I just pulled out both 502 Gen 6 motors in my Fountain. (25hrs since rebuilding and using the ZZ502 cam) My boating summer was short! Both motors are getting water back into the combustion chamber, rusting the valve seats over time and making this worse. One motor had lots of milky water in the upper end. Compression was down 40-50lbs on the middle cylinders (3,5, 4,6) on both sides of both motors. Also, I'm using RM exhaust manifolds and single plane merlin intakes.
I tore the engines down, checked the degree on both the cams and found they were at 104.5* so definitely too far advanced.
My question is: Would retarding the cam 5* from the 106* position solve the reversion problem or should I look to get 2 new cams? I need to be able to reuse the stock roller lifters and was thinking about a cam with 114* LSA. I spoke to Comp cams and they informed me that more duration overlap on the exhaust side would cut down on reversion. I don't quite understand this. Does this make sense and can someone explain this better? The cam they recommended was 224*/234* @ 0.50, .537/.537 lift and 114* LSA. Any experience out there would be of great help right now.
Thanks much

The ZZ502 cam is vary similar to the HP 500 carb cam. It can work and will make good power. You need to have exhaust risers that mix the water in farther down stream; preferably right near where it goes through the transom..

Changing the installation centerline will solve the problem. I am running the following Crower cam with Stainless Marine manifolds and no reversion issues on a blower application that idles for skiiers constantly....

Gross Lift Int. Exh. Dur-Int. Dur.-Exh. Centerline
Crower: .707 .727 270 278 112

Other than that, go with a better exhaust manifold....Like Stainless Marine or a header like CMI maybe? Just a thought.

The cam you are running is just fine.........

Has any one ever plumbed water to the manifold and then out of the manifold and by passed the risor and then plumbed the water out over the side? Would this work or the rubber boots connecting the exhaust to the silent choice and Y pipe get too hot?
Ghittner: The duration specs on your cam, are they @ 0.50?

how did it end up at 104.5* in the first place??

I believe advancing the cam centerline will help with reversion, not make it worse. Advancing lessens the amount of time the exhaust valve is open while the piston travels down the bore on the intake stroke.

None the less, installing a cam with the centerline called for is generally optimal. After that, extend your water mix as far as necessary to get rid of the milkshake. A pcv valve can help some here as well.

The rubber hose would get too hot. How long is the rubber hose? Can you get a new watercooled tail pipe to replace partof the hose? But, that would probubly cost as much or more than swapping cams.

this is not true.... advancing the cam only moves all of the lobe events sooner, ie: it closes the exhaust valve sooner after TDC
if that is a 112* lobe sep grind it was installed 7.5* advanced on the IC method, if it had 4* advance ground in then it was a 3.5* advanced install, nonetheless, somebody screwed up... 112* lobe sep should really have no reversion issues at all, unlesss the water dump theatre is real close to the riser elbow

Interesting, so closing the exhaust valve sooner after top dead center does not reduce the amount of time the valve is open while the piston travel down? Must be some kind of new math.

Reversion is a direct result of the amount of time the exhaust valve is open while the piston is travelling downward. Lessen the amount of crank degrees the exhuast valve is open on the piston down stroke and you lessen the amount of reversion. Advancing a cam reduces reversion, retarding increases reversion.

BTW, both of my engines reverted water till milkshake until I extended the dry section of my exhaust. My cams are 112 lsa.

BT

i'm sure we're saying the same thing, his concern is the lobe ITSELF is changing events... static overlap in and of itself is one issue, static lobe seperation is another, static IC installed # is another

112* advertised LS, installed at 104.5* IC tells me something is installed wacko

unless of course i'm reading this wrong..