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QJET CARB

  • Tiny
  • fishstu
  • 1986 Chevrolet
  • V8
  • 2WD
  • automatic
  • 45,000 miles

I have a 1986 454 with a Rochester QJet 4bbl carb in a 35ft MH. I had a sticky throttle on hills when I floored the pedal for several mins. I also noticed the last 200 miles a slight back fire on initial acceleration. I installed the remanufactured QJet about 5 years ago and haven't had any problems. I decided to throw a rebuild kit on it since I had it off fixing the sticking secondary butterfly on the throttle plate gasket, due to the back fire. Simple, right? Couple new gaskets, new rubber on the accelerator pump, blow out some ports and a new needle and seat. I re-installed the QJet onto the motor and had to screw in the idle screw so much it's running from the primary curcuit and dripping in the venturies. I started at 2.5 turns out on the Idle-mixture screws and in or out nothing fixes the rough idle and black smoke out the exhaust. I have checked umpteen times for vacuum leaks and can't find any. Beside the sticky throttle on load and occasional bask fire it was running well. What could I have changed to effect the idle this way.

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Wednesday, June 22nd, 2011 AT 8:05 AM

11 Replies

  • Tiny
  • caradiodoc
  • Expert
  • 25,635 posts

If it's dripping from the venturies, that's has nothing to do with the mixture screws. That suggests the float is set too high which will cause a real rich mixture; more so than the mixture screws themselves. A quick way to prove that is to pinch off the rubber fuel hose where it comes into the fuel pump. As the fuel is used up in the float bowl, the dribbling fuel will stop and the running will improve.

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Wednesday, June 22nd, 2011 AT 8:19 AM
  • Tiny
  • fishstu
  • Member

Maybe my terminology is wrong the idle stop screw on the side of the carb that the accelerator linkage rests on is barried, that is the spring is completely compressed just to keep the engine running. As that screw operates the primary butterflys it opens it up enough to begin using the primary curcuit. Is the idle curcuit clogged? Am I somehow starving air and running rich?
The factory setting for the float is 3/8in, how much should I lower it, and still run OK?

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Wednesday, June 22nd, 2011 AT 8:33 AM
  • Tiny
  • caradiodoc
  • Expert
  • 25,635 posts

That 3/8" float measurement is just a guide to get you in the ballpark. Once you lower it to where you no longer see raw fuel running in, you will very likely not notice much difference in the way it runs if you lower it a little more. However, the lower you go, the leaner the mixture will be so you will have to tweak the mixture screws a little. Basically, if you get it too low or too lean, you'll feel the hesitation and stumble on acceleration but fuel mileage will improve slightly. If you have it too rich, but not to the point of running over, you won't notice it in the engine performance. It's doubtful you'll even see black smoke from the tail pipe. The only way to find that wasted fuel is with an exhaust gas analyzer.

I don't think your idle circuits are plugged. Those are the same jets used for high speed when the metering rods lift up. Half of the cylinders wouldn't run if one of those were blocked.

Something else I forgot about, and now my memory is failing me, I think there should have been two small metal balls in the accelerator pump circuit. One of them is held down by gravity, but when you press the gas pedal the rushing fuel pushes that ball off its seat to let the gas squirt into the venturi. If that ball is missing, vacuum will continue to draw in raw fuel and cause a flooding condition. That will appear to partially clear up if you lower the float since it's harder to suck that fuel up the passages and into the air flow but it will still be flooding at higher vacuum such as when coasting, and you'll have a severe stumble at low vacuum when accelerating.

If you pinch the fuel hose and the engine smooths out within about half a minute or less, that suggests the float is just too high. If it continues to act flooded or way too rich until it almost runs out of fuel, check for a missing check ball in the accelerator pump circuit. As the fuel is used up in the float bowl, the engine will run better and better until it stalls.

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Wednesday, June 22nd, 2011 AT 9:23 AM
  • Tiny
  • fishstu
  • Member

OK, I lowered the float. The first time to 1/2 inch(ran a little better), then to 9/16 inch and does run better. Don't have to bury the idle speed screw at the linkage any more just to keep it running. Although it's a definite improvement I continue to see black smoke out the exhaust at idle. The smoke clears right up if I throttle up. It doesn't take long, once I return to idle for the black smoke to return( 30 sec). I hate to lower the float any more, it hardly drops now. Also I bench set the idle mixture screws to 2 1/2 and while running I slowly turned each one in 1 1/2 turns. I didn't notice any change and just stopped. Should I be going in or out to clear up the black smoke or just keep lowering the float? Or might I have something else going on.
Thanks for the help.

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Thursday, June 23rd, 2011 AT 1:32 AM
  • Tiny
  • caradiodoc
  • Expert
  • 25,635 posts

Did you pinch the fuel hose to see if it clears up? Did you make sure the check ball is in place for the accelerator pump?

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Thursday, June 23rd, 2011 AT 2:03 AM
  • Tiny
  • fishstu
  • Member

Yes the accelerator pump check ball is in place and clean. Could the new ball in kit be the wrong size? Too small? As far as the pinch test I understood that to check if float level is at question, and since this is in a 35 ft MH and have some 30 feet of hard fuel line, I thought trial and error float adjustment would work ok, which it seems to have helped. I will try the pinch and just give it extra time.

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Thursday, June 23rd, 2011 AT 4:03 AM
  • Tiny
  • fishstu
  • Member

Hey what if the arms on the Power Piston Assy that support the primary metering rods got bent during overhaul? What if the Idle curcuit which is dependant on those metering rods to "meter" the fuel at idle were too high allowing extra fuel(raw fuel)to enter the curcuit. What if that caused a rich mixture and black smoke out the exhaust and bottoming out the idle speed screw just to keep it ruuning. What if I leveled those arms evenly across the Power Piston Assy, would that fix the black smoke and rough idle?

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Saturday, June 25th, 2011 AT 4:28 AM
  • Tiny
  • caradiodoc
  • Expert
  • 25,635 posts

Now you're taxing my memory. I can't remember what closes the metering rods. They're either linked to the throttle shaft or they ride on a cam, ... Or a spring opens them up by pushing on a piston during acceleration, and vacuum pulls that piston back down when it's high. I've had some older Chryslers that I've put together with one of the metering rods off-center and not in the jet. That jammed both of them fully open. If yours uses that piston, be sure vacuum can get to it to pull it closed. That could be a problem if you replaced the base gasket and they provided the wrong one. I've seen the wrong gasket cause a vacuum leak and high idle so I suppose it could cause your problem too.

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Saturday, June 25th, 2011 AT 5:01 AM
  • Tiny
  • fishstu
  • Member

Yea after trial and error with the float too many times I figured there was something I was missing. I went ahead and timed the ignition(to make sure and it was fine)then meticulously, slowly went through every part of the carb. I found one of the Power Piston arms was bent higher than the other. I inverted the Piston on the bench and made the needed adjustment to square it up. Put it back together and it purrs like a kitten. No more black smoke, no more rough idle and I had to back off the idle speed screw to get to 750 rpm. I never had to "Pinch" the fuel line 30+ feet away to diagnose anything.
Thanks for the help
Fishstu

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Saturday, June 25th, 2011 AT 5:32 AM
  • Tiny
  • caradiodoc
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Yippee! Give yourself a pat on the back.

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Saturday, June 25th, 2011 AT 6:14 AM

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