Trevor Baker (22/134 Wight Seal) had that problem on his 1GM10. The only symptom was that he was only getting about 1hp out of the engine! The elbow looked fine from the outside. Inside the sleeve that should direct the cooling water down away from the engine had developed a leak in the weld that let cooling water back into the exhaust port resulting in a blockage of black concrete (presumably salt and carbon) reducing the port to about 1/4 inch diameter!
[img]http://i144.photobucket.com/albums/r189/geoffatstanpit/Yanheadold.jpg[img][/img]http://i144.photobucket.com/albums/r189/geoffatstanpit/Yanexhaustnew-1.jpg[/img]
If that worked OK you should have pics of the elbow and port as was and the same new.