My brother is running a motor I put together a few years back. I'm having a tremendous amount of difficulty dialing in the junky Holley 670 Street Avenger in.

I'm new to carbs other than Holleys... my 6.0 LQ9 runs a Proform carb but he's rather hung up on using a Quick Fuel.


I built the motor about 4 or 5 years ago but from what I remember it is a 350 Vortec (L31).
Steel shims (9.5:1 compression) .
Edelbrock RPM Air Gap dual plane manifold.
Comp Cams XR276HR cam (224/230 duration, .536"/.544" lift, 110 LSA and 106 ICL) with roller rockers. We are talking about 6,000 rpm shifts.

Car has a full exhaust (mid length headers, off road y pipe, etc). 3.73 gears, built 700R4 with 3,000ish stall. Full weight 1987 Camara RS (maybe 3,300lbs?).


I keep telling him vacuum secondaries . The 750cfm carb seems a bit big to me but his friend is telling him 750 with mechanical secondaries.
Any opinions?

 

zraffs, I have run a few qf carbs. I have one on a 355 9.5 motor. I have raced other motors, engines over the years. The latest is a 650 4 corner idle. From the box to the Dyno
- no issues, no adjustments were required. And, I mean none. The motor made 403Hp at 5350 and 423 tq. This is my 2nd Qf carb, the other is on a 470 stroked Stage 1 Buick.. That one is a 950 4 corner idle. 562 hp, 588 Tq. I have messed with Holleys, demons, Qjets, carters. You can basically name it. The qf. Definitely outshines all others. Personally, I have had nothing but issues on a vac secondary. It is a struggle to get them to open when you want them to.
As the Dyno guy says, you bring me a motor to Dyno with vacuum secondaries carb on it and I will tie the secondaries open so you can see what your motor can do.. I have had every motor I have ever built or had built on a Dyno for break in and adjustment. (40 years of racing), every one did better with mechanical secondaries.. Remember, you can adjust mechanicals to open when you want them too. You can only do so much, and too little with a vacuum set-up. My opinion.. Take it for what it is.. Byron

 

Interesting. I have used both secondaries. I've never had any problems with vacuum secondaries in the past... this is actually the first Holley I can't remotely tune... if I take the off idle hiccup out, it returns much harder higher in the RPMs. I told him i would keep trying and he said he'd just buy a new carb. For the minimal driving he does with the car I suppose a mechanical secondary won't be a bad choice.

I love my Proform. It shares the main body of the quick fuel carb from what I've been told. I will have to call quick fuel and see what they suggest.

 

i've had good luck with AED carb's...and there a little cheaper

 

If you insist on using a vacuum secondary carb, then at least get the adjustable type that meters the vacuum signal instead of changing springs to alter opening characteristics. Much better approach to achieving the desired goal.

But, having said that, the only application I can think of that would benefit from a vacuum secondary is a small tire class where you need to limit launch power. Otherwise, use a double pumper.

 

So what about on a strictly street motor were fuel mileage isn't a concern?

 

I'd go double pumper. Having had the experience once on the street of the vacuum secondaries staying open momentarily after I let off the gas, I decided I didn't like them for the street anymore.

 

This information may be coming too late but I changed out the primary metering block with a billet QF metering block ($80) and nearly all the issues everyone has (including myself) with tuning the Avenger line of carbs vanished. Any performance issues I have now are just due to the carb (570cfm) being too big for the stock little 305. The reason? Emulsion orifices. The stock block on the avengers only have 2 while the billet has 4 for each primary. Plus you can adjust your power valve fuel orifice size as well as your idle air bleed jets. Makes tuning with a wide band a science instead of an art.