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Q-Jet Carb Adjustment

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by Smitty, Feb 19, 2001.

  1. Smitty

    Smitty 1/2 ton status

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    Need a memory jog on Q-Jet carb adjustment. Shouldn't the engine die if the mixture screws are turned all the way in and the engine rpm is low?

    Smitty
     
  2. Chewy

    Chewy 1/2 ton status

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    Yes, turning the screws all the way in basically turns off the fuel flow. Does the engine still run with the screws in?

    [​IMG] '87 Beast
    Colorado, USA
     
  3. Smitty

    Smitty 1/2 ton status

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    It still runs. Turning the screws doesn't seem to make a difference. It's one of the carbs that had the plugs removed so you can get in to adjust the screws. When I went to adjust it one screw was out 4 turns and the other one was out about 1/16th of a turn. The truck sat for a while and might have a problem with the float level. It isn't running too bad but I started checking after I failed the emissions test (in a big way). I've messed with Holleys quite a bit in the past but haven't had to deal with a carb that has so many vacuum ports before. Probably has to do with the CA emissions on the truck.

    Smitty
     
  4. Grim-Reaper

    Grim-Reaper 3/4 ton status Author

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    Start looking for the vacume leaks. Can of Carb cleaner works great for finding them. Spray the area with carb cleaner where you think there is a leak and the rpm's will change.
    If the float was stuck it would either die or it would pour gas all over the top of the engine till it caught fire.
    I think what your going to find is a vac leak and a improperly adjusted or hung choke. The Choke hung would account for no control with the mixture screws. Once that is fixed you shoud have full control and with a vac gage on a central port (like the one out of the side at base). adjust till highest vac on both screws and then give a 1/4 turn in to lean it out at idle and that should pass emissions.

    Diging it in the dirt with my K5's
    Grim-Reaper
    <A target="_blank" HREF=http://grimsk5s.coloradok5.com/>http://grimsk5s.coloradok5.com/</A>
     
  5. HarryH3

    HarryH3 1 ton status Author

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    Grim, your post reminded me of an "experience" I had with a brand new Holley carb several years ago. I had installed it on the 360 in a '79 4WD Dodge van that I had, just a few days before. Go out to head to work and the engine fires then dies. Crank it some more and it doesn't want to start at all, then I'm smelling gasoline. #@*&$!.

    So I yank off the inside engine cover and remove the air cleaner. Hit the key and watch as a 1/4-inch wide stream of gas jumps straight up about 12 inches out of the float bowl vent tube! [​IMG] Turned out that one of the floats had a tiny pin hole in it, so it was sitting on the bottom of the float bowl, leaving the needle completely off the seat. [​IMG]

    <font color=black>HarryH3 - '75 K5</font color=black>
    <A target="_blank" HREF=http://ThunderTruck.ColoradoK5.com>http://ThunderTruck.ColoradoK5.com</A>
     
  6. Smitty

    Smitty 1/2 ton status

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    Grim,

    Thanks for the info. I found a small vacuum leak this morning but not enough to make much of a difference. The choke is wide open and I've got the air cleaner off right now to make the rough adjustments. I wasn't thinking that the float had the needle valve wide open, just that the float level might be too high. I haven't had a Q-Jet open since high school and you might say I've slept a few times since then.

    Didn't have a whole lot of time to mess with it today because of the rain but I'll hit it tomorrow. This thing is bugging me, I don't see a fuel mist when I look down the primaries but this thing has to be getting fuel from somewhere. Is there the possibility of the secondaries providing the small amount of fuel to run at idle? If I back the throttle screw all the way out and have the idle mixture screws in all the way it will still run around 400 rpm, not very even but it does run.

    Smitty
     
  7. Grim-Reaper

    Grim-Reaper 3/4 ton status Author

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    Wel depending on the Q-jet you may still have a choke problem. The linkage may still be running the carb rich and you really can't tell untill you mess with the linkages between the choke bowl and the carb body. The choke butter fly up top may be open but the mix linkage may still be hung. Also depending on the carb they have either holes or a slit for the idle jets. When you cranked down the idle stop did you try to adjust the mix screws then? It may be that the stop was set to high and it was creating a syphon if it was the slit style port or it was high enough to get the off idle port to kick in. That a jet a bit higher on the side above the idle jet but it bypasses the idle screw to give off idle transistion before the main jet kicks in.

    Diging it in the dirt with my K5's
    Grim-Reaper
    <A target="_blank" HREF=http://grimsk5s.coloradok5.com/>http://grimsk5s.coloradok5.com/</A>
     
  8. Smitty

    Smitty 1/2 ton status

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    Grim,

    Guess these things have really changed since I worked on them. I tried adjusting the mixture screws after I backed off the throttle since I was thinking it could be pulling fuel through if the plates were open far enough, no luck. I'll check the linkage on the choke side, the butterfly opens and closed by itself as it warms up but there might be something hung-up. I priced a kit last night and found one for under $15. I need to pull the thing off to change the base plate gasket anyway.

    Smitty
     
  9. Grim-Reaper

    Grim-Reaper 3/4 ton status Author

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    Most parts stores sell a book titled "Rochester Carberator tuning" or something close to that put out by haynes. Good book with some really good set up and rebuilt tips. Well worth buying. The quad basicly is the same but they made a few changed to the idle circuit and choke set ups. That book covers all the styles they built. I think yours is old enough that it doesn't have all that electric mixture control BS on it. My personal feeling is about 79 is the perfect Quad. takes about 30 minutes to convert to electric and i have NEVER had a stall issues with it on hills. It will suport all the emmission crud like evap, AIR and other junk all the way up to 86. If the emmisions Nazi's are clueless that this is a good replacement for the electric versions.

    Diging it in the dirt with my K5's
    Grim-Reaper
    <A target="_blank" HREF=http://grimsk5s.coloradok5.com/>http://grimsk5s.coloradok5.com/</A>
     
  10. dyeager535

    dyeager535 1 ton status Premium Member

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    Did you try tightening down all the screws on the top of the carb, as well as the carb to intake bolts? They can and do loosen up over time, which creates a nice vacuum leak, as well as "cross bleeding" internally. Do not overtighten any of them of course, the Q-jets is rater "soft" but just nice and snug. You will know if any were loose.


    Dorian
    My K5 and Chev/Olds tech/links page: <A target="_blank" HREF=http://www.dorianyeager.com/index2.html>http://www.dorianyeager.com/index2.html</A>
     
  11. Smitty

    Smitty 1/2 ton status

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    First off I'd like to say thanks to everyone for the help. I pulled another Q-Jet apart for the practice today, figured I'd better before I tore into the one on the truck. It's been a long time since I'd had one apart but there's not too involved once you get into it. The last time I had one apart was in 78 so it has been a while. The carb I've got doesn't have anything electric except for the choke and the anti-diesel solonoid so I'm lucky in that aspect.

    I'll quit beating this thing to death, thanks again

    Smitty
     
  12. dyeager535

    dyeager535 1 ton status Premium Member

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    Smitty, just noticed you mentioned the anti diesel solenoid...I'm not sure if thats what it is, or an AC kick up, (you might be right) but any chance that thing is hitting the linkage all the time, keeping the primaries open slightly? Mine would cause my truck to diesel, as in never shut off, until I unplugged it. You probably already tried this, but another easy to do, might fix idea.

    Dorian
    My K5 and Chev/Olds tech/links page: <A target="_blank" HREF=http://www.dorianyeager.com/index2.html>http://www.dorianyeager.com/index2.html</A>
     
  13. Smitty

    Smitty 1/2 ton status

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    Found a couple of problems when I pulled the carb apart. One of the gaskets had a large chunk missing, there was some grit in one of the ports for the idle mixture screws(it wasn't really bottoming out), the intake manifold had been dinged up in a few places so there wasn't a flat surface for the carb to mount, someone had used RTV to seal both sides of the carb base gasket and a lot of rust on the throttle shafts.

    It was a blast getting all of the RTV out of everything! Took most of the day just to clean everything.

    It runs like a top now. Pulls a steady 19" of vacuum at idle. Idles smooth, almost too smooth--I thought it had died at the stoplight when I was test driving it. Seems to have a lot more power off the line too.

    Thanks to everyone for the advice. Now I just need to see if it will pass a smog test.

    Smitty
     

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