2001 Mercury 50hp four stroke cuts out at 4200 rpm

[Ranger620]

My mechanic is pulling his hair out trying to figure out whats up. I bought the 50hp motor off ebay its very clean used very little. He cleaned the carbs and has adjusted it to idle perfect, fuel pressure perfect, changed plugs and wires any ideas? Any help would greatly appreciated

[JUST-IN-TIME]

test fuel pump and new gas tank
check for spark