So I changed the impeller today as well as the Prop. Went from a 21 pitch vengance prop to a 19 pitch.. Thankfully the overheating at idle problem is gone. Engine runs awesomely cool now and that brought a huge smile on my face. Felt like quite the achievement fixing it on my own for 50 bux versus paying the boat yard 450+.. I replaced all gaskets impeller and impeller housing.. Engine temp at idle hardly goes beyond 130 degrees! Now my only other mechanical issue is the lack of top end speed and performance from my boat. The engine is new, I installed the 19 pitch prop today while doing the impeller and thought that I would get quite a few RPM more from the engine as well as a lot more speed.. I thought that dropping the pitch would make the holeshot slower but what I realized is that the holeshot felt much much stronger! I was quite surprised at this.. I gained about 500RPM as well and 3-4 mph on the top end.. The boat is still a slow poke though, it tops around 40mph.. That is at just over 3/4 throttle.. I then decided to put the throttle to WOT and when I did the engine began to bog and sputter, when i felt this I immediately backed off and when I did she backfired just a bit.. Now I know this prob will never be a 65-70mph Boat but I fully expect it to run at least mid-high 50's as it did when it came from the factory according to Wellcraft and a few reviews from the 80's that I dug up online.. What gives? The boat runs strong at every RPM except WOT.. even at WOT it wont pull more the 4K RPM and then it bogs and loses power!? Do you guys think this is a fuel delivery problem? A carb problem? Im stumped as to why she wont pull the full 4800-5K RPM with a brand new engine and a much better prop? Should I drop down even further to a 17Pitch prop to gain the extra RPM? Or should I be looking elsewhere in the actual engine compartment for my problem? I guess I dont know much about what engine the previous owner put in the boat other than it is a new longblock 350 mercruiser that appears to be stock.. It has corsa exhaust and silent choice as well as an updated serp belt pulley system.. Starts right up and sounds absolutely beautiful at idle and under load.. Oil pressure is perfect, and she holeshots like a champ now with the smaller pitch prop.. The 21 was KILLING it in the holeshot and on the top end... Im Lost!! Sorry for the long rant and looking forward to any and all suggestions..

Are you going lean? Sounds like you are running out of gas and that is a lean pop you are hearing. What's the timing?

I would check several things. 1st start basic with the gas. Make sure you are using fresh gas and also an additive. If you are using "Pump Gas" the ethenol can play funny things with older engines. I run an additive each tank, I forget the name but I get at at West Marine that aids with the ethenol and if there is any water in the tank.

Second check your fuel filter or even replace. That's a cheap fix and something I do every year as well. I remember in my Scarab one, I went thru three filters in one year do to something that was in the tank from the PO. Just gummed everything up.

Next is the carb. Could need an adjustment or could need rebuilt. Either way not all that expensive. Hope that helps.

good suggestions thanks all! I will replace fuel filter and check for basic tune up as well..

Some things not right,find it before you change props again.Might also want to check your fuel pressure.
This is what I would do.
1) compression check (doubt its internal but rule it out)
2) fuel filters and pressure check.
3) adjust/rebuilt/ or replace carb depending on what it needs
4) plugs/ wires/ cap and rotor unless they are new
5) timing check also check distributor to make sure advance is working (actually this should be higher on list)
6) check ignition system bad coil? would also show up under load