[Mo]

I agree with Al...

My thoughts on this one:

I would go right to exhaust manifold outlet. Remove the hose off the outlet closest to the hot exhaust section (the rear one) and try to put your finger in it. It may even look like there is water there so stick you little finger or something down in the hole and ensure it is clear. Another hint to this being the issue is if there was a smell of rubber. This would occur when the rubber on your exhaust hose heats up with the disproportionate amount of water and hot gases in the exhaust. If that turns out to be the case you will need to remove the exhaust manifold and clean it all out. Remove carb, choke attachement, throttle attachment. Disconnect hose off the end of the hot exhaust. If the hot exhaust breaks during removal it needed to be replaced anyway...again, not a major. Remove the 3 bolts holding the manifold...slide it to the side an inch or so to clear the studs and then move the whole manifold and hot exhaust forward and toward the cabin...keep it all together and pressure wash the manifold clean ...you could bring it to a rad shop either and have them clean it out...should be easy for DIY er though.

If that checks out double check the impeller. Some impellers don't fit correctly and leave a space between the cover plate and the impeller itself...there are cases where they were actually the wrong size. MMI has the right gear. It needs to be a tight fit. If there are wear score marks on the plate you can actually secure the plate and sand it with a course sand paper on a random orbital sander until flattened out...then put it back on with gasket or permatex and it will work better.