Best to run a valve up to your seat to turn on the water, those bassett springs suck. I went dry cause I had so many issues with the water

I just put on a set of basset headers with the supplied basset tee valve all new it's installed correctly my concern is the water seems to come on to early around 1200-1300rpms should I have a gate valve in front of the tee valve I run a gate valve at the pump and I run a 5/8 dump   


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Do you have a gate off the pump? What temperature does the engine run?

If the motor runs really cool at idle, throttle down on the gate valve until you see about 150°F and the header valve should come in later.

If the header valve still comes in too early, you need a slightly larger dump fitting. 500hp needs apt a 3/8"dump outlet.

GT

The dump fitting at the transom is 3/8 with a5/8 hose the motor runs at 140-150 and I do run a gate valve at the pump hp is closer to 600 should I change the dump fitting to something bigger 1/2-5/8 maybe. When I get off the gas and bring it dwn to idle it wants to stall until it clears the water out of the headers 


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Go to a 1/2" dump fitting. If it's an aluminum one you can drill it out. Maybe go to 9/16" first. I'm guessing that 1/2" will be the ticket.

500 hp is about the limit on a 3/8".

GT

The Bassett "T" is a small on/off regulator, it keeps water out of the exhaust below about 1500-1800 RPM, that's normal. In the -8 fitting on the "spider" (that's the small lines that screw in the headers themselves) there is a double male that connects it to the feed hose. That fitting, if correct, has a divider in the center, and that divider has a hole from .080" to .120" diameter to regulate the amount of flow into the "spider". It sounds as if you either don't have that proper fitting, or the hole is too large. These exist because there is a great deal of difference in the pump pressure and block pressure found in jet boats. What was fine with your near stock engine will be too large when the new engine spins the pump faster and to a higher pressure.   
Always replace the calibrated spring and ball in the beginning of every season too, they rust and corrode.  Timinator