Register now to get rid of these ads!

Technical Q-Jet woes

Discussion in 'The Hokey Ass Message Board' started by 57 HEAP, Dec 11, 2015.

  1. So I have a stumble on my daily. SBC with Q-Jet (pre computer style) remanufactured from Jeg's and installed about three years ago. Ran fine until recently. Crack the throttle and the things bogs. Even slight pressure changes on the gas pedal to maintain steady speed produces a hesitation.

    Took the thing apart and sprayed all the holes and grooves and etc. with carb cleaner. Didn't see any trash or water in the fuel bowl. Put it back together and no change. Idles fine around 800. Steady vacuum at about 15 inches.

    Ideas? Thanks.
     
  2. theHIGHLANDER
    Joined: Jun 3, 2005
    Posts: 8,185

    theHIGHLANDER
    Member

    Ignition may be suspect, but before that how's the fuel delivery? Pressure is one element, volume perhaps the most important. If those 2 are good the power circuit is the culprit if it's a lean bog, secondary upper butterfly valve spring/setting if it's a rich bog. The power circuit is usually controlled by a spring loaded valve that raises as vac drops. One of the old backyard tricks was to stretch the spring some to make the power valve react quicker. How about the accel pump? Getting a nice full shot when it's called for under foot?
     
  3. LOL didn't someone once say that most carb problems were ignition and vice versa?
     
    squirrel likes this.
  4. Dan Timberlake
    Joined: Apr 28, 2010
    Posts: 1,304

    Dan Timberlake
    Member


  5. Larry T
    Joined: Nov 24, 2004
    Posts: 7,661

    Larry T
    Member

    Did you check the accelerator pump shot? That or stuck metering rods are usually the culprit for Quadrajet off idle flat spots, if it is actually the carb.
     
    AHotRod likes this.
  6. squirrel
    Joined: Sep 23, 2004
    Posts: 47,998

    squirrel
    Member

    Does it also have an HEI? they used to have problems with the advance mechanism wearing out. But yeah, start with a tune up, including checking the compression and valve action.
     
  7. The car is a 1984 with 305 and Computer Command Control (GM). At the time I swapped to a non-computer carb, I also changed distributers. New unit from Jeg's, but did not replace plugs or wires. I put maybe 4,000 mi on the car a year. I have replaced the fuel pump, but don't know exactly when at the moment. So I think that isn't the problem. I can see fuel squirting into the carb when I move the linkage, but is it enough? I think not.

    Stuck metering rods and power circuit. I have a manual I need to look at to see what exactly you guys are referring too. I also plan to pull some plugs while the engine is running to check for a problem there.
     
  8. squirrel
    Joined: Sep 23, 2004
    Posts: 47,998

    squirrel
    Member

    If you can see fuel squirting when you move the linkage, then the accelerator pump is working.

    Also look for a vacuum leak.

    Fuel pump issues usually show themselves at full throttle, after you've been going a little bit, it will act like it ran out of gas. I doubt it's a fuel pump issue.
     
  9. farmboat
    Joined: Aug 13, 2006
    Posts: 287

    farmboat
    Member
    from Lucas, KY

    check fuel filter
     
    onetrickpony likes this.
  10. onetrickpony
    Joined: Sep 21, 2010
    Posts: 467

    onetrickpony
    Member
    from Texas

    Change the fuel filter. You may have picked up a load of water at the last fill up, also.
     
  11. Lets see you have put 12K give or take on an engine that you bought and switched out the carb and distributer but didn't put plugs and wires on it and gawd knows how many miles those plugs and wires had on them at the time? maybe try plugs and wires before you go after the carb? I think I probably would.
     
    slack likes this.
  12. ^^ Bought the car new and it has about 180,000 mi. Not sure when the plugs and wires were done, but I have it written down. I should think that 50,000 miles on plugs and wires is not too much. I think I'm in that range.
     
  13. OK then. @ 50K they couldn't possibly be the culprit. What are they gapped at? I mean today not when you installed them.
     
  14. squirrel
    Joined: Sep 23, 2004
    Posts: 47,998

    squirrel
    Member

    50k miles is nothing on a new car. But it was a lifetime on an old one...
     
    Johnboy34 likes this.
  15. saltflats
    Joined: Aug 14, 2007
    Posts: 10,622

    saltflats
    Member
    from Missouri

    180.000 miles
    That puppy has to be tired
    Did the problem start all of a sudden or has it be slowly
     
  16. AHotRod
    Joined: Jul 27, 2001
    Posts: 11,309

    AHotRod
    Member

    It's most likely the accelerator pump is tired and needs to be replaced.
     
  17. Problem started suddenly.

    Look at it again today. Pump shot looks good. Idles great. I looked at #1 plug and it had spark when held against the exhaust manifold, but that was the only one I checked. Fuel filter about two years old, didn't see any water in the carb when I took it apart. Put some Sea Foam in the tank, maybe there is some water in there. I think I will try wires and plugs, but at this point I feel I'm grasping straws.
     
  18. squirrel
    Joined: Sep 23, 2004
    Posts: 47,998

    squirrel
    Member

    Look at all the plugs, before you get too far...
     
  19. Larry T
    Joined: Nov 24, 2004
    Posts: 7,661

    Larry T
    Member

    I agree with Jim, check the easy stuff first. If that doesn't fix it........................................ metering rods.jpg
     
  20. Larry T
    Joined: Nov 24, 2004
    Posts: 7,661

    Larry T
    Member

    Actually, you can check to see if the primary metering rods are stuck by taking a small screwdriver and use it as a probe through the vent tube of the carb. If you can push the rods down and they spring back up with the pressure from the screwdriver, they're probably ok. If they feel solid, they're stuck down and need to be freed. I have been able to do this without pulling the top of the carb with a screwdriver or piece of wire with a small hook on the end of it. But most of the time the top has to come off and the power piston cleaned.
     
  21. Does it have a fuel filter behind the fuel inlet nut? If so, have you changed it lately? They are small, and once they are clogged full of junk, the engine will barely run.
     
  22. Joe H
    Joined: Feb 10, 2008
    Posts: 1,008

    Joe H
    Member

    Check the linkage at the accelerator pump, if it doesn't move 100% with the throttle, you can get a flat spot even though the pump works. Also, if the pump doesn't work with slow throttle opening, the leather or plastic cup could be starting to wear out on the accelerator pump plunger.

    I would pull the carb for a complete cleaning in a soak tank. Q-jets work great when clean, and spray cans usually do not work like a good soaking does when it comes to cleaning.
     
  23. Are you still running the stock intake with an EGR valve? I seem to recall these C3 305s having an issue with the EGR valve sticking, maybe open, maybe closed. Can't remember exactly now. That or a leaking vacuum hose or bad thermal switch that kept the valve from actuating at the proper time. It made them feel lazy and lean off idle much like a bad accelerator pump would do.
     
    Larry T likes this.
  24. I'll check the metering rods. I have had the top off and cleaned the carb out, it wasn't really dirty. So I may have a stuck rod. Still have the EGR, I'm planning to make a block off for that. I have an in line filter between the carb and fuel pump. Think it has about 6,000 mi on it.
     
  25. Larry T
    Joined: Nov 24, 2004
    Posts: 7,661

    Larry T
    Member

    Before you take the top off the carb again, plug the vacuum line going to the EGR valve and see if it changes the way the car runs (easy stuff first).
     
  26. First of all Merry Christmas and thanks to all!

    Installed new wires and plugs today and my problems seem to be gone. Since slight movement on the gas pedal would create a stumble, I was thinking it must be the carb. After tearing it apart, I did find the return spring for the accelerator pump weak, so I replaced it. And I had pulled out the distributer and not set the timing with a light, that was something that added to my confusion. The timing was too far advanced and that created acceleration problems.
     
  27. Johnboy34
    Joined: Jul 12, 2011
    Posts: 1,381

    Johnboy34
    Member
    from Seattle,Wa

    Thanks for coming back and letting us all know..
     
    hipster likes this.

Share This Page

Register now to get rid of these ads!

Archive

Copyright © 1995-2021 The Jalopy Journal: Steal our stuff, we'll kick your teeth in. Terms of Service. Privacy Policy.

Atomic Industry
Forum software by XenForo™ ©2010-2014 XenForo Ltd.