I have a 454 Magnum (Q-jet carburetor) with an Alpha outdrive. Motor starts fine, idles steady, and makes good power initially but after 20 minutes of 3/4-to-full throttle motor sputters and losses power.
When this happens the motor will start, idle fine, and runs steady up to 1500 RPM but anything more brings back the sputtering. Shutting down for about 30 min. makes the problem disappear, but it comes back again at about the same run time (almost like clockwork).
I thought vapor lock initially but the easy starting and smooth reliable idle never goes away.
The motor is running at normal temps always.
The mechanical fuel pump is almost brand new and is rated at 120 GPH at 5.5 - 6.5 psi.
The water separating fuel filter was just changed.
The fuel lines are not stock (CG-approved stuff from aeroquip) but I ran them as far away from the hot stuff as you can get.
The carb is a rebuilt aftermarket marine unit - not the original but looks like an exact duplicate. (Not sure about jets or pump sizes but it runs great until the problem kicks in). The choke is functioning properly and it opens long before this problem occurs.
Any ideas???
When this happens the motor will start, idle fine, and runs steady up to 1500 RPM but anything more brings back the sputtering. Shutting down for about 30 min. makes the problem disappear, but it comes back again at about the same run time (almost like clockwork).
I thought vapor lock initially but the easy starting and smooth reliable idle never goes away.
The motor is running at normal temps always.
The mechanical fuel pump is almost brand new and is rated at 120 GPH at 5.5 - 6.5 psi.
The water separating fuel filter was just changed.
The fuel lines are not stock (CG-approved stuff from aeroquip) but I ran them as far away from the hot stuff as you can get.
The carb is a rebuilt aftermarket marine unit - not the original but looks like an exact duplicate. (Not sure about jets or pump sizes but it runs great until the problem kicks in). The choke is functioning properly and it opens long before this problem occurs.
Any ideas???
