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Author Topic: Quadrajet accelerator pump problem  (Read 2109 times)
mikefam
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« on: May 08, 2013, 03:55:24 PM »

I have a poser for the group. I've rebuilt more Q-jets than I can remember and have never seen anything like this. The carb is completely rebuilt including new throttle valve bushings and runs great. There is no hesitation and it accelerates smoothly and it idles great but something very odd is going on.

When I turn the engine off the accelerator pump is not working at all making it very hard to start cold! I turn the engine off and quickly pump the throttle while looking down the carb throat and it is not pumping at all. I start the engine and looking down the throat I can see that the pump is working fine. Shut it off again and no pump. I can see the pump moving up and down so the only thing that I can figure is that the float bowl is going empty as soon as I turn the engine off but if that is the case then where is the gas going? The engine does not flood and restarts easily when warm and when I loosened the fuel line at the carb there was still pressure in the line.

I'm about to take the top off the carb but thought that someone here might have an idea before I do that. Any takers?

Mike.
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68 Convertible w/327 275hp donor engine from a 67 Impala and TH350
mikefam
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« Reply #1 on: May 08, 2013, 06:23:12 PM »

I forgot to mention that the accelerator pump was working fine and this problem occurred after the car sat for a month or so.

Mike


t
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68 Convertible w/327 275hp donor engine from a 67 Impala and TH350
Mike S
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« Reply #2 on: May 08, 2013, 06:35:56 PM »

 The first thing that comes to mind is the accelerator pump check ball hung up. The reason it seems to work with the engine running is possibly due to the slight vacuum in the area so atmospheric pressure will act on the fuel level in an attempt to equalize.

Mike
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67 LOS SS/RS L35 Hardtop - Original w/UOIT
67 NOR SS/RS L35 Convertible - Restored
mikefam
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« Reply #3 on: May 08, 2013, 06:57:07 PM »

Mike:

Thanks for the input, and that sounds possible. Would you think that it is stuck closed or stuck open?

If it's stuck open I might be able to hear bubbles blowing back into the float bowl when I pump it. I'll have to try that tomorrow. I think that if it IS stuck open that it would at least pump a little when the engine is off.

If it's stuck closed that would explain the fact that it is dead dry with the engine off but I would expect a big flat spot in the acceleration. I'm having trouble accepting that it runs so well with the pump not working at all. I just rebuilt the carb and everything was perfectly clean and working but the engine is newly built and the builder tuned the carburetor on the dyno so he is the last one to take the top off.
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68 Convertible w/327 275hp donor engine from a 67 Impala and TH350
mikefam
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« Reply #4 on: May 09, 2013, 03:57:00 PM »

The first thing that comes to mind is the accelerator pump check ball hung up. The reason it seems to work with the engine running is possibly due to the slight vacuum in the area so atmospheric pressure will act on the fuel level in an attempt to equalize.

Mike
Mike

You were right on about teh pump not working at all. I was mistaken about it pumping with the engine running. When I took the top off the carb and removed the piston from it's bore it came out in pieces. The rubber had seperated from the plastic head of the piston and it was just AFU. I replaced the pump and it's working normally.

Mike
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68 Convertible w/327 275hp donor engine from a 67 Impala and TH350
hotrod68
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« Reply #5 on: May 22, 2013, 03:53:43 AM »

Junk the Quadrajunk and put a Holley Spread-Bore on it    lol      IF originality doesn't matter.
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HotRod'68  1968 SS350 coupe undergoing frame-off resto/rod. 386/350/4.11s
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169INDY
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« Reply #6 on: June 05, 2013, 07:30:19 PM »

E-T-H-A-N-O-L
 Huh
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Jim
68 SS/RS L35 Th-400 LOS
69 Pace Car L48 Th-350 LOS
Stingr69
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« Reply #7 on: June 07, 2013, 04:29:54 AM »

E-T-H-A-N-O-L
 Huh

Agree.   I think you can find replacement accellerator pumps that are compatible with ethanol fuels but they do not come standard in the carb kits. Lars Grimsrud would know more about it. You might try emailing him? 

V8FastCars@msn.com


-Mark.
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169INDY
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« Reply #8 on: June 08, 2013, 12:27:39 PM »

E-T-H-A-N-O-L
 Huh

Agree.   I think you can find replacement accellerator pumps that are compatible with ethanol fuels but they do not come standard in the carb kits. Lars Grimsrud would know more about it. You might try emailing him? 

V8FastCars@msn.com


-Mark.

This outfit has some:
http://quadrajetparts.com/rochester-quadrajet-parts-quadrajet-accelerator-pumps-c-128_22.html

Jim
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Jim
68 SS/RS L35 Th-400 LOS
69 Pace Car L48 Th-350 LOS
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