Need some carb help

So my 81 vette has a crate 350 in it with a 800cfm carb (previous owner had the work done). Well i noticed it had a bog (almost like it was flooding out the engine) in it when you gave it an instant full throttle and i noticed the exhaust from the car had some black in it so it was running really rich. I took it in and had it tuned up and it runs a lot better now but there still is a bog at low rpms when you give it an instant full throttle. I was reading on car craft saying that a good size for a carb on a 350 would be 600-650 cfm or there are adjustments i could do to my current car like rejeting and so forth. I don't know a whole lot about carbs so any info on that would be very helpful and also if there is something else that im over looking that could be a causing this problem. There is about 2000 miles on the new engine so i didnt have any idea what it could be besides an oversized carb. What do you guys think? Also im looking at putting new weatherstripping on the car and was wondering if there is any tips or anything i should be aware of that might make my experience with this a little easier with less headaces. And one last thing... My antenna is stuck up and how do i go about getting it to go down. Would pulling the motor off and sliding it down manually work? It doesnt really matter to me to have it to pick stations up at the time. I'm going to be working on a sound sytem at a later time and will deal with getting it to work properly then. I just want to get it down so i can get a car coverly completely on it and so i dont get some idiot riping it off in the parking lot. Thanks ahead of time for all of your help!
 
Blake

    I am not a carb expert either but it sounds like you are having a problem with the accellerator pump. My Holley 750 had a flat spot when I got on it and my mechanic ended up reworking the accellerator pump, he actually wanted it to deliver more fuel on acceleration.

Can't help you with the antenna or the weatherstripping.

sounds like good stuff. How do i go about checking the accelerater pump?

Hi Sweetness224, the 800 CFM carb is more than the engine can use, 600 - 650 would be much better suited to the 350. Depending on the brand of the carburetor - Holly, Edelbrock, Rochester, some fine tuning may get you to a more decent running package. The accelerator pump is one area of the carb that can be tuned. Also the type of secondary throttle actuation (mechanical vs. vacuum) is a key component to drivability. Try to figure out what brand and type of carb you now have and get back with us. Many times in an attempt to produce more H.P., people will "over-carb" the engine, that is what your dealing with at this point. I'd think about a new carb. better sized to the 350.

In my experiance, the carb tune and timing are crucial to smooth acceleration. And yes, I agree, sounds like your engine is over carbed.
My 454 only calls for a 650 to 750 cfm. I could go to an 850, but then I'd just be wastin' gas.

Im away at school atm and my baby is back home in the garage but i 85 percent sure its an edelbrock. After your guys info i do think its over carb, anyone got a good suggestions on some good carbs to but on it, im roughly at 350-400 hp right now. I want something that would be good for drivability too but yet good power.

Here's some carb links:
http://www.lbfun.com/Corvette/Tech/vettetech_files/Lars%20-%20tune%20a%20holley.pdf
 
http://www.lbfun.com/Corvette/Tech/vettetech_files/Q-Jet%20Tuning%20Paper.pdf
 
http://www.lbfun.com/Corvette/Tech/vettetech_files/Selecting%20A%20Carburetor.pdf
 
http://www.lbfun.com/Corvette/Tech/vettetech_files/Carburetor%20Tech%20Info.pdf
 
Haven't read all of these yet but here's my $.02 worth.
 
You can get away with a somewhat oversize carb if it has vacuum secondaries.  Since you said you get black smoke when accelerating, it sounds like the carb is jetted too rich.  If you have a Holley, the accel. pump is tunable for shot size, duration, and how it comes in.
 
Unless you are interested in racing, I'd recommend getting a Q-jet.  They're easy to work on and setup.  Good performance and economy.    Of course that assumes you have a stock type manifold for spread-bore type carbs.

intake is a torquer 2 i beleive, at least thats what the local corvette shop said

According to Summit Racing, that is a single plane manifold which by design works best in the upper rpm band.  Would recommend looking into a dual plane manifold for street use.

So swaping intakes and a new carb in the 600-650 cfm range should give me a lot better performance?

.....and better mileage.

Thanks for all the info guys. When i get it out this next week or so since its starting to get nice around here... finally... I'm going to see if i can find out for sure what carb and intake it is exactly so i know where to go from there. Is there any specific spots where i can find some model numbers on the intake and carb?

Of course the other thing to check when you get the "stomp it bog down" symptom is timing.  Not only base timing, but check the vaccuum advance goodies to make sure they are working ok.  Often, the extra shot of fuel will arrive before the vaccuum can catch up enough to advance the timing to take advantage of it.  Kind of a flooding while running kind of thing.  Once the revs come up a bit, the vaccuum comes up too, and advances to make the burn.

bill.

Thats another good point, but i had a local classic car resto shop work the timing which made it run better but still has that bog in it. So i think the timing is okay. I'll have to double check that though.

If you don't want to go back to stock, I'd check Edelbrock cuz they have some good dual plane manifolds and their version of a Q-jet which is supposed to have some improvements.  Of course you need to know if your cam is stock too because if the PO put a single plane manifold on with a big honkin' carb, he might have put in a cam to take advantage of them. 

If you do decided to go stock, I can look up the P/Ns for you.  They may be outdated but your parts guy can probably get an updated number.  Of course they may no longer be available which means the search is on.  That's half the fun. 

I am probably risking a butt chewing from my friends here... but here goes.  If it all possible try to find a manifold with the same height as a stock one.  Single planes do work best at higher rpms, but I believe Edlebrock is making a new style double runner that is low enough to fit under a stock hood.  I used a taller Edlebrock two plane mainfold on my 454... and opened a huge can 'o worms trying to shoehorn it under the hood.  I had to use drop base air cleaner from a '69, and finally found a 2.5' taller stock style hood that would clear the intake.  These guys are right on carb size also,  650 max is all you need.  If you decide to go with the Edlebrock, order the off road spring loaded needle seats, it helps keep the right amount of fuel in those side mounted float bowls.  You will definately see the difference when you accelerate.  You might also want to check into a Pertronix Ignitor II breakerless ignition kit and a hotter coil.  A hotter spark than stock means better burn in the cylinders. Worked for mine!

Like I mentioned before, you can use a carb in the low 700 cfm range if it uses vacuum secondaries.  The original Q-jet was around 730.  I can't remember the exact number and too late to look it up.

I have a 79 L-48 with some mods and a holly 600 is more than enough.  With just a small block 350, I am still able to get a nice chirp out of the tires when shifting to the next gear.  Stock 3 speed auto transmission.  I have a different intake, it says performance on it I think and nice exhaust but I am not sure what is inside the engine.
 
The only draw back to the holly 600 is the fuel intake is on the driverside of the carb, meaning you have to run either a rubber fuel line from the fuel pump or manufacture a fuel line your self.  I have the rubber fuel hose, not sure if they sell a metal one that will go from the fuel pump over to the driverside of the the carb.

As I have previously posted, GM used steel fuel lines for a good reason, to prevent fires.  Since you can get steel tubing that already has the correct fittings on it, it's easy to make your own.
 
Just need to get a good tubing bender and the correct length of tubing and you've got a hard line from the pump to the carb.  Highly recommended.

Hi Sweetness224, congratulations on getting a Vette on your own at a young age.
I thought that most stock Q-Jets were rated at about 750 cfm so 800 cfm is just a bit too large, ( someone may correct me on that ), when you first take off you are runing on the primary side of the carb, as rpm increases the secondaries start to open. At 3000 to 3500 rpm they are only open about half way, as you reach the red line they are still only open about 95%. Now you shift the secondaries close a little untill you again reach the red line, then you shift into high gear and near the end of the run everything ( primary and secondary are wide open).
73shark hit the nail on the head if you are not using vacuum secondaries than the they open too fast and your car bogs.
If you drive for fun a duel plane manifold and a smaller carb will give you a nice fast ride.
See if you can find out what stall speed your converter is and what rear gears you have, and do you have headers?
PG.

Hi again, the weatherstripping replacement is not a hard job, just get some 3M general purpose adhesive cleaner (at the auto parts or paint store) and have some patients getting all the old stuff off.
Just cover the seats and carpet with something like a old towel, and get black adhesive to put the new weatherstripping on with.
 
I have a fixed antena so someone else will answer that.
Have fun and enjoy your ride, PG.

One other thing about the weather spripping, tape it in place dry then apply the adhesive, that way it won't be an inch too long when you get to the end of the door. PG.

If you are thinking about replacing your carb here is a pretty neat website thats great if you dont know what size would workbest for your car.
http://www.holley.com/applications/CarburetorSelector/CarbSelection.asp
hope it helps

Like 73shark says:

quote:


You can get away with a somewhat oversize carb if it has vacuum secondaries.

And like Lar's says in one of his papers:

quote:


Most Q-Jets are 750 cfm carbs. This is more airflow than most small block engines can ever handle. Yet, GM used Q-Jets on everything........ The secondary airvalve on the Q-Jet effectively makes the Q-Jet a variable-cfm carb.

Many Edelbrock carbs are Rochester Q-Jets knock-offs with a few improvements, and this is quite possibly what you have. If so, you should be able to keep it, and just make the necessary adjustments.

Stay away from any new Holley carb.  They are junk.  The quality that previsouly went into Holley carbs is gone.  I have had two brand new ones in 12 months and they were both junk.  Both made the car sputter, burp, jerk, jump and flat spot.  The first one was a 575 and the second was a 650.  Both were Street Avengers.  I retuned the accelerator pump the vacuum secondary spring, rejetted both of them and still each of them gave me nothing but fits.  I was beginning to think that I had a vacuum leak somewhere else on the car.  I plugged all of the vacuum accessories and still no improvement.  I finally got sick of fighting with the Holleys and installed a 625 Road Demon.  By god, it was the Holleys.  Right out of the box, the only thing that I did was set the four corner idle, adjusted the electric choke and set the idle at 700 RPM.  The car now runs perfect.  That being said I have a 750 Holley on the wifes 69 Camaro that has been on there since 2000 and it still runs perfect.