Carb guys in here?

[bcjjones]

I have a 95 c1500 2wd with a 28” tire running a posi with 3.73 rear gears and an AR5 5 speed swap trans.

Motor is a fully built/forged 383, aluminum promaxx max 200 heads, dual plane intake, 10.6:1, Crower roller cam 236/240 .555/.559.

Stout motor, gunna add a small 100 shot of nitrous at some point soon. Street driven truck with little to no track time.

Currently running a street avenger 770. Runs good. Did a secondary tube mod on it to help the secondaries open sooner. Has the quick change secondary spring pod and has one of the lighter springs.

Based off the set up, weight of the truck and the manual trans, would y’all choose a mechanical secondary over a vacuum? I found a really good deal on an almost new pro systems 750 mechanical secondary I’m thinking of trying but figured I’d ask opinions first?

 

[Erik the Awful]

I can go either way on this, but I think a lot of guys don't like vacuum secondaries because they don't know how to adjust them properly. If you have a good deal lined up on a mechanical secondary carb, why not?

 

[Hipster]

Same here on vacuum vs. mechanical. I don't understand this "tube mod" because in my experience when vacuum secondaries open too quick it causes overfueling and the lightest spring in the kit is usually too fast.

On the used carb, what was built for somebody else's combo might not be right for yours. So it's a gamble. A second reason I don't really mess with used carbs it because it may have been tampered with like a "tube mod" or some other goofy mod.

 

[Erik the Awful]

Most carburetors don't have changeable emulsion tubes, aka air bleeds. Typically a 'tube mod' means removing the factory pressed in tube and threading the hole to allow screw-in tubes.

Carb tuning in a nutshell:
Jets control mixture at low and mid-range rpm.
Emulsion tubes control mixture at high rpm.

Throw in metering rods and it gets even more fun. I'm no pro at carb tuning. I'd have to sacrifice some chickens to get it right.

 

[Schurkey]

There's an entire sub-section devoted to carb issues.

LSX + Carb Swaps

This section is for info on LSX or carburetor swaps into the OBS trucks.

www.gmt400.com

No way would I recommend a double-pumper on a street-driven vehicle. A vacuum-secondary can be tuned to run as good as a double pumper on the street, with most engines--and with better fuel economy.

As said, jets on a Holley are tuned for part-throttle use. Heavy-throttle enrichment is done with the power valve, and the power valve channel restrictions. Too many folks dick with the jets instead of the power valve/channel restrictions, then they can't figure out why the fuel mileage is terrible and the throttle response is less than optimum.

 

[Supercharged111]

There's an entire sub-section devoted to carb issues.

LSX + Carb Swaps

This section is for info on LSX or carburetor swaps into the OBS trucks.

www.gmt400.com

No way would I recommend a double-pumper on a street-driven vehicle. A vacuum-secondary can be tuned to run as good as a double pumper on the street, with most engines--and with better fuel economy.

As said, jets on a Holley are tuned for part-throttle use. Heavy-throttle enrichment is done with the power valve, and the power valve channel restrictions. Too many folks dick with the jets instead of the power valve/channel restrictions, then they can't figure out why the fuel mileage is terrible and the throttle response is less than optimum.

They do jets and idle screws and think they're done. Not even close to close.

 

[Frank Enstein]

Leave it a vacuum secondary. Put a 245 to 250 @ .050 cam and we'll talk double pumpers!

More in a few minutes!

 

[Frank Enstein]

Step 1) Buy this book.

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Step 2) Buy a good vacuum gauge like this one. https://www.summitracing.com/parts/wmr-w80594
Step 3) Read the book.
Step 4) Go down the rabbit hole with Alice!

This book is written in English. For Humans!

No Alien Hieroglyphics. No Chinese Algebra!

It's a fat magazine with a bunch of pictures.

You will learn very quickly why you want to keep the Vacuum Secondary Carburetor.

I suggest getting;

A jet kit

Reusable bowl gaskets

Nylon bowl screw washers

A .025" shooter and a pin vise and number drills from #61 to #80 (but not right away)

And if funds allow a wide band O2 sensor. Not necessary but it can help you get it tuned faster.

The guys with street cars fooling around with their carbs every weekend do not know how to tune!

And finally tape "My Buddy's" mouth shut before you buy useless stuff that wastes money and doesn't help.

 

[Hipster]

I have Vizard's version of that book floating around somewhere. Very comprehesive.

 

[Frank Enstein]

David's book is very good but it builds on the Dave Emanuel book. I haven't seen a replacement for the one I recommended. The publishing company idiots discontinued this book. I screamed at them for 5 years and it ain't coming back. Fortunately I have sold 2000 copies of that book so there are still some out there used. The one I copied the image from was less than $14!