3/8" line should be used on all V6 engines. 5/16" is for the V4's. Sounds like a fuel problem. The old-style dual premix pumps should provide adequate fuel to run a high compression 235, so should be just fine for your 150 (165 hp version). Agree, when at WOT, have a friend constantly prime the fuel hose bulb-this will force extra fuel into the carbs. If it runs better as long as you constantly prime the fuel hose bulb, its, a starvation problem. Could be: 1. wrong pumps (undersize) 2. undersize fuel hose from the tank (5/16 instead of 3/8"). 3. bad ball valves in the fuel hose bulb. (try a factory 3/8" hose.) 4. loose fuel pickup inside the tank, permitting air to get sucked in at the top where the tube meets the tank pickup head. 5. Incorrect fuel hoses to the carbs-generic hoses which will bend (restrict) when bent into 90 degree curves. These should be special hoses which are sold by part number by Bombardier. 6. You could have a fuel restriction inside one of the carbs-which could include debris in a jet. The reason for the up/down rpm's is that the demand for fuel outstrips the engine's ability to supply it to the carbs. Will run fine at WOT for a while, till the carbs run out of fuel. Then it will slow, allowing the pump to catch up and refill the carbs. Then the rpm's will increase again. This will repeat over and over-any time over 4000 in your case. Again, make sure you have the correct part number dual pumps (they all look the same) and the correct fuel distribution hoses when you converted from VRO to the dual pumps.
