Carburetor Question

[jayb]

If your build is similar to Jim's, I'd get the 750, or maybe even a 700.  I think an 800 may be a little big...

[JimNolan]

I wish I knew why mechanical secondaries on an automatic car get such a bad rap.  The old rule of thumb that you should use vacuum secondaries on an automatic and mechanical secondaries on a stick car is a really gross oversimplification, IMO.  I like the positive action of mechanical secondaries, and I've never had a problem tuning a mechanical secondary carb to work on an automatic car.  Jim's combination is pretty mild, and the vacuum secondaries obviously work well, but mechanical secondaries would work fine too.  And I've always found that while vacuum secondary carbs need some signficant tuning to work on most combinations, mechanical secondary carbs are pretty much a bolt on and go proposition.The only thing they really give up is mileage (and who cares about that LOL!).

Jay,
   You're right I'm sure. The only thing I know about vacuum secondary's or vacuum advance for that matter is what I've been told. The only thing that has led me to my understanding carburetors is word of mouth and the factory installing these items even on their performance cars. I've fought these carbs for years trying to figure out why they stumble, making my own accelerator pump cams and changing nozzles or power valves so I could have performance as well as drivability. I should read your book more often.

AMDSCOOTER.  A 10L weight is equal to 20 degrees of advance. I'm used to welding up my own using a predetermined limit opening to get what I want, so I'm just used to signifying the results in what you find imprinted on the weight. If I ran a serious performance car just for drag racing etc. I'd get a MSD distributor or equivalent to have better control. As it is, my car winning on the drag strip is TOTALLY dependent on my ability to repeat my dial-in, no matter how fast or slow it is.

PS. This 750 is probably the best "out of the box" carburetor I've ever bought. I set the float levels, idle adjustments and secondary spring. I am well pleased with the carb. Thanks My427stang for telling me what to get.

[amdscooter]

^^^ Duh..  me<== smacks head. 10L = 20 degrees. I should have remembered that from when I pulled my stock dizzy and the options were 13~18L. I'm really glad to hear you have yours dialed in.  I'm going to take the lazy route and get a 700~750 double pumper with an electric choke and hopefully be able to call it good under the hood for now. I need to get my buckets redone so I have somewhere to sit this summer as I'm driving it around. There is a spring poking out the front on the drivers side keeps scratching the back of my leg.   

[Rory428]

First off, a properly working Holley with vacuum secondaries will not produce a "kicking in" feel. Since they open smoothly, under load, as the airflow is needed, they will offer a smooth, linear surge as they open. Also, there is no reason why a vac sec Holley carb will not make the same power as a equally sized double pumper. My 454 Fairmont has gone high 9`s with an ancient 3310-1 Holley 780 vac sec carb that has NOT been heavily modified. Hell, it still has the choke tower in place, the only "mods" are a bit of experimenting with sec diagphram springs, accelerator pump cams and squiters, and jetting, as well as having a broken off  baseplate mounting ear welded back on, hardly anything trick. I have tried several other carbs over the years, including 750,800,and 850 DPs, a 830 annular DP, a 950HP series, a 1050 Dominator, as well as 960 and 1050 "3 barrels". None of these carbs were any quicker at the dragstrip than the 780 vac. Also the 780 had the best idle quality and drivability in the pits and while staging the car. So the old rumor that a double pumper is always better for a manual trans car seems to be a myth as well.