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73-87 Chevy _ GMC Trucks => Engine/Drivetrain => Topic started by: silver stepside on October 04, 2005, 09:22:00 PM

Title: Carb question
Post by: silver stepside on October 04, 2005, 09:22:00 PM
How do the butterflys open on a four barrel carb? Is there some sort of rod or linkage? I finally got my truck up and running and the throttle response and everything is good, but when i step on it it goes but feels like theres more power. I looked under the hood and noticed there is nothing there to open them or any sort of connection with the throttle cable, any pics would be great, i believe it is a rochester carb. Sorry to be so long thanx. - Dennis

Title: Re: Carb question
Post by: roundedline on October 05, 2005, 11:08:00 AM
It is vacuum operated based on the needs of the motor.

Chris Lucas
www.73-87chevytrucks.com
www.captkaoscustoms.com
Project Su
Jimmy 2WD Project

Title: Re: Carb question
Post by: silver stepside on October 05, 2005, 06:47:00 PM
Thanx Chris for the info i'll check all the vac lines make sure everything is good. - Dennis
                    P.S.  I'm still gonna order a set of tailpipes from you just havent got around to it fixing everything else on the truck :p . Thanx again.  

Title: Re: Carb question
Post by: roundedline on October 06, 2005, 10:51:00 AM
Wrong type of vacuum.  It is engine vacuum.  The motor basically "pulls" the secondaries open.

Chris Lucas
www.73-87chevytrucks.com
www.captkaoscustoms.com
Project Su
Jimmy 2WD Project

Title: Re: Carb question
Post by: FULLSPEEDJUNKY on October 06, 2005, 07:36:00 PM
look for a small vacuum operated diaphram on the passengers side of the carb. they do go bad. remove the air cleaner, start the motor and open the thottle quick ly and see if the secondarries are opening, you will also notice a nice groan.:lol  

Title: carb question
Post by: silver stepside on October 06, 2005, 08:54:00 PM
Thanx speedjunky i'll try it tomorrow. Can i replace just the diaphram if it is bad?  

Title: Re: carb question
Post by: RSBAD454 on October 07, 2005, 11:53:00 PM
There are no diaphrams on a Q-jet....the rear secondaries have air valves above them. These open fast or slow depending on how the spring is adjusted. they expose a fuel discharge port to reduced air pressure and in turn pulls a shot of fuel to reduce lean bog from air valves opening. As air valves open with demand from secondary throttle plates, they rotate a cam that pushes a arm up proportionately lifting secondary metering rods out to let fuel into main secondary discharge nozzles.
Secondary valves are linked to primaries and open when primaries and fully opened. If you ahve a problem with full throttle power or transition from primaries to secondaries, then adjust secondary air valve tension.

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