[70 RR]

I have a 70 Roadrunner 440/6 w/4 speed and 4.10 gears. When l really get on it, the motor bogs and there is a slight hesitation. After a second or two of this, the motor seems to get caught up and runs like hell. I have checked the timing and set it to 12.5 btdc. The holley carbs are new with an Eldelbrock aluminum manifold, E Street aluminum heads and a little hotter than stock cam. Also Mopar electronic ignition, new Accel wires and new Champion RC12YC plugs. Any thoughts? Thanks.

 

[70 RR]

Welcome to the forum.
Have you checked the springs in the secondaries? It is very common for them to be too soft and allow them to open too quickly, causing a bog. There is one in the front and one in the rear, Holley makes a spring kit with color coding and they can be changed to adjust the speed of which they open. Start with the heavy and medium spring, one in front, other in the rear, then go to the medium spring changed to the medium light, then the medium replacing the heavy.....you get the idea. Get the secondaries to open before the bog, the bog is opening too soon, vacuum will force the seconaries open so go to the bog, then back the spring set back one stiffer to stop it. Works every time.

Thanks, I called Summit and they talked me into 2 spring kits when I probably only needed one. I guess the idea is, the middle carb opens first, then the front and finally the rear. Correct? At the moment, the front and rear are opening too soon and bogging the motor. This really helps, thanks.

 

[70 RR]

Well, not exactly. The primary center carb is 360cfm, so you run around with a 2bbl carb most of the time, the secondaries work together at the same time and are 1000cfm total, working on the vacuum signal to the amount needed to produce the power needed. There is a rod from the center carb and a slot that closes the secondaries down when the gas pedal is not engaged and should be adjusted as follows: with the engine off, both secondaries closed, the rod should not be tight, but just a little slop so there is about .010 clearance of the rod before the secondaries engage. Both front and rear carbs have to be adjusted equally so they will engage equally otherwise a lean or rich condition could result, something you don't want. With the throttle opened on the primary carb, you can then (engine off) move the rod so it smoothly moves through the slot on the carb to open the secondaries equally, if one opens before the other, adjust the connection between the two secondary carbs so they move equally, readjust clearance to the center carb. The two needles of the center carb should be adjusted at about 1.25 turns out from gentle bottoming out, indicating the jets are correct, if it goes above 1.5 turns out then it is too lean, below .75 means too rich, adjust the jetting accordingly to make that part correct. A smooth idle and smooth vacuum signal on a vacuum gauge, divided by 2 and add 2 gives the proper setting for the power valve, figuring your vacuum with a mild cam should be around 14 inches of vacuum, gives you 7 plus 2 is a 9.5 power valve, 12 inches would be 6 plus 2 or 8.5 power valve, which allows the vacuum to drop to a little under half the engine pull before it kicks in. Once everything is adjusted, recommend finding a shop with a exhaust gas analyzer to ensure she isn't running lean at different rpm, makes the valves live longer and prevent heating problems, amongst burning the engine itself up by running lean. The real frustrating part of the 6 pac setup is that the front carb has to be removed in order to remove the bowl of the center carb to make jet and power valve changes, I usually remove one of the two bottom bolts of the bowl and catch the fuel in a spraycan cap so gas doesn't go all over the place. Keep us informed of your progress, and I can help you with everything else.

I will try and find a shop with a gas analyzer and see where I stand. After changing the vacuum secondary springs, the motor is running great and no bog. I would like to make sure that its getting the right mixture though. Thanks.