tell tale problem

[Orlando]

ok, i got the engine running the right direction wich was the previous post on this motor,, it is a nissan 15hp ns15b2, but i dont think this problem is model relevent. it had been sitting for a few years when i got it, i rebuilt carb, and replaced the entire ignition system. when i was working on it i knew the impeller was shot because it had been sitting, so i only ran it a couple minutes at a time. so i havnt let it overheat badly. i replaced the impeller the other day, and run it in a 50 gallon barrel, pumps water fine at idle, good steady stream from tell tale, however if i increase rpms say from 700 to say 1500 (about halfway from the slowest setting to the start position on the tiller handle)the tell tale stops almost immediatly and it looks like exhaust coming out, water also stops coming out the exhaust ports on the back, then the motor starts to warm up, but if i lower the rpms again the water starts flowing again and the engine cools right back down. there was no thermostat installed when i bought the motor, and i have a new one that is currently back out of the motor. no difference. this is the first time i've really messed with an outboards cooling system, so any suggestions are greatly appreciated.