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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.
 

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Welcome to the forum. Sounds like the secondaries need adjusting, they are opening too soon and bogging, leaning out. Get yourself a vacuum secondary spring kit, contains about five or six different springs in it, all color coded. Start with the two heaviest, as in heavy in the rear vacuum canister and medium heavy in the front. The process is to slow down the secondaries from opening too fast and going lean (the bog), by swapping in the next lighter spring for the previous one until you start to bog once again, then go to the previous setup. So numbered one(heavy) through five (very light), start with 1 and 2, then go to one and three, then two and three, then two and four, then three and four. You will get to the point where they open according to the vacuum signal and not too soon, whcih is what you want. Let us know how it goes.

A second thing you will want to do, also, is slow down your vacuum advance springs, too. Big blocks don't like fast advance, so you should have a heavy spring and a medium spring in it, total advance of 38 degrees, too much will give you a little ping sometimes.
 

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Hey dana44,

Thanks for your input. A friend of mine mentioned to me the "total timing" of 38 degrees. I will experiment with your suggestions this weekend.

70 RR
 

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I was thinking about that last night and couldn't remember if I had added that or not. You or your friend know how to figure that out, right? The butterfly portion of the rotor shaft that is part of the mechanical advance has a number stamped on it, which is either 14 or 17. Double this number is the mechanical advance available, so 28 or 34. From there the 28 is best so you can have (or should have) no more than 10 degrees at the crank, so you should back yours down just a few degrees, kind of hard on the starter at this point, especially when warm, and then the heavy and medium springs under there will make it nice and smooth, especially with those 4.10s in the rear. Oh, and too much advance tends to have a history of both bad gas mileage, and broken pistons, so not worth pushing it.
 

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dana44 said:
I was thinking about that last night and couldn't remember if I had added that or not. You or your friend know how to figure that out, right? The butterfly portion of the rotor shaft that is part of the mechanical advance has a number stamped on it, which is either 14 or 17. Double this number is the mechanical advance available, so 28 or 34. From there the 28 is best so you can have (or should have) no more than 10 degrees at the crank, so you should back yours down just a few degrees, kind of hard on the starter at this point, especially when warm, and then the heavy and medium springs under there will make it nice and smooth, especially with those 4.10s in the rear. Oh, and too much advance tends to have a history of both bad gas mileage, and broken pistons, so not worth pushing it.
My friend has way more knowledge on this subject than I do. I ordered a couple of the vacuum spring kits and will consult with him on the rest. I hope to get on it this weekend, I will give you an update as soon as I do. Thanks again!
 

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There's also a bleed hole on the wall of the venturi of outboard carbs. This hole is drilled into the vacumn signal port feeding the diaphram that the spring pushes against. If that hole size has been changed it will impact the opening point of the secondaries. Check it for dirt.
 

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If the carbs are new it shouldn't be an issue. On the other hand, the hole would be used to adjust the resistance on the back side of the vacuum diaphram and it should not have been altered under most circumstances.
 

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Discussion Starter #8
stroudtom said:
There's also a bleed hole on the wall of the venturi of outboard carbs. This hole is drilled into the vacumn signal port feeding the diaphram that the spring pushes against. If that hole size has been changed it will impact the opening point of the secondaries. Check it for dirt.
My carbs had the second strongest spring (brown) in both the front and rear carbs. I swapped those for black springs and the car ran perfect. Would you suggest putting the brown spring back in the front carb, or leave well enough alone. By the way, I appreciate the help!
 

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You can enjoy it the way it is, or you could experiment just a little bit and see if there is a little bit more to eek out of her. This is kind of where the exhaust gas sniffer comes into play, make sure it isn't on the lean side.
 
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