Author Topic: The Carb You Love to Hate  (Read 1185 times)

Offline JohnnyPopper

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The Carb You Love to Hate
« on: November 14, 2022, 07:30:16 PM »
So just spent the afternoon rebuilding the venerable/despised Rochester Quadrajet.

Notice that the intake manifold is a split plenum, with a little crossover, I mean very little.

However the thick base plate gaskets are open, cancelling any discreet performance from each side of the carb.

Was this because the problems with performance had to be cured by opening up to a common plenum?

Or have they always been this way?
1957 Apache 3100 235 Inline 6, 3 on the tree
1973 C-20, 3+3 454 4BBL TH400  Water Injection
1978 K-10, 350 4BBL TH350 NP203 M.M. Part time Kit/Hubs
1980 C-10 under construction

Offline Stewart G Griffin

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Re: The Carb You Love to Hate
« Reply #1 on: November 26, 2022, 07:04:03 PM »
What type of intake do you have?

Offline JohnnyPopper

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Re: The Carb You Love to Hate
« Reply #2 on: November 27, 2022, 12:25:11 PM »
Stock split plenum as in one side is raised and the other is lowered. One common passage between sides.
1957 Apache 3100 235 Inline 6, 3 on the tree
1973 C-20, 3+3 454 4BBL TH400  Water Injection
1978 K-10, 350 4BBL TH350 NP203 M.M. Part time Kit/Hubs
1980 C-10 under construction

Offline VileZambonie

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Re: The Carb You Love to Hate
« Reply #3 on: November 27, 2022, 06:38:15 PM »
You need to realize that your engine is only as strong as all of its cylinders combined. To think that each cylinder is always performing the same and doing the same work is about the same as thinking all of the people at your job put in the same amount of effort. While the divider helps the runners equalize pressure, the area atop of the plenum helps stabilize overall performance. You want all of your cylinders to have the ability to drink from the richest well at all times. Your fuel metering device can only do so much and is not a computer controlled direct injection system.  The gaskets you are running are what you want.
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              ⌠ŻŻŻŻŻ'   [☼===☼]
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74 GMC, 75 K5, 84 GMC, 85 K20, 86 k20, 79 K10

Offline JohnnyPopper

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Re: The Carb You Love to Hate
« Reply #4 on: November 27, 2022, 07:44:41 PM »
VZ, that is the most cogent response to an otherwise confusing circumstance. Thank you!

If you know, was the original Qjet designed to provide a discreet fuel delivery to each side of the manifold? Or was designed to deliver a sum of both sides? Ergo the open plenum base gasket?

Finally, the base gasket had an openness at the front of the carb that the replacement did not, i.e. there is a crossover the base plate between sides of the primaries that were uncovered and subject to vacuum conditions. The replacement gasket sealed the crossover. I'm going to posit that the crossover should be sealed. Your thoughts?
1957 Apache 3100 235 Inline 6, 3 on the tree
1973 C-20, 3+3 454 4BBL TH400  Water Injection
1978 K-10, 350 4BBL TH350 NP203 M.M. Part time Kit/Hubs
1980 C-10 under construction

Offline VileZambonie

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Re: The Carb You Love to Hate
« Reply #5 on: November 28, 2022, 05:57:24 PM »
There have been countless design changes in the dual plane manifolds but most people don't pay attention to them as they take them off and pitch them in the scrap bin. I am sure the subtle differences were there for emissions complaints, nevertheless, most choose the open design for optimal performance and fuel economy.
,                           ___ 
                         /  _ _ _\_
              ⌠ŻŻŻŻŻ'   [☼===☼]
              `()_);-;()_)--o--)_)

74 GMC, 75 K5, 84 GMC, 85 K20, 86 k20, 79 K10

Offline philo_beddoe

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Re: The Carb You Love to Hate
« Reply #6 on: December 04, 2022, 07:44:06 AM »
I love those Rochester Q-jets. Ever since VZ dialed mine in about 5 years ago, I have not touched it, literally. And the engine runs very smooth.
Behold, the day of the LORD cometh, and thy spoil shall be divided in the midst of thee.  Zechariah 14:1