I have just fitted a 750 vac secondary to my pontiac trans am gta L98 auto...it has Dual point vac advance dizzy set at 8" btdc and a performer manifold a mechanical pump a xe268 cam with high rev lifters and hardened ball pushrods have not fitted headers yet still stock at moment.
i have set the idle mixture using a vacuum guage and it runs sweet at 800rpm.
i took it down the highway today i put 36 litres of gas in and punched it a few times short 1/4 bursts, when i punch it hard it bogs and it only done 128 miles on 36 litres of gas which worked out to be only 17 miles per gallon which i dont think is good how do i get rid of the bog and do i need to drop a few sizes on my main jets????
Any ideas????:hail:

 

The bog is the secondaries opening up before the engine can use it. There are two ways to adjust it:

1) The "classic" way, change the secondary diaphragm spring to a heavier one.

2) A Quickfuel Adjustable Secondary Housing (Jegs has them). Instead of messing with the spring, you adjust the vacuum signal via a needle valve.

Of the two, #2 is the right choice, because it adjusts how soon the secondaries open without affecting how many RPMs it takes to fully open them.

Either way, you want to adjust it so it doesn't bog and you can't feel the secondaries opening (if you can feel them opening, it's bogging).

 

What you are saying is true but the Bog is while the vehicle is stationary ie : at traffic lights and i hit the gas trying to pull away and its embarassing it bogs in the first few feet of pulling away and sometimes stalls so i end up blocking the highway and having to start the car again...

 

Whole different story. That's not typically called "bog" but stumble or dead spot.

That sounds like accelerator pump either not working or not being sufficient. With the engine off and air cleaner removed, look down the primary venturi and open the throttle by hand. You should see two healthy squirts from the shooter as soon as the throttle starts moving.