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I've run into a problem with my 712.
It first happened at highway speeds 55-60 mph with sudden loss of power and a chuffing backfire from the exhaust. It didn't return for a few weeks and now it happens almost every time I drive the truck.
The first few miles are fine with the truck running great. It idles well and doesn't smoke or backfire. Just after I get up to 50-55 mph it starts up again. and keeps acting like it's running out of fuel.

The fuel filter is clean and I rebuilt the fuel pump, but that hasn't helped. I was about to tear into the carbs but I am not anxious to do that.

Any thoughts?

Chuck

Before looking at the carbs, make sure all fuel lines are good and sealed, cloth covered hose can be deceptive. Next check the tank for debris by removing the sending unit and look inside the tank with a flashlight.

Chuck,
Had the same problem with my 712 last year; turned out to be hairline crack at the tank and at the fuel filter connection. When cold, all was well. When warm, crack opened and sucked air. Look at the easy, cheap stuff first.

Sounds like it could be the carb float levels too low. Starving at the higher RPM's.

55- 60mph sounds a little fast in the 712. check the fuel filter same problem here but it was at speeds closer to 50mph.