Monday Mailbag: Tracking Down Causes of Full-Throttle Power Loss
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.
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Now that makes a lot of sense, my 428 pontiac Engibe does the same thing , thank you
I put a big block in my 57 Chevy. It did that until I changed the sock in the gas tank.
The inline electric pump pre mechanical pump is the restriction in the. Fuel delivery.
I think it could be an overly rich condition causing this problem all the same, especially with the 4150 and the low engine temps. As another Facebook reply recommended, I’d put O2 sensors on it. You’d be able to tell immediately if it is too lean / starved or too rich.
I have a very similar set up, with very similar problems. 64 GTO 455, 6x heads, RAIV port matched and the big valves with Crower cam around 292 adv dur. & .520 lift. (approximately, been 10yrs since I installed it) At 3500 rpm she pops &bangs & falls on her face. I have just purchased new comp cams valve springs, & a roll of 3/8 stainless fuel line. Now, I just need to find the time to install it…
Bad coil? Timing gears off a tooth? Bad wire between coil and distributor? (wires will break withing insulation) Try unplugging vacuum advance, stick as golf tee in the hose and drive it that way.
I have a 66 gto W/3x2s after and after installing a small Comp. Thumper Cam, ( sounds great ) I had a lot of trouble with acceleration . Put new fuel pump (no) changed timing (no) new external fuel filter (no) what the problem was not enough vacuum created from cam . Cut a few coils off the spring inside the middle carb that is actuated by vacuum , I don’t remember any part names ! Did the trick also took out that bronze looking internal gas filter ! This motor is no monster but strictly sounds, runs, and looks great !
Agreed that it could be fuel related but I have seen these ponchos run through the rpm range with just an upgraded mechanical pump and stock 5/16 line. One problem we have run across with the same symptoms is valve lash settings. Yes, Pontiacs are suppose to be torqued but once you have milled heads and roller tip rockers you sometimes run into too much preload on the lifters. Out cure has been to use poly locks and adjust them like a sbc.