Reversion is an idle-only thing. Exhaust gas velocity overcomes it off-idle.

30 degrees is a little too conservative on timing. If you have excessive compression for your fuel, less timing won't realy compensate for it. If you have too much cam for your compression or your exhaust's ability to outflow, it will also be a dog. You really need to find out exactly where you are on these issues before you can get a true understanding on what's going on inside your engine. Those RPM's are way low for a small block. It should be able to run 5800 forever. If it has steel cranks, good rods & 4-bolt blocks, add another thousand.

6800?? I don't know many people running near 7 grand. Actually, I've never met anyone running those RPM's in a small block powered boat.

Merc specs are in the 4800-5200 RPM range. add a few performance related parts and you might be in the mid 5000 range but any higher than that usually requires to much cam for stock marine exhaust.

Like said, you will only get water at idle unless you have MAJOR problems. Being you are having intake valve problems is definitely heat related.. could be caused by bad fuel, wrong timing, wrong plugs, propped wrong, etc... All these things will cause cylinder temps to go up. Do you have any internal or external flaps? If not this will also be a water on the plugs problem if the idle is set to low or the water is able to back up the exhaust at idle or sitting with the motors off in choppy water. Also, how much exhaust fall do you have? Do you have any spacers to get the riser higher? From what I remember of the 28 Checkmates they set low in the water. It sounds like you have multiple problems here. Start with one and fix it then to the next. Once you know you have the problems fixed correctly, leave it alone and enjoy boating!

6800 with 4-bolt, forged crank & good rods. I had a set on my 30 Velocity raceboat and ran the 2 full seasons at those speeds. Not occasionally either.

5800 on the light-duty components.

On a race boat I agree. You have a Bravo behind it? Standard SBC/Alpha 1 combo at those RPM's won't last long.

It's unusual to have the intakes tulip. Intakes are cooled by the incoming fuel air mixture. Must be a lot of heat or low quality valves. Make sure you use good valves like Ferrea or Manly severe duty. Exhausts tulip and then if there is a lot of reversion they shatter usually taking your whole engine with it. If you like to run hard, use inconel exhaust valves. I agree with the guys that say richen the fuel mixture. You might also want to put a fuel pressure guage on it to make sure it isn't starving for fuel at high rpm's. Getting the rpm's up into the low to mid 5000's will reduce the chance of detonation also.

TRS. You're right- an Alpha isn't anywhere near a performance drive.

I have had the valves replaced twice, and I'm now looing into spacers to get my riser fall higher, the 28 checkmate dose sit very low in the water, also I have external flaps on the 4 inch tips. thanks for the info

My boat has Brovo 1's it's and 1989 checkmate maxxum, I belive that the motors were 330 EFI, and then changed over to carbs, bored 30 over, mild cam's just a valve eater.....I'll get it, you guy's gave me alot to look at.

So you have no internal flappers?