klein

Hi guys i need your opinion on this topic.

I have 86 Iroc-Z that has a swaped 350 SBC with offenhouser intake and Holley 4010 double pumper 600 CFM.

My transmission is TH350, i am not sure about the rear end ratio but the gears are short, my top speed is 120 Mph at 5000 rpm.

Anyway i was thinking to change the carb to a 600 CFM vacuum secondary...Is it worth to do it?

I have no bog or anything the car runs and pulls fine but its very mild close to stock engine, it made around 200 Hp and 370 Nm at the dyno.

The "problem" i have is that when the engine is cold it runs very rich, the smoke hurts my eyes. Also when it warms up and you floor it it always makes a small puff of black smoke when secondaries open....

I thought that vacuum secondary will be better for all around driving and performance...what do you think?

I was looking at Holley 600 CFM street warrior carb.

Jorlain

iTrader: (4)

Better is really in the eye of the beholder. A vacuum secondary carburetor is more forgiving for the most part, but a properly sized\tuned double pumper can work just as well.

The puff of black smoke is likely due to you mashing the pedal - you get a pump shot plus you're cranking the secondaries wide open - which leads to rich condition momentarily. If the carburetor is properly tuned, and with some practice, you should be able to roll into the secondaries without mashing the pedal to the floor (without performance loss).

I'm guessing you just need to tune that carb... and maybe fix an exhaust leak (the exhaust should come out somewhere behind you, not in the cabin of the car). :P

RedLeader289

iTrader: (2)

I'd stick with the double pumper. And properly tune it. No need to spend money for no reason.

I prefer the mechanical action of the double pumper, I know that the secondaries are going to open every time.

305sbc

iTrader: (3)

If you don't race, then a vacuum secondary carb is a good option. Get one with power valve protection so it doesn't blow out, then learn how to tune and adjust the carb. There are some great books out there for that. Holley has a ton of adjustability.
The Vac carb should be better for your fuel mileage as most of the time the secondaries stay closed and you don't need so much pump shot.

skinny z

A few things:
A vacuum secondary can be every as fast at the dragstrip as a mechanical secondary carb. No ifs, ands or buts.
If fuel economy means anything to you, then the vacuum carb is the way to go. Every movement of the throttle adds fuel to the air stream via the accelerator pump. If you have two pumps, you're adding twice the fuel. The problem is compounded with a manual transmission (although I see that's not the case here).

Now, that said, the best thing you can is to invest in a wide band O2 sensor and gauge. Then you can actually see what the tune is and make changes accordingly. You may find that you can bring your double pumper in line and eliminate the overly rich condition. Beyond that, if your mileage isn't to your liking and you know you have a decent tune, then you can make the switch to a vacuum carb.

JamesC

iTrader: (5)

If you haven't completed Sofa's holley tune, check the sticky at the top of the page. My 600 vac was turned so poorly (by someone who supposedly knew what they were doing) that performance and mileage suffered badly. Though winter has halted my tinkering for the moment, the results below speak for themselves:

https://www.thirdgen.org/forums/carb...ey-tuning.html

JamesC

pancherj

Timing first...then carb.

NoEmissions84TA

iTrader: (1)

And once you get that sorted out, you may want to swap to an overdrive transmission.

Schurkey

http://www.chevelles.com/techref/Adj...tic_Chokes.htm


Be aware that "eye-burning" exhaust can be from a too-lean mixture causing lean misfire. Misfire for ANY reason will produce high Hydrocarbon emissions, which is what burns your eyes.