Question: I’m having a problem with my engine bogging at 5,000 rpm in Second gear and 4,000 rpm in Third. I think it could be one of two things: my intake manifold or rearend gear ratio. It’s a 454 with Edelbrock oval-port heads, a Comp 268 cam, a 750 Holley, a TH350 trans, and 3.07 gears with 30-inch-od tires. The intake is a Performer 2-O. What about this combo can I change to fix this problem?Kevin Williamson,  Mobile, AL

Chevy Engines

Possibilities: Bog is a vague term that may mean different things to different people. Here, I interpret what you call bog to mean the engine flattens out and doesn’t want to rev or pull at rpm in gear. Yes, Chevy Engines aka “Rat motors” will run better with a Performer RPM intake and 3.55 or 3.70 gears, but the existing hardware should not be causing the car to flatten out at 4,000-5,000 rpm. Almost certainly, you have a fuel-delivery or tuning problem. A marginal fuel-delivery system can be crutch-ed in the lower gears because low gears provide a torque multiplication assist. In the higher gears, the engine must accelerate the weight of the car on its own, which can put a marginal fuel-supply system over the edge.

So check out that fuel pump. Plumb in a pressure gauge and monitor the fuel pressure. Anything below 3.5 psi on the top end is unacceptable-it indicates the pump or fuel-supply system is not keeping up with the engine’s fuel requirements or the fuel filter is clogged or restrictive. A high-flow inline fuel filter is preferred over the restrictive sintered-iron fuel-bowl inlet filters. The fuel line should be at least 3/8 inch or larger; try to keep bends to a minimum.

If fuel pressure is OK, check the pump output volume. A high-perf fuel pump should fill up a 1-gallon bucket in well under a minute. Finally, check the pump’s vacuum draw. As read on a vacuum gauge T’d into the fuel line back by the tank sending unit, there should be no less than 13 inches of vacuum. Low vacuum here means the sock, strainer, or screen inside the  gas tank is clogged or has collapsed.

OK, so the fuel-supply system checks out . . . what’s next? Examine the carb. On a Holley, the primary and secondary floats should be adjusted so the fuel level is just even with the bottom of the sight plug. Be sure the throttle-linkage is correctly adjusted to yield full-throttle and that the secondaries fully open. On vacuum-secondary Holleys, it may be necessary to fine-tune the secondary opening rate using a softer spring. Many vacuum-secondary Holleys come from the factory with a black spring that is too stiff, preventing the secondaries from fully opening. (Note: Vacuum secondaries will only open on a real-world running engine with the car in gear and/or under load.)

With the carb basic adjustments sorted out and no fuel-delivery problems, if the car still flattens out in gear at rpm, ease off the throttle and then get back into it. If the car now picks back up and pulls, it’s a sign the carb’s main system is too lean. In other words, you need to rich en up the primary and secondary main jets.

If you run all of these tests, than nothing at all wrong with the fuel system?

There are many other reasons a bog down can occur, to many to get into in this post. The problem lies in the intake system most likely. It may not be a bad idea to check the timing and valve adjustment too. There are other articles on our blog which may help address this issue, please feel free to search. Our main job at GotEngines.com is to sell engines, that is our specialty, we have had a lot of success with our blog and helping people with questions find the right answer. Call us @ 1-888-344-8044 for any further inquiries.