Time for a new Carb(advice needed)

[gadget73]

How small can those things be had, though? The Rochester carbs available on the aftermarket I've seen (Quadrajets) have usually been 750cfm.

oh yeah, http://www.thermoquads.com/ 800 cfm up to 1100 cfm. Those things are monsters, the rear bore is over 2" per barrel.

[Archangel]

How small can those things be had, though? The Rochester carbs available on the aftermarket I've seen (Quadrajets) have usually been 750cfm.

There was also an 850cfm version, forgot which engines got it tho. However you're missing the fact that Q-jets allow engine to pull only as much air as it wants/needs - essentially on a 305 you can floor the pedal and make the butterflies go WOT, but you're still gonna have the air door open just halfway cause that's as much as the engine can handle. Actually you should know that already, don't Edelbrocks use an air door too?

As for how small the go - my Holley being a spread-bore is a 650cfm unit and is the smallest in the 4165 family, next one up is 800cfm and the big one is 950cfm.

[Pirate]

I am beginning to suspect that it simply didn't run right 'out of the box'.

That Holley should have worked just fine, had someone actually listened to what I had said to do with it originally, provided everything else was operating properly.

Here is the simple skinny with vacuum/mechanical secondary carbs: If your uncle/cousin/nephew's former roommate suggested you get something Holley-based with a mechanical secondary, then he is an idiot. Mechanical-secondary-based carbs, unless it is one of the Edelbrocks, is quite possibly the worst carb you can put on there. Why?

1. Automatic transmission. Do you have a 5000-rpm stall convertor? No? Then your motor won't be lauching at anything above 2500 rpm. You do not need two sets of accelerator pumps at that low of an RPM, not on a 306, unless someone in your general 20-mile radius knows how to lean it down to where the pair of pumps can work properly. I can do it, but it takes me at least a week of experimentation to get it right.
Dual accelerator pumps come in handy with stick-shift applications, especially when launching at 4000 rpm or so, under maybe a 1/4 throttle, and then flooring it right after....at that higher RPM, the 306 can handle the fuel load that the dual accelerator pumps are capable of, without bogging the motor.
In fact, I will not recommend a mech-secondary carb for any automatic, unless there is so little vacuum signal, where the vac secondaries wouldn't work. I can make a double-pumper work on an automatic, but not very many people can, however, and the extra tuning headache isn't worth it. Somebody simply thinks that tossing a double pumper at this car will cover up for another problem at this point...


Good luck, however.

[SLYDAWG]

Carb tuning

I must agree with much of what has already been said here. Unless that carb is warped and does not mate well to the throttle plates, or the float bowls don't match up, there is no reason a rebuild kit would not straighten things out.

I must also agree with NOT using a double pumper on a heavy car with a small engine. My 81 bracket car is putting out an estimated 350hp and it has a vac. sec. carb on it. The secondaries can be adjusted to open at any time, much better for a dd.

If you are dead set on buying a new carb, I recommend the Avenger series from HOLLEY, vac. controlled sec., jetable sec. instead of a metering plate, 50cc acc. pump, adj. floats, quik change spring kit, reusable gaskets. All the std. Holley parts fit. Co$t in the $300 range. The 670 is big enuff, as this is what I have on the racecar.

[Mr. Land Yacht]

well..what I got is 600cfm...but I wont say anything more beyond that.

[gadget73]

mmmhmmm, what they all said. Huge carbs, and mechanical secondaries, and definitely double pumps are not going to perform better, or probably even as well as what you currently have can perform with the right adjustments. Holleys are very tunable. This is good and bad, but with the right work its absolutely possible to make one run extremely well.


The only reason that the big cfm OEM carbs (Thermoquads, Quadrajets) work as well as they do on even small displacement engines is the very nature of the vacuum secondary setup. Actually TQ carbs use an adjustable spring to tension it, Qjets have weights. Edelbrock/CarterAFB carbs also use weights on the air door. The more the door opens, the more it pulls on the metering rods, and thats your secondary fuel mixture. Not terribly complex, but it does work rather well and makes for an easy to tune and rather streetable motor. The motor just won't get more air than its capable of pulling, so it runs smooth without having a toilet flushed on it whenever you hit the go pedal.

[1987cp]

hmmmmmmmmm.................... given your propensity for shopping at SummitRacing.com, I guess logically that would limit possibilities to one of the following:

Edelbrock 1406
Edelbrock 1400 (emissions-legal)
Edelbrock 1409 (marine application)
Edelbrock 14064 (1406 with EnduraShine)
Holley 4160 Marine
Holley 4150 Marine
Holley 4160 Adjustable Float (dunno what that means ....)
Holley Custom Speed Shop 4160 (chrome/purple)

Any guesses from the peanut gallery on which one it is?

[Archangel]

Mike, adjustable floats when applied to Holley means that there's the screw-and-nut deal on top of the fuel bowls that allow you to adjust the floats (and thus fuel level) from the outside, which is how you want it. "Non-adjustable" floats are actually still adjustable, but you need to remove the fuel bowls for that, and the adjustment itself ain't too accurate as it's done by bending a the tab that moves the needle into the seat.

[Pirate]

hmmmmmmmmm.................... given your propensity for shopping at SummitRacing.com, I guess logically that would limit possibilities to one of the following:

Edelbrock 1406
Edelbrock 1400 (emissions-legal)
Edelbrock 1409 (marine application)
Edelbrock 14064 (1406 with EnduraShine)
Holley 4160 Marine
Holley 4150 Marine
Holley 4160 Adjustable Float (dunno what that means ....)
Holley Custom Speed Shop 4160 (chrome/purple)

Any guesses from the peanut gallery on which one it is?

I think he is going to go with the Holley custom chrome/purple 4160, and buy purple/chrome lace Speedo underwear to match.

Ick.

[Mr. Land Yacht]

I bought from a website that Scott recommended...the prices were cheaper.

[Archangel]

I actually like that 4150, it seems to be the only carb of the list to have a secondary metering block vs. the silly plate, and it's got the external float adjustments too - the bad thing tho is that the secondary linkage is way too short to allow any modifications to opening point, it also seems pretty linear once it starts moving the secondaries.

Colin, enough of this, what the heck did you get? Or at least when is it scheduled for delivery, so we know when to expect news about it?

[1987cp]

Hmmm..... so the selection of suitable 600cfm doodads from this unnamed website is likely to be slightly different than what I listed. We shall anxiously await news of your purchase!

[Mr. Land Yacht]

well, I just checked my email and it happened to be shipped already..it shows it In Transit for Wednesday 10/17.
so I'll prolly have it wednesday nite.

[Archangel]

tease :p

[Mr. Land Yacht]

well, to tease you a bit more, I'll tell you that it does have a chrome-like finish.