holley carb problem

hi all new at this so sorry for the mistakes.i have a 750 dbl.pumper 4779 holley on my 406 pontiac that i have bought new a year ago but only used for a month or two then it set the rest of the time till i got my engine back together. now it has a bad spluttering just off idle at about 2000 rpms while driving down the road at a steady speed.i rebuilt it but it still doing the same.changed the jets power valve from 6.5 4.5 still no change any ideal ?also if ican't get it fixed was thinking of buying a 750 mighty demon are they any good thanks.

Try lowering your float levels, sounds like thay might be a bit high giving you a rich condition too early.

Tommy, going to another carb will probably get you right where your are now. There's nothing wrong with the Holley, but you need to tune it a little, and in some cases a lot. Dano is right about the float levels. You can buy some see through sight level screws that can make it easier to adjust float levels. If they're not adjusted correctly it will run like a diesel or cause some flat spots. I've had problems with accelerator pump screws being too tight and it also caused richness. When you set them like Holley recommends, they can allow fuel to dribble going down the road caused by your foot on the gas peddal. There are also air adjustments screws, but thats fine tuning. I'd suggest that you get a Haynes Holley Carb book, or one like it, as there is a lot of good information in it. Don't let my ramblings confuse you, these carbs are not that hard to set up for you car. When you change something, do only one thing at a time. Don't change jets, power valves, etc.. Adjust the floats and drive it to see if it improved. Then go to next adjustment. That way you'll know what adjustment made improvement and what didn't. Let us know how it worked out.