Q: I have a 455-cubic-inch Pontiac with a pair of cylinder heads from a 400. The compression ratio is estimated at 9.3:1. The engine was completely rebuilt by a professional engine shop with forged pistons and crank, good valve job, roller tip rockers, double valve springs, etc.
Here are some of the components I have:
- Holley 4150 carb, 750 cfm with #71 primary jets, #76 secondary jets
- Edelbrock Performer RPM manifold
- 67 gph electric fuel pump, 120 gph mechanical fuel pump
- Camshaft: 231/243-degree duration at .050-inch lift, .480-inch lift, installed straight up
- 1 3/4-inch headers and 2 1/2-inch dual exhaust with crossover pipe
- Electronic ignition with Accel Supercoil and 8mm wire
- 160-degree thermostat
The problem is, the engine cuts out completely at 4,500 rpm under full throttle. At 3/4 throttle, the engine will rev through 5,500 rpm. The car idles fine and runs fine until it hits 4,000 rpm, when it starts coughing. At around 3,500 rpm, the power kicks back in and engine runs fine until it hits 4,500 rpm and starts the cycle all over again.
Here is what I have done to correct the problem, none of which cured it:
- Adjusted the timing set 10 degrees
- Replaced the ignition with a stock points-type
- Tried a Quadrajet carburetor
- Fed the carburetor with just the electric fuel pump
- Disconnected the secondaries — engine revved through 5,500 rpm but with less power
- Replaced the secondary jets with #80s
- Checked electric fuel pump for proper operation
What else can I look at?
A: We think your problem is fuel-related. Check the fuel pump pickup inside the fuel tank. The factory installed a screen at the end of the pickup to keep rust from getting into the fuel line and fuel pump. If this screen is clogged, it can become a major restriction.
The second thing to check is the fuel line. If you are still using the stock 5/16-inch line, we would upgrade it to at least 3/8-inch, with 1/2-inch line being even better. You will have to increase the size of the pickup line as well.