These are my SM exh manifolds. Pardon the debris on the bottom...that's from 30 yrs of sitting and getting the engines out.

But you can make out the white blooming Aluminum oxide spots on the walls.

As for rubber exh tip flappers...I don't think they create much of a resistance to flow. There is both exh gas mass and water mass moving at high rate even at idle that keeps them open.


Mercruiser engine with acrylic exh to visualize reversion.

The 236° @ 0.50" is on the larger side, and the exh is ever larger, despite the wider LSA.

Possible test option is to run the engines ona steep boat launch to get the riser angle up. See if it still gets water in the oil.

Or idle for a while at normal level and pull a riser off. Should see evidence of moisture in the manifold.

The center 2 cylinders will be affected more than the outer 2...with the SM manifolds.

I was JUST watching that video trying to teach myself a little more about reversion super cool video!

im just REALLY confused as to why it’s BOTH engines again!! Everything and I mean EVERYTHING is new except for blocks , intakes , manifolds and risers. And carbs. I’m at the point I really want to take it somewhere. I’m in Tennessee and pickings are slim and Eddie would be my choice but I know he is busy as absolute possible. Anybody want to make a trip here? I’d pay for time and travels or something?

I’ll run her again tomorrow and immediately pull a riser back off and see what it looks like inside?

Have any tips or tricks on pressurizing the system while in the boat with air? Is that even possible?

thanks man!

I was looking at the eng pics early in your other "Operation 87 Cafe rescue" thread. Now I see the new engs have stock recirc pump/thermostat.. My bad.. Should have read the entire thread... Still would be good to get water pressure readings at speed.

You're in fresh water if I remember...or you wouldn't be running the new heads in salt.

What are the engine temps like?

Is there merit to restricting the raw water cooling? Or reducing the flow volume through the exh manifolds...while keeping them cool, but having a second transom dump.

The pill to take may be getting different risers that have more slope.

The new heads raised the exh ports up 3/8" at a 45°, so up maybe a 1/4" ish.

Another option would be a spacer at the dry exh man to riser joint to raise them up, get more angle.

Of course....this is if the reversion is the issue is from an on the aggressive side cam, and a shallow riser slope.

won't attempt to say what the cause is as i'm not an engine guy either. Curious tho as to why you have this style distribution/thermostat housing with 4 hoses ? To me it looks like the type that fed the older mercury exhaust manifolds and risers separately even tho there was water passages from the manifold to the riser, (the rear or top hoses fed risers and they had check balls) think they fed water as the thermostat opened.


Are you feeding the manifolds from front and rear ? All the water goes thru them anyway so i'm not saying it's the cause.


so if you removed and plugged the T then fed the manifolds with 1 hose each they would get water at start up and hot water would get introduced as the thermostat and check balls fluttered.

just asking, not pointing to a cause, funny to me tho how the manifolds are plumbed. Are those manifolds supposed to have those reversion/resonator doo-hickey cones in betwwen the manifold and riser like the 496 manifolds did ?


hope you get it sorted out on the quick

Those tails, are they dry to the tip like the gil tails or only patrially dry just past the elbow ?

These stainless plates with the cone went in between both manifolds and risers, any moisture that got in and ran back down would get trapped and sizzle off with the engine running.

These photos give a little more clarity to your question I think.

that T does have the little plastic balls in. Maybe this will help make more sense for everyone. As I don’t understand why they have two hoses to the manifolds

does the plumbing of my manifolds make since? Is there to much water why do I have two hoses going to them? Seems like the more I dig around here on OSO I have seen several with just one? Or am I wrong?

With raw water and fresh water cooling you need exh manifold water flow to keep them cool until the thermostat opens up.

Then you have the 2 hoses feeding the the exh manifold. Not sure if the thermostat re-directs the flow through the 2nd hose, as it diverts flow to engine then to exh manifolds.

You only have 1 feed out of manifold into the risers...that creates a restriction, vs the 2 input anyway.
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I don't think it's setup wrong, but you may be getting a lot of water from the raw water pump.

Is it just the photo or are the exh man to riser hoses pinched to make the corner?