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73-87 Chevy _ GMC Trucks => Engine/Drivetrain => Topic started by: rainman15 on January 12, 2011, 07:34:06 pm
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My truck is a 305 4 barrel quadrajet, and is all original under the hood. It runs great, no missing or sputtering or anything like that. My issue is when I'm accelerating so long as I have my foot no greater than half way down, it runs great and everything seems to be normal. But if I push the accelerator further than half way down the noise of the engine seems to deepen but it loses power rather than continuing to accelerate. I let back up to half throttle and it picks right back up again, running great and pulling strong.
I changed my fuel filter. I checked my timing. While doing these items I run the truck for a while with the air cleaner off and opened it up quickly to full throttle using the linkage while watching the carb. The top two flaps covering the back barrels never opened. I replaced the vaccuum line to the diaphram that their linkage goes to, and manually checked the diaphram to make sure it worked and it does. They still don't open when I go full throttle. For that matter I can't even see the back lower blades open when I go full throttle. I checked that by holding the top blades open and jammed the linkage to full throttle and the lower blades in the back didn't open. Aren't they supposed to open at full throttle?
I'm no carb expert. You guys seem to be pretty well versed in this quadrajet stuff. What do you'all think?
Steve
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OK. First thing's first. Let's establish that your secondaries are opening at the throttle body (base plate). There is a secondary stop on the passenger side of the carb that prevents the secondaries from opening if the choke is on. Take a look at the pics below.
Secondaries can open:
(http://i208.photobucket.com/albums/bb247/1985C10/secondarystopopen.jpg)
Secondaries locked:
(http://i208.photobucket.com/albums/bb247/1985C10/secondarystopclosed.jpg)
With this in mind check the operation of the secondaries at the throttle body.
Next it is hard to get the airflap to open while revving in neutral with the choke pulloff installed. No, I am not advocating that you remove it. The pulloff puts deliberate delay on the opening of the air flap to make it more progressive and to prevent bog. If you can tap the airflap with your finger (engine off) and it opens (there should be a light spring tension) it's probably OK and opening under high load. It should open easily and smoothly with light spring tension. Sometimes the springs for the airflap break. If it doesn't feel "springy" that may be the issue.
Now for what I think your problem might be: If you hear the engine grow set of lungs my guess is the secondaries are at least trying to come on line, but can't do so because your POE is not functioning properly. POE (pull over enrichment) acts like an accelerator pump for your secondaries, except there is no pump, rather, the air stream draws fuel through a little hole (just above the airflap). It is not uncommon for the tube which feeds this little hole to fall out of the airhorn. Here are some more shots of the Q-jet I'm rebuilding to shed some light on what I'm talking about.
POE discharge hole:
(http://i208.photobucket.com/albums/bb247/1985C10/POEdischargehole-1.jpg)
Airhorn - what's missing here?:
(http://i208.photobucket.com/albums/bb247/1985C10/aurhornmissingPOEtube.jpg)
There it is:
(http://i208.photobucket.com/albums/bb247/1985C10/POEtubeinmainbody-1.jpg)
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Wow, that is an extremely helpful post. Thank you very much for your help and the detailed pictures.
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Here is a book that will help you tremendously if you decide to start tinkering with the Q-jet:
http://www.amazon.com/Rebuild-Modify-Rochester-Quadrajet-Carburetors/dp/1932494189/ref=sr_1_1?ie=UTF8&qid=1294963205&sr=8-1
I have a copy and highly recommend it.