I just finished going through 4 Q-Jets over the weekend, and thought I would list the problems that these 4 "normal" carbs had. Just for your entertainment, here are the problems that 4 carbs had:
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• Accelerator pump rod installed in incorrect lever arm location
• Accelerator pump that is not fuel compatible installed in the carb - pump diaphragm swollen and completely seized in the pump bore - accel pump inoperable
• Choke pulloff bleed calibration hole drilled out, eliminating secondary airvalve control
• Float needle clip installed sideways, jamming the float
• Extreme low float level (very lean)
• Incorrect seals installed on the choke intermediate shaft
• Idle mixture screws fully backed out, attempting to compensate for a lean condition with no idle air/fuel control
• Bent linkages, bent throttle lever, and bent throttle shaft
• Missing fuel inlet fitting
• Missing vacuum nipples
• Wrong float installed
• Broken power piston retaining collar
• Missing primary rod retaining spring
• Secondary throttles not closing due to altered throttle linkage, causing fuel to discharge from secondary nozzles at idle – massive rich condition
• Secondary linkage altered to limit secondary opening to ˝ throttle
• Bent secondary throttle shaft
• Sloppy secondary throttle shaft bushings, preventing consistent return-to-idle speed after WOT operation
• Damaged/altered throttle lever, requiring weld repair
• Bent secondary airvalve shaft
• Severe damage and erosion of dichromate finish on all components
• Missing choke cover
• Secondary airvalve tension spring loose and damaged
• Corrosion on all steel surfaces
• Incorrect hardware installed throughout carb
• Extreme low (lean) APT setting
• Accelerator pump diaphragm collapsed - accel pump inoperable
• Incorrect inlet seat installed
• Undersized jets installed
• Choke intermediate shaft seals missing
• Secondary shaft spring disconnected, intentionally disabling secondaries
• Incorrect choke coil lever installed - choke cannot operate
• Upper Idle Air Bleeds filled with epoxy, making idle circuit grossly rich
• Broken secondary metering rod hanger cam
• Bent & destroyed secondary airvalve spring
• Bowl idle vent assembled and installed upside-down - inoperable
• Primary metering jets damaged
• Grossly misbalanced idle mixtures
• Plugged fuel filter
• Defective choke pulloff - spring is too tight, and bleed restrictor has been drilled out making the pulloff inoperable
• Secondary throttle linkage bent & altered to prevent secondary opening
• Bent & binding secondary airvalve shaft
• Defective (bent & altered) secondary airvalve spring
• Wrong power piston spring installed - spring from ball point pen installed
• Stripped needle/seat threads in bowl
• Cracked float bowl
• Stripped fuel inlet threads in bowl
• Choke cover housing screws broken off in choke housing
• Idle air bypass passages plugged with lead plugs
• Incorrect throttle plate installed with undersized idle air bypass bleeds, causing "fuel dribble" from the main discharge nozzles at idle
• Idle air bleeds drilled out oversized, producing a non-correctable lean idle mixture

Did you notice that not one of them had a "leaky bowl plug" causing fuel to "drain out of the bowl overnight?" Yet several owners suspected the "leaky bowl plug" problem due to difficult next-day cold starts. Those problems were mostly caused by bad, Chinese-provided accelerator pumps (sold all over eBay, Amazon, and by all the chain store auto parts outlets). Yes, the junk pumps are blue... just because they're blue does not mean they are pump-gas compatible... watch out...


Lars

 

Might be good to show us the right way to install the choke shaft seals. Some Qjets have 2? None of the DIY videos I have seen show how to install them.

 

Holy cow!

What a long list of issues.

Glad you are here to help Lars.

 

I thought my Qjet was fine, but I sent it to Lars just in case he decided to quit doing this. He had quite a list of things needing attention. Car runs like a scalded ape now!
Thanks again Lars!!

VS

 

That's quite the laundry list of issues with just 4 carbs!
The one's I found interesting was worn secondary bushings, I understood that because the secondaries weren't used all the time that the casting didn't wear as badly as the primary side.
The bypass air plugged with lead plugs....why do that?
They are all interesting issues, and are probably expected now after ham-fisted mechanics have a go at "Tuning"......I know I'm guilty of that many years ago (removing parts that I figured weren't needed, and mix and matching jets and rods.)
Did anything surprise you, or have you seen it all before?

 

Forrest -
I'm surprised myself at how many Q-Jets are coming in with worn/sloppy secondary shafts. You wouldn't think this would cause a problem worth paying any attention to, but it actually has more impact than worn primary shafts:

When the primary shafts are worn, the fact that the primary throttle blades are not fully centered in their throttle plate holes doesn't cause a problem unless the wear is really significant: The primary blades are held partially open by the idle speed screw, so they are never required to be fully and completed sealed closed.

On the secondary side, however, the throttle blades must seal up completely in their throttle holes, with no (or very little) air gap (air leakage) around the blades. The area above the secondary throttles is the plenum between the throttles and the secondary airvalves. The secondary fuel discharge nozzles respond to low pressure created in this venturi plenum. If there is air leaking past the secondary throttle blades at idle, there will be a low pressure area created in this plenum, and fuel will dribble out of the secondary discharge nozzles, creating a massively rich idle condition. Usually, the throttle blades will center themselves and seal up when the engine is "off." With engine running, they stay centered and sealed. The moment the secondaries are cracked open, the throttle shaft slop will allow the blades and shaft to move slightly, shifting the shaft and blades out of alignment with the bores due to the slop in the shaft. When the secondaries close with the engine producing vacuum, they don't center up in their bores, and you end up with erratic idle speed and a rich-running condition after the secondaries close.

To test for this condition, you can simply do the following: Start the engine and let it idle. Don't flick the throttle into the secondaries - let them remain closed and untouched. With your finger, push down firmly on the forward edge of the secondary airvalves, forcing them open against the pull of the choke pulloff. Opening the secondary airvalves at idle should have no effect on idle speed or idle mixture if the secondary throttles are fully closed. If the engine dies, or if a dramatic change is noted in idle rpm when you open the airvalves, you have a throttle blade alignment problem. If all is good for this first test, let the airvalves close, and now "flick" the throttle into the secondary side (making sure the secondary lockout lever is retracted to allow the secondary throttle shaft to open). Visually verify that the secondary throttle shaft is rotating and opening momentarily. Flick it a couple of times and then let the engine return to idle. Is the idle speed and quality the same as it was before? If not, push the airvalves open with your finger again. If this either kills the engine or dramatically changes idle speed and idle quality, you have a sloppy secondary throttle shaft that is allowing the shaft and the blades to "****" in their bores and not re-seat after WOT application. This will require repair of the secondary throttle shaft bores.

Many of the late 1970's M4M carbs are designed and set up to run very lean. Carb builders who don't know what they're doing will identify that they have a lean-run idle condition, and they are unable to back out the idle mixture screws far enough to richen the carb up. Rather than identifying and fixing the real problem (oversized IABs), they plug the idle air bypass system. This does two things: It eliminates the idle air source (which is like a calibrated vacuum leak), and it requires the idle speed (throttle opening) to be increased, putting the carb into the transition circuit at idle. This draws more fuel through the carb since the engine is now "sucking" fuel from both the idle circuit and the transition circuit. Often, it will pull enough air through the venturi at idle (due to the throttles being open too far) that the carb will dribble fuel from the main discharge nozzles at idle, further richening the carb up at idle. This often causes the idle mixture screws to become unresponsive, and it will often cause a stumble or hesitation going from idle into main discharge since there is no "enrichment reserve" provided by the transition circuit. But the lean-run idle problem is solved... Good job, incompetent carb builder..!

Lars