Have you verified there is actually water on your plugs? You could remove the rave valve housings and see if there is water on the raves. Real easy - two bolts each, you don't even disconnect the air pressure lines, just lift them out as an assembly. Once they are out, you can look right into the cylinder and see the piston. If you take off the plastic cover for the PTO coupler, you can turn the engine over by hand and get a real good look at the edge of the piston and top of the piston.
I recently had a head gasket fail on my 951, and it showed itself by oil/water seeping out from under a head bolt. The compression was still about 125 psi. The base gasket under the cylinder block can also leak, as can the exhaust manifold gasket and head pipe gasket. If the head pipe gasket leaks, it will sometimes only do it when you're near top speed. That's because at high speed the jet pump is forcing more water into the cooling hose and making more pressure. It can spray out of the head pipe gasket right into the carburetor. Its a very common thing on the 951.
During my last engine repair, I discovered that my engine drain line was full of sand and completely plugged. Thats the clear 3/8" line that connects to the crankcase not the 1/2" black line that comes from the head. Its under the exhaust pipe coming off the side of the crankcase down low. Once I cleared that, my side pisser had less force. So that showed me that the engine drain line relieves some of the water pressure in the engine. Less pressure on the head pipe gasket = less chance of failure. Yours could be clogged too and possibly increasing the chances that your head pipe gasket could spray water at high speed. After a high speed run, look at your engine and carbs to see if they are wet.
Something else that recently happened to me was air bubbles in my fuel line. Even though the fuel system was clean, a connection was allowing a small amount of air in. You could some intermittent air leak like that. You could splice in some clear fuel line to observe the fuel flow.
jjsinaz is right about the fuel cap. If you have a failed fuel system check valve, as you use fuel a vacuum will be created in the gas tank and your fuel pump will pull less fuel. Have you tried running with the cap off?
I recently had a head gasket fail on my 951, and it showed itself by oil/water seeping out from under a head bolt. The compression was still about 125 psi. The base gasket under the cylinder block can also leak, as can the exhaust manifold gasket and head pipe gasket. If the head pipe gasket leaks, it will sometimes only do it when you're near top speed. That's because at high speed the jet pump is forcing more water into the cooling hose and making more pressure. It can spray out of the head pipe gasket right into the carburetor. Its a very common thing on the 951.
During my last engine repair, I discovered that my engine drain line was full of sand and completely plugged. Thats the clear 3/8" line that connects to the crankcase not the 1/2" black line that comes from the head. Its under the exhaust pipe coming off the side of the crankcase down low. Once I cleared that, my side pisser had less force. So that showed me that the engine drain line relieves some of the water pressure in the engine. Less pressure on the head pipe gasket = less chance of failure. Yours could be clogged too and possibly increasing the chances that your head pipe gasket could spray water at high speed. After a high speed run, look at your engine and carbs to see if they are wet.
Something else that recently happened to me was air bubbles in my fuel line. Even though the fuel system was clean, a connection was allowing a small amount of air in. You could some intermittent air leak like that. You could splice in some clear fuel line to observe the fuel flow.
jjsinaz is right about the fuel cap. If you have a failed fuel system check valve, as you use fuel a vacuum will be created in the gas tank and your fuel pump will pull less fuel. Have you tried running with the cap off?
Engine heats up... compression goes down and so does performance. I've seen engines with missing and broken rings get over 130psi. To me any degradation in compression is a sign of problems. I am quick to do a top end. I'd rather pay now... than pay later and pay more. LOL 
