When I run it on the hose, it's nose high as my driveway is a small hill. The angle of the tails id have to measure, but they're far from flat. Figure the V band sits level with the engine.

Bravo 1

Your correct it's 8.5 overlap. Add the intake valve closing to exhaust valve opening to get it. 8.5 isn't that bad NA. I don't know what boats typically run. In my blown and turbo applications on street I run 3 degrees over lap max. Intake centerlines are higher than your 110. It should do it on more than 1 cylinder I would think.

So when I noticed it was when I started to get my boat on plane early (before allowing it to get to 160⁰), it was under powered and missing. After a few seconds it did clear up. All the summer long I always idle out to the channel and let the thermostat open before I get on it which allowed time to clear out the water and start firing. Never noticable of a miss. Knowing I had an issue, I put 8 new plugs in and all of the old ones had rust on them. So it's definitely not one side or narrowed down to a cylinder or two.

What is your idle speed in neutral ?
in gear in the water ?
what icl/ecl did you put your cam in ?
what is your initial ignition timing deg ?

anywaybyou can make your tailpipe pic better ? I’m old with eyes getting weaker and my ohone doesn’t show pics that great…..i’m unable to see what your describing with the tailppioe.

So when I installed the cam,I did degree it on 110⁰ ICL so the exhaust "should have been 118⁰. Ignition is set at 32⁰ full advanced and I believe the idle timing is 10⁰ or 12⁰ but never looked at that. I don't have Idle issues or starting issues so I never felt the need to change the springs or bushings to adjust the curve it amount of advance. Idle in neutral is 900 to 1000 and in gear it's 100 rpm drop so 800 to 900.

That's a reasonable slope. With a stern low attitude, you'll likely have an even better angle.

Some risers have a dam at the bottom, just after the apex, to stop water flow into the manifold ports.

Mercruiser had them on their cast iron risers....then decided to remove them..."what did they meed to be there for"...then a bunch of people were having reversion issues with stock systems.

Makes me wonder if that internal flapper I was told to remove, I don't need it if I keep the external flap was helping stop reversion.

Danimal,

I was led to believe that it's not so much the lobesep itself as it is the degrees past TDC when the exhaust valve opens. It stands to reason that, if the exhaust valve is closed by the time the piston starts back down, there literally CAN'T be any reversion. I'm not versed enough, but, do the math and see where that happens.

On the internal flap... While it would seem that flap might be helping to prevent backflow, it is really just for the purpose of preventing water from being wave driven up the exhaust track while the engine is not running. If you think about it, there are four cylinders contributing to the flow, both exhaust gasses and water, in an exhaust manifold. If a cylinder is going to pull air/water in due to cam timings, that flap won't help.

Thanks. Brad.

Just for clarity, I'm speaking to a metal dam on the bottom of the riser apex.

Assuming you have through transom exh?

Flappers on the tips are important...waves can drive water up the exhaust risers. Stern wake striking the transom will do it as well. Even prop exhaust systems have the internal flappers on the Y pipe system to arrest that...despite the prop being under water a ways.