Learn me somethin' 'bout carbs... - Team Camaro Tech
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post #1 of 22 (permalink) Old Sep 5th, 14, 07:09 AM Thread Starter
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Learn me somethin' 'bout carbs...

I was looking around Ebay for a used 4bbl, 390 CFM carb for my 250 and saw prices all over the map. I've rebuilt my 1 bbl without much trouble and know I can handle rebuilding a 4 bbl if I have good instructions (which aren't hard to find) so I'm not worried about that. Rebuild kits are about $50 so I'm fine finding a used carb that needs TLC and rebuilding it if the price is right.

So... why the massive price differences?

For instance I just found this: http://www.ebay.com/itm/4-BARREL-650...16fe8f&vxp=mtr

Yeah it's a 650, not a 390. But if I rebuilt it myself can I change the jets to make it squirt less and work with my 250? For $56 shipped would it be worth a try? Is there any reason NOT to put a 650 on a 250 with a Clifford intake, headers and 2.5" exhaust?

What about this: http://www.ebay.com/itm/Vintage-Holl...b2efce&vxp=mtr

Same questions but even cheaper.

Here's a 390 CFM Holley for a few dollars more: http://www.ebay.com/itm/Holley-390-C...e8dd29&vxp=mtr

Not know enough about carbs I'm leaning towards something like this. Yeah/nay?

Give me a computer, a fuel rail, and some squirters and I can work with you but these old metal boxes with floats just confuse me...

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Langdon split header to a 2.5" exhaust
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post #2 of 22 (permalink) Old Sep 5th, 14, 07:41 AM Thread Starter
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Re: Learn me somethin' 'bout carbs...

Also electric vs. mechanical choke? My old stock exhaust manifold had a mechanical choke on it (spring attached to a rod). What would be required to do an electric choke? If I went to a mechanical choke what the heck to I connect it to now that I have aftermarket headers?

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Offy intake
Holley 390 CFM carb
Langdon split header to a 2.5" exhaust
Mild cam
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post #3 of 22 (permalink) Old Sep 5th, 14, 07:49 AM
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Re: Learn me somethin' 'bout carbs...

I would go for the Holley 390 cfm you posted the eBay link to. It's a good deal as new they sell for $450

Send the seller a pm and offer maybe 200 shipped.

The 600 is just too big to make work. It's not the jet size. It's the cfm. You won't get the velocity needed for it to work.

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post #4 of 22 (permalink) Old Sep 5th, 14, 07:51 AM
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Re: Learn me somethin' 'bout carbs...

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Originally Posted by Gbauer View Post
Also electric vs. mechanical choke? My old stock exhaust manifold had a mechanical choke on it (spring attached to a rod). What would be required to do an electric choke? If I went to a mechanical choke what the heck to I connect it to now that I have aftermarket headers?
All you need is 12v switched for the electric choke. 5 minute job. Best way to go.

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post #5 of 22 (permalink) Old Sep 5th, 14, 08:02 AM
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Re: Learn me somethin' 'bout carbs...

Agree with John. You want the electric choke and the 390 will have smaller barrels than the 600/650. This means better response on the small cube motor. The stock 1bbl carb for that engine was probably 200CFM or less. Can't wait to see you get this 6 going!

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post #6 of 22 (permalink) Old Sep 5th, 14, 08:03 AM
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Re: Learn me somethin' 'bout carbs...

I agree with the recommendation to go with a Holley 390 CFM carb, that would seem a very good fit for your needs.

Another alternative is a Quadrajet. Although it is a 750, it has small primary bores for good low speed tuning, and the top plate on the secondaries can be tuned to restrict the airflow to just what the engine needs by simply adjusting the spring tension. This is not unique to the Q-Jet, but it is one example of a carb that you can do this on.

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post #7 of 22 (permalink) Old Sep 5th, 14, 08:20 AM
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Re: Learn me somethin' 'bout carbs...

Quote:
Originally Posted by Gbauer View Post
I was looking around Ebay ...

So... why the massive price differences?

Supply and demand is some of it. A 390 is a small cfm carb and not in much demand, but those that need a 390 have to fight for the few that were made. The condition is the other deciding factor; at least if you exclude those deluded individuals that have no idea as to the value of items).

For instance I just found this:

Yeah it's a 650, not a 390. But if I rebuilt it myself can I change the jets to make it squirt less and work with my 250? For $56 shipped would it be worth a try? Is there any reason NOT to put a 650 on a 250 with a Clifford intake, headers and 2.5" exhaust?

You can not change the main jets to reduce the flow of air through the carb. It is fixed by the size of the venture cast into the main body. If you reduced the amount of gas flowing through the carb it would run lean and it will either over heat your motor or put it into detonation breaking the pistons. A 650 is designed for a 350 V8 and is too big for a 250 L6.

As to the price I buy 650 cfm 4160 series carbs for $25 at every swap meet I attend. Add shipping and it is definitely not worth the price.


What about this: http://www.ebay.com/itm/Vintage-Holl...b2efce&vxp=mtr

Same questions but even cheaper.

Same answer with the added problem of a Ford carb not having the linkage on the passenger side of the carb to actuate your transmission kick down linkage if you had a PowerGlide or TH400 tranny.

Here's a 390 CFM Holley for a few dollars more: http://www.ebay.com/itm/Holley-390-C...e8dd29&vxp=mtr

Not know enough about carbs I'm leaning towards something like this. Yeah/nay?

A Holley 390 cfm four barrel was made specifically for six cylinders and VW four bangers that are raced as gassers.


Give me a computer, a fuel rail, and some squirters and I can work with you but these old metal boxes with floats just confuse me...

Actually I have been a Holley aficionado ever since GM abandoned Carter AVS carbs back in 1963. However for a six cylinder motor I have to agree that EFI is a much better solution. This is because of fuel distribution issues caused by the long log manifold with a centrally located carb. To optimize your motor you need an aftermarket Offenhauser three one barrel carburetor manifold and three one barrel Rochester carburetors off of a four one barrel Corvair flat six. Then you would have to make your own linkage to actuate the three carbs.



This would still be easier than trying to adapt the EFI off of a newer "Atlas" in line 250 cube six cylinder engine out of a new Chevy SUV or van. The easiest solution of course would be to just pull the complete motor and computer with the factory EFI wiring out of a new Chevy and stick it into your Camaro. Definitely be unique at a car show. Probably cheaper also since the in-line engine is unloved and as such cheap in bone yards.


A turbocharged Atlas six cylinder DOHC motor in a Camaro



The DOHC with covers removed:

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post #8 of 22 (permalink) Old Sep 5th, 14, 08:22 AM Thread Starter
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Re: Learn me somethin' 'bout carbs...

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Originally Posted by Vega$69 View Post
All you need is 12v switched for the electric choke. 5 minute job. Best way to go.
12V switched to what?

Does the carb have a delay built into it where it shuts off the choke after so much time? I'm not sure I understand how the electric choke works.

Sold:
1968 Base hardtop 250 with a P̶G̶ built 2004r
Offy intake
Holley 390 CFM carb
Langdon split header to a 2.5" exhaust
Mild cam
Custom interior
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post #9 of 22 (permalink) Old Sep 5th, 14, 08:25 AM Thread Starter
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Re: Learn me somethin' 'bout carbs...

Quote:
Originally Posted by Vega$69 View Post
I would go for the Holley 390 cfm you posted the eBay link to. It's a good deal as new they sell for $450

Send the seller a pm and offer maybe 200 shipped.

The 600 is just too big to make work. It's not the jet size. It's the cfm. You won't get the velocity needed for it to work.
Thank you!

The carburetor sizing calculators are saying about 350 CFM so I'm sure a 390 would work well with the Clifford intake.

Next question: If I'm going to increase the intake and exhaust valve diameters, smooth out the ports and add a larger intake would that change the sizing of the carb?

Sold:
1968 Base hardtop 250 with a P̶G̶ built 2004r
Offy intake
Holley 390 CFM carb
Langdon split header to a 2.5" exhaust
Mild cam
Custom interior
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post #10 of 22 (permalink) Old Sep 5th, 14, 08:34 AM
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Re: Learn me somethin' 'bout carbs...

Head work is a good idea. But the six cylinder head has a lot of work beyond just larger valves to even begin to make the same power. To achieve that goal you have to actually go to a V8 head which of course isn't offered anywhere so you have to make your own (they are made by others and offered for sale but they are not cheap).





I suggest you read Leo Santucci's book on hot rodding the six cylinder motor to learn more about this subject.

http://www.amazon.com/Chevrolet-Inline-Six-Cylinder-Power-Manual/dp/1931128154/ref=sr_1_fkmr0_2?ie=UTF8&qid=1409927591&sr=8-2-fkmr0&keywords=Leo+Sanducci%27s+Hot+to+hot+rod+Chevrolet+six+cylinder.

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post #11 of 22 (permalink) Old Sep 5th, 14, 08:34 AM Thread Starter
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Re: Learn me somethin' 'bout carbs...

Quote:
Originally Posted by Larger Dave View Post
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Thanks, Dave, but I want to stick with the born-with drivetrain. I know it will never be a monster but it will be good enough to cruise and enjoy.

As to the intake manifolds: I've been looking into it quite a bit and from what I've read and seen online there isn't really an advantage to going with (3) 1bbl carbs vs (1) 4bbl. The heads on these engines are weird. I had a long talk with Tom Langdon about it when I bought the headers. I was going to make a custom intake manifold with my neighbor, inject fuel right before each intake port, and put a GM throttle body up front to control it. Turns out GM tried this for about a decade and could never get it to work right. Tom was part of that team. Everything I suggested GM already tried and showed it wouldn't work.

It's the damn siamese ports on the head that's the problem. Unfortunately there's no way to fix it short of dropping $2,000+ and, well, it's a 6, not a V12.

Tom said a single 4bbl works best. His experience with the L6 is pretty darn deep.

...even though I'm not blazing a new trail it's still fun!

Sold:
1968 Base hardtop 250 with a P̶G̶ built 2004r
Offy intake
Holley 390 CFM carb
Langdon split header to a 2.5" exhaust
Mild cam
Custom interior
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post #12 of 22 (permalink) Old Sep 5th, 14, 08:36 AM Thread Starter
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Re: Learn me somethin' 'bout carbs...

Quote:
Originally Posted by Larger Dave View Post
Head work is a good idea. But the six cylinder head has a lot of work beyond just larger valves to even begin to make the same power. To achieve that goal you have to actually go to a V8 head which of course isn't offered anywhere so you have to make your own (they are made by others and offered for sale but they are not cheap).





I suggest you read Leo Santucci's book on hot rodding the six cylinder motor to learn more about this subject.

Chevrolet Inline Six-Cylinder Power Manual: Leo Santucci: 9781931128155: Amazon.com: Books.

Big Dave
Looks like we're racing on the replies.

As said above: I'm not looking to build a drag car. Just a fun cruiser with a bit of punch. I'd be happy around 250 rwhp. Right now I won't even put it on a dyno because I don't want to be embarrassed....

edit: already ordered the book. Should be here soon...

Sold:
1968 Base hardtop 250 with a P̶G̶ built 2004r
Offy intake
Holley 390 CFM carb
Langdon split header to a 2.5" exhaust
Mild cam
Custom interior
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post #13 of 22 (permalink) Old Sep 5th, 14, 08:37 AM
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Re: Learn me somethin' 'bout carbs...

Quote:
Originally Posted by Gbauer View Post
12V switched to what?

Does the carb have a delay built into it where it shuts off the choke after so much time? I'm not sure I understand how the electric choke works.
The choke closes when it's cold. You push the pedal one time and it sets the choke and idle stop. When you turn the key on the 12v starts heating the resistance wire/spring (think toaster) after a given time the spring unwinds and the choke is fully open. When you turn the engine off it will stay open until engine cools off.

Here's a more detailed explanation

http://www.hotrodders.com/forum/how-...work-7696.html

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post #14 of 22 (permalink) Old Sep 5th, 14, 08:48 AM Thread Starter
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Re: Learn me somethin' 'bout carbs...

Quote:
Originally Posted by Vega$69 View Post
The choke closes when it's cold. You push the pedal one time and it sets the choke and idle stop. When you turn the key on the 12v starts heating the resistance wire/spring (think toaster) after a given time the spring unwinds and the choke is fully open. When you turn the engine off it will stay open until engine cools off.

Here's a more detailed explanation

http://www.hotrodders.com/forum/how-...work-7696.html
Perfect! I understand completely!

You're absolutely right. That's DEFINITELY the way to go.

OK so I'm looking for a 390 CFM, electric choke carb. I want reliability and street-ability. Holley? Rochester? Demon? Any specific models?

You guys have been very, very helpful. I would have made a huge mistake if I hadn't asked here first.

Sold:
1968 Base hardtop 250 with a P̶G̶ built 2004r
Offy intake
Holley 390 CFM carb
Langdon split header to a 2.5" exhaust
Mild cam
Custom interior
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post #15 of 22 (permalink) Old Sep 5th, 14, 08:49 AM
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Re: Learn me somethin' 'bout carbs...

Quote:
Originally Posted by Gbauer View Post
12V switched to what?

Does the carb have a delay built into it where it shuts off the choke after so much time? I'm not sure I understand how the electric choke works.
12V switched to something downstream from your ignition switch so it is only hot in the ON position. The 12V applied to the choke coil causes it to expand. It expands over a period of time, say 5 minutes. During this time the choke is on, fast idle occurs and the choke butterfly slowly moves from closed to open. When you kill power to it by turning off the car the coil contracts to it's original size(after a period of time) and the cycle will start over next time you start the car.

Drawbacks- leaving key ON will result in choke being energized and may not work as intended after a period of time with a cold engine.

- Electric choke may operate when the engine is warm enough not to require it. A divorced choke activated from engine heat would not engage at that time.

The good news is Holley electric chokes are easy to adjust by loosening 3 screws and turning the choke housing.

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327 L30, K-K, Deluxe int, tach & gauges, 12 bolt posi, 4 speed.

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