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Old 07-21-2004, 03:03 PM
Terje Terje is offline
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Isn't the 600 CFM for his current carb a maximum capacity for that carb?

Even if the maximum volume his engine will ever suck at full throttle is 350 CFM, the proper air fuel mixture is still around 14 to 1 (I don't know the exact ratio. might be 14.2:1)

A good quality properly tuned carb should deliver the correct air fuel ratio at all air flows.

That formula calculates the minimum capacity you should use, not the maximum.

In other words, if you put a 350 CFM carb onto an engine capable of sucking down 700 CFM, then at full throttle, the engine will run fine, but with only half the capacity it is capable of.

A 350 CFM capable engine with a 700 CFM carb should run at full power at half throttle. Giving more throttle on such a setup should have no effect.

On my carb (Rochester Q-Jet) the secondaries are activated by the airflow through the carb, and thus the airflow determines the fuel flow maintaining the correct 14 to 1 air fuel ratio.

What should be happening is when the engine is at idle, and I suddenly floor it, the carb will gradually increase the throttle as the speed of the engine increases and thus the airflow gradually increases.

jpd37, I believe that if your carb is properly tuned, it should run fine on that engine.

Originally posted by
If there is a bog or a feeling of a surge when you hit 1500-2000rpm then the secondary spring is opening too soon or too late.
Willys, I've got a stock 350 with a Q-Jet in a 1977 C20 I just bought. I just got it to pass smog by replacing all the vacuum hoses and putting the carb together correctly.

Either the idiot before me or some mechanic of his put the choke lever on wrong so the choke was stuck, and also put the fast idle cam upside down.

Anways, at this time, when I'm driving slow and floor it, the power increases slowly at first, then there is a sudden increase in power.

I guess you're calling that, "a bog or a feeling of a surge when you hit 1500-2000rpm".

So, my "secondary spring is opening too soon or too late."

How do I go about figuring out which and adjusting it? There is a screw that I believe tunes the spring that holds the secondaries closed. I think maybe I should adjust that.

I have the a Rochester Q-Jet manual on request at the library, but it's taking forever for them to get it.

Last edited by Terje; 07-21-2004 at 03:03 PM.
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