this I had to drill something lol.

[trivialpursuits]

Circle on the left picture shows where the factory put a slot on the 300x. All other models don't have this.
I merely suggested replicating the hole.


Right side picture is the main "water in" conduit.
I posted that to show that if you don't drill the side holes. 100% of idle exhaust must pass thru (each side) that area, instead of diverting it out of a secondary location.

Bumping a slightly old thread for clarification:

Does the 300x tuner have that hole on both sides of the can? or is it only on the Port side as shown in the pic with the green circle?