Polaris 500 is a Dog

Hello everyone, over the summer I acquired a 1995 Polaris RXL 650, but the motor was shot(pto crank bearings were bad), so I decided to swap in my 500 engine out of my old Sled which ran great last year and out ran my dad's 600 yamaha. I had to fabricate a mount for the engine but the clutch center to center is correct at 12 inches. The other day when I took it out, it feels okay when taking off but anything past the low end, its a complete dog. The sled tops out at about 40mph and has ZERO power. I checked the carbs for plugged jets and they're fine also tried swapping the secondary clutch from my old Sled to see if that was the problem but it didn't change anything. Only thing that is different on the Sled is that I'm using the exhaust from the 650. I don't know if that might play a role but I did take the can off and just run it with no can for a few minutes but it made no difference. I am going to try to adjust the belt deflection tomorrow to see if that helps. When the sled is jacked up off the ground, although it bogs and is hesitant, it eventually revs out to its normal 5500 RPMs(the tack is off) and sounds normal. I'm just not sure what's going on with it. Also compression is 120psi which is what its been for the past 3 years. Sorry for the long post. Any help and/or advice would be appreciated. Thanks

Polaris 500 is a Dog