Bang after fuel bleed

how often do you use this engine,yanmar,s have issues with the exhaust mixing elbow where the internal and external welds crack and allow water back into the exhaust valves and the bang is often a sticky valve . check for rust on the back starter side of the engine and also release the 4 bolts holding the mixer and push the mixer back and look for water or white powder.it is very hard to see the cracks by eye.both my engines cracked before 400 hrs.i replaced my mixers with cast stainless mixers that seem ok so far.The cracking is due to the weld process of stainless steel and re welding will only warp and delay the problem .

thanks all. pretty sure elbow was replaced by previous owner. its just the coincidence of it occuring directly after bleeding fuel and repriming. prior to fuel filters change and bleeding i drained raw water , changed impeller and restarted without issue. water exhaust all ok. after fuel reprime it strarted and ran for a few seconds, long enough for me to think it there was no air in the system, then bang stoppd dead