Rochester 2 barrel vacuum ports - Pontiac GTO Forum
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post #1 of 11 (permalink) Old 04-16-2017, 09:12 PM Thread Starter
 
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Rochester 2 barrel vacuum ports

I have a 67 Lemans 326, 2 speed auto trans. When I bought the car it had an Edelbrock 1406 and a performer intake manifold. I am no machanic, most of what I know is from asking questions and watching youtube videos. I tuned it the best I can and it drives okay, but it has only modest get up and go from a start and very poor mpg, about 8 around town. I have the original rochester 2 barrel and manifold. I would like to put the rochester back on and maybe get better mileage so I can at least enjoy cruising around town and not break the bank. What I need is pictures or diagrams of what hoses goes to which vacuum ports on the rochester. Thanks for the help.

Last edited by gcstone; 04-16-2017 at 09:57 PM.
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post #2 of 11 (permalink) Old 04-17-2017, 06:35 PM
 
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The Edelbrock carb is rated at 600 CFM's - and that is wide open. Probably a little big for your application, but not 8MPG's bad unless the choke is not fully opening, idle mixture screws are adjusted too rich, the jetting is set rich or the carb is just flat out not set-up correctly and you are dumping gas into the engine. Your plugs would probably be fouling out at that rate. The factory AFB was rated at 500CFM's, but you should be able to get the 600 to work. Also, distributor timing is very important on Pontiacs. This has been covered in depth in calibrating the distributor's advance curve. The vacuum advance is also an import part of the set-up and is aimed at providing improved gas mileage - it must be working correctly.

If you swap to the original 2Bbl, you could still be disappointed and get poor gas mileage if the ignition/distributor/vacuum advance turns out to be your problem.

Here is the Edelbrock instructions for setting up their carbs. The 1406 is found here and you will see how to set the carb "leaner" with both smaller jets and smaller metering rods. You can also change out the step-up piston spring that raises the metering rods. If you have the wrong one, this will cause the metering rods to be raised up to soon and run rich. They give you the factory set-up your carb came with and then each step makes the carb "leaner." http://www.edelbrock.com/automotive/...ers-manual.pdf

You also want to know what kind of vacuum the engine is pulling. A vacuum test can also tell you a lot about the engine. So my recommendation is to do a few checks before swapping out the carb. It may be a carb problem, it may be an ignition problem, or it could be something completely different not related to carb/ignition.
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post #3 of 11 (permalink) Old 04-17-2017, 08:02 PM
 
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Those are all excellent points. Just another thought if you like a snappy throttle and fast response is to not overcarbuerate the engine.

Holley makes a brand new 2 barrel street avenger that will make your 326 very snappy and responsive. Now from 70 mph to 100 mph you won't have anything,....but that carb is real nice and about 350 bucks brand new.

PJ is right about the timing and rochesters are great if all set up and rebuilt right and you can handle it, but that 2 bbl Holley street avenger is hard to be. We all tend to discount the importantance of the velocity of the air pulling through the Vacumn Venturi of the carb...

That velocity makes a strong draw on the mixture and creates a very snappy throttle response,...of course as PJ said timing etc must be correct as well. But a engine overcarbed, ez to do on a 326 will be sluggish as the air has no velocity through the centuries, just sort of passing by slow and it effects everything, undercarb a little,....too big won.t help.

Racers and guys with stroked 455 killer engines giant valves etc they need big volume carbs. As far as Vacumn, really depends on needs, power brakes? You have an auto trans modulator, Vacumn blend doors and should have full vac to distributor. You can tee those off pretty ez.

Make sure you have an operating PCV system as well....!

You will get it and that 326 will surprise you!
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post #4 of 11 (permalink) Old 04-17-2017, 09:02 PM Thread Starter
 
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Lemans guy, I am interested in your idea. I am not interested in building a race car but I would like a better throttle response and MPG than I get now. Like I said I am not really a mechanic and would like something that is basically ready to go right out of the box with just a little adjusting. I am looking at the Holley site, which CFM? Will the Holley connect to my throttle setup or will that need to be modified? Also, what are your thoughts on changing the trans to a th350?
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post #5 of 11 (permalink) Old 04-17-2017, 09:45 PM
 
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Gc, the 2 bbl Holley Street Avenger 350 CFM will give you the snappiest throttle and great mileage. It is a bolt on tight out of the box.

I bolted one on my 326 and have Air fuel meters in both exhausts, ....it plugged in as a perfect 13.8 at idle, light thro title cruise is 14.3...

Sometimes 14.7...that is all a computer will get you, but I have my timing right....as far as throttle set up they have a Chrysler throttle cable thing looks Like a u shape I think, and maybe a part I grabbed off the 2 bbl rochchester and it fir right up...

The TH350 is what I put in. I had the Pontiac 300. Huge difference and money well spent. It is a bolt in, you may have to make sure you have the right ujoints but that is easy.

A Holley street avenger, a TH350, I put in a Dual Gate shifter in the 350, makes a great cruiser,.".real responsive, and gas mileage is good, I never rally notice it....

But again timing is everything there.....
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post #6 of 11 (permalink) Old 04-17-2017, 10:18 PM Thread Starter
 
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Lemans guy, I currently have my timing set at 12 btdc and that is the where it seems to run the best. I am seriously considering your suggestion, I may do this very soon. I was already thinking about the trans swap and got a quote from a local shop that has been recommended. What about the rear end gears, the rear end code is WC which I believe are 2.78. I want to be able to cruise on the highway around 70 mph.
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post #7 of 11 (permalink) Old 04-18-2017, 06:28 PM
 
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2.78 is a great cruising gear. 12 BTDC is base timing....The. Their is Centrifigal timing and Vacumn advance timing.... you need all three set right

Google How to optimize spark timing in Pontiac,,,,,give you some good look at the weights and springs etc in Distributor

2.78 and the TH350 and the 2 bbl Street avenger and correct timing will give you an ice smooth solid cruiser and real peppy and responsive, good gas mileage as well.

Make sure on the carb you have someone help you secure the gas lines with appropriate gas line fittings and clamps. You can get custom fittings or if you use rubber and steel like I do use OEITKER clamps they never leak...you don't want leaky fuel lines. Screw type hose clamps for heater hoses are not really that secure. They can fail. Even though some guys use em for years.

You will get it and it won.t be terribly expensive to have it really great..

2.78 years and TH350 should cruise at 70 about 2800 to 3000 RPM. Just perfect really if your timing curve set right you will be getting 46 degrees of spark advance and barely touching the pedal!
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post #8 of 11 (permalink) Old 04-18-2017, 09:26 PM Thread Starter
 
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Thanks for the advise. I ordered the Holley street avenger today from Jegs. When I bought the car I could not seem to get the timing right. I simply bought a new stock distributor and installed a pertonix ignition module and coil. The car seems to like the 12 timing and I figured that was it. I will look up more on the timing. I should be getting the new trans around Fathers Day.
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post #9 of 11 (permalink) Old 04-24-2017, 06:22 PM Thread Starter
 
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Okay Lemans guy, I am off to a good start. I got the Holley, it did need an adapter to go from the Rochester bolt pattern to the Holley but I got lucky and found one at my local speed shop. Every thing seems to run fine, a little better throttle response, that is good, so now to see if it helps with my MPG. Next problem is going to be that I need to plug the holes in the manifold that the old choke heat tube came up from. I was thinking of using a couple of self tapping metal screws with a little sealer. I was also told that I would need to cover the heat passage at the manifold gasket. Any thoughts? After that is done I can finish any adjustments on the carb and then I will study up on the timing.
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I have run the cars both ways,..with the heat crossover blocked and not blocked. I would strongly recommend not blocking the manifold crossover. The Holley electric choke works fine at startup on cold winter days, the crossover blocked is not a good idea on a street car basically,

Racers may need it, but you will run better leaving it as factory. Yes agree you can plug the manifold hole any number of ways. As long as it's a tight seal, good to go. It gets hot, usually just a metal plug.

After that timing is everything!
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  Pontiac GTO Forum > The 1964-1974 Pontiac Tempest, Lemans & GTO > 1964-1974 Tempest, LeMans & GTO Engine Tuning and High Performance

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