I have been chasing this problem for a while now. Its a 1989 regal commadore 280 with twin 470 4 barrel carbs pertronix ignition. They start cold fine and run normal. After a 30 to 40 min ride they will not idle. When returning to idle they seem to loadup and stall. Then when restarting have to clear out at high idle return to idle and same thing happens. Once they cool off a bit about 30 min they run normal again. I ran new wire from the key to the distributor for good 12v and carbs were rebuild professionally. Any help would be greatly appreciated.
A couple of things you ca do. Insulate the fuel lines from the pump to the carb with the aluminum foam backed the race cars use. Go online and go to Jegs.com. The other thing is when you had the carbs rebuilt, or did you just exchange the carbs for different ones. The 470 4 bbl is a special carb made for Mercury by Rochester. The internals of the carb –fuel passages, air bleeds and so on are smaller than the ordinary Qjet carb. Mercury had two qjet carbs for the 470, an early one and a later one. They are interchangeable. They have some plastic bits inside the ordinary qjet doesn’t.
Thanks for responding. I spoke with the carb guy and they are marine spec for that motor. I checked the fuel line. One has an updated rubber line and the other has a steel line. I dont understand why both motors have the same problem. They have separate system for just about everything. checked the fuel tank for water thats good. I am going to change the coils and distributor pickup and see if that helps. Also was thinking of changing the ignition breakers. thinking if they heat up maybe the voltage drops when at idle??