Thread: carbs for the street

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hi,
I keep seeing eveyone talking about that vacuum secondaries are the best for the street and i was wondering what is the differance between that and the mechanical secondairies?
Im asking this because i have been trying to get my carb all set in and i have a 750cfm holley 4150hp carb, with mechanical secondaries and was wondering if there is any tricks or anything to make it better for the street. I just had the carb fully tuned and everything, but just wondering if it will ever run good on the streets.

thanx,
david

Ther's a lot to be said for Holley double-pump mecanical secondary carbs. There is a seperate, full-size fuel supply for the secondard bowl assuring that extended secondary use won't result in fuel starvation. Because there's two pumps and more positive and sooner opening secondaries, you don't have gas hangin in the rear floatbowl till it gets stale. You also usually get a real rear metering block, which makes tuning the rear as familiar as tuning the front. They are the better choice for an engine that has poor vacuum.
Having said all that, I've got a couple of vacuum secondary holleys that work fine. I don't really save any gas with them but I suppose the potential is there

well i do have poor vacuum in my engine, and i have vacuum advance hooked up but im wondering if its even helping and deciding if i should really keep it hooked up or not. Just learning the whole thing on timing with mechanical and vacuum advance.

thanx
david

A little more info on your engine would help, what CI, cam specs, heads, ignition, etc.

Originally posted by daveid
well i do have poor vacuum in my engine, and i have vacuum advance hooked up but im wondering if its even helping and deciding if i should really keep it hooked up or not. Just learning the whole thing on timing with mechanical and vacuum advance.

thanx
david

Some vacuum secondaries are factory set to start advance at 6" of vacuum and they're all done at 12" of vacuum.

Mid 60's SBC as an example, but other SBC's have a different vacuum advance curve.
Just depends on what you're trying to do.

A fairly radical cam in an SBC will idle approx 9-11" of vacuum so you have enough vacuum to work with as far as the vacuum advance goes.

Here's a fairly long article on timing.
http://www.jalopyjournal.com/forum/s...light=Got+Time

It's also on this board, but I can't find it.

its a chevy 454 with msd ignition system, edelbrock rpm heads, with a 292 cam, 570 lift. My vacuum is only 5hg at idle.

thanx,
david