i dont really understand them but some say there good and others say there bad whats the deal on them. another thing is if the cab has an electronic choke it has a vacum secondarie it seems like. im looking for a new carb but i dont know what to get i want to get somthing around a 600 - 650cfm.

 

I dont like Vacuum secondaries. if you have low vacuum, they dont work right, as in the case of my impala, and we had to bend rods and stuff to get it to work. I like mechanical, because they'll always work. If you have a 289, i wouldn't go over 500 CFM, 600-650 is too much for that small motor.

 

I like mechanical seconds better too, no springs or stuff to mess with trying guess if its right or not.

Carter AFB 600 with manual choke and manual seconds is an excellent carb I recommend it

Gun

 

Vacuums have always said to have been better for lower performance motors. It gives you the fuel when there's a demand for it. Mechs open all the way at full throttle and give too much fuel off the line.

From Summit

 

Mechanical FTW!!! Oh, an dont over carb your engine... as an example, im running a 650 on my engine... and the engine isnt exactly a light weight...

 

The Carter AFB has a counter weighted baffle over the 2nd ventures, when you stomp on it the butterflies open all the way but air flow through the ventures comes on easy because as air volume is regulated by the baffle which opens fully only when sufficient air velocity has been reach.
Its very effected and simple way to prevent hesitation.

Gun

 

Mechanical secondaries are a lot easier to tune in. Vacuum secondaries can open when you don't want them to, such as going down the highway a higher RPM's.

The only downside with the Holley DP is the richer idle circuit, but that can be easily changed.

 

So many answers, so many variables, so little time...


Vacuum works fine. you just have to know how a little bit about them to get them working just right. Double pumpers use more fuel because when you move the throttle two pumps are shooting gas in instead of one.

Both types will open while going down the highway at high rpm, if you are getting into the secondaries you are into the carb pretty hard. The mechanical wont close if there is no demand where the vacuum will close if there is no demand saving fuel.

Holley 4150 carbs are size sensitive, iyou have to be pretty close to get it to work right. A vacuum carb, the 4160, is much more forgiving of over or under carbueretion.

IMHO the best holley vacuum secondary carb is the 780 that came on 428 fords in trucks and other higher performance engines. you can still buy them new. they have metering blocks rather than a plate, so you can change jets in the rear as well as install a power valve. I have one of these carbs and it is a monster on any engine I have put it on. it runs great and I have tricked it out just a tad, a quick change spring kit so I can put a lighter or heavier spring in the secondary dashpot, clear sights for the bowls, and the PV and jets depend on what the engine likes. yeah its a bit big for a 302 but they work wonders on a 351 and larger.

I tend to stay away from the avenger carbs and the single feed 650s, they work good but they dont have the adjustability I like in a carb. And the transefer tube is a pain. I loathe carter/edelbrock carbs. havent found one yet that works good on anything but a small chevy.

if you have a street car, get a vacuum carb and a quick change kit. they are easy to install and you can fine tune the secondaries that way. No bending of stuff needed..

 

The secondary accelerator pump only activates with the secondaries. If you need the secondaries to cruise down the highway, you have other problems (like a serious lack of power). Vacuum secondaries open with the air velocity coming through the primary barrels. They will basically open at a set rpm. Anytime you exceed that rpm, they are open. This is not true with DP's.

 

yes that is true.. but you still move two pumps if you move the throttle too far.. that was the point, not that they work all the time.