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'Off Idle' Hesitation


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Any thoughts on this one?.......My '83 Avanti (RQB3655) has recently developed a 'just off idle' hesitation or 'bog'. This showed up about two months ago,..the day I changed out a leaky mechanical fuel pump. This car has been converted to non-electronic carb (Quadrajet) and distributor...this conversion was done years ago, and the car has always ran and responded very well. In the past two months I've changed the fuel filter in the carb inlet, changed the in line filter ahead of the fuel pump, removed and cleaned out the inside of the carb very thoroughly, then adjusted and reinstalled it on a new base gasket, changed the spark plugs, cap and rotor. As I drive this Avanti daily, I have to keep it reliable, hence the reason I threw the tune up at it. I notice an unusual amount of play in the primary throttle shaft when I jiggle the linkage....could there be a vacuum leak at this shaft? All hoses, etc., are definitely A-OK. THANKS!

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Carburetors are voodoo as far as I'm concerned, but the float level could be off or a problem with the accelerator pump can cause bogging down. Quadrajets are excellent carbs but it takes someone who knows and understands them to get one sorted out. Using an air/fuel meter will be a big help as well.

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You sound as though you know a bit about carbs... have you tried looking down the primary throat while jazzing the throttle linkage? You should get immediate strong double streams of gasoline squirting into the throat when you pull the throttle linkage.

If not, the accelerator pump is bad. That's a pretty simple fix. You can purchase an accelerator pump separate from an entire rebuild kit, but it does require removing the top of the carb, and it's easy to drop bits & pieces into the intake manifold, so best to remove the carb and make the fix off the car; it's also possible to break/ruin gaskets when disassembling a carb that's been installed for some time.

Have a look at this link: http://www.camaros.net/forums/archive/index.php/t-67610.html

If you are mechanically inclined, it's fairly easy to do a complete rebuild on a quadrajet with instructions that come with the kit, and there are websites that have instructions, even videos on youtube; for example...

http://www.crankshaftcoalition.com/wiki/How_to_rebuild_a_Rochester_Quadrajet_4MV_carburetor

I rebuilt the quadrajet on my '80 last year, using a kit from this vendor:

http://quadrajetparts.com/rochester-quadrajet-parts-quadrajet-rebuild-kits-quadrajet-rebuild-kit-1979-q4009a-p-558.html

Unfortunately, the website I referred to for rebuild instructions is no longer available, but generally the instructions that come with a rebuild kit are sufficient. Take a lot of digital pics as you disassemble the carb, in case you need to refer to them later.

Be especially careful removing/installing the fuel inlet tube fitting, because the large, fine-thread fitting is easy to cross-thread and if you chew up the threads on the carb body, there's no easy home repair to fix that.

Here are some quotes from a Corvette expert ("Lars") in 2002 regarding problems he's seen with Q-jets stumbling off idle:

Jammed accelerator pump check ball
Results in: Off-idle stumble or hesitation, hard cold-start.
Comments: This is a common problem in carbs that have been allowed to “dry out” for a while. Sediment in the bottom of the fuel bowl will lock up the check ball as if it were set in concrete. This will prevent any accelerator pump discharge.

Accelerator Pump Rod Installed in Wrong Hole in Pump Arm
Results in: Off-idle stumble or hesitation.
Comments: There are two holes in the accelerator pump lever arm for the lever arm rod to engage into: Inner and Outer. The outer hole produces a leaner pump shot, and can cause a lean stumble on engines requiring a robust pump shot. Corvettes and performance cars always used the rich, inner pump arm hole.

Jammed accelerator pump
Results in: Off-idle stumble or hesitation, hard cold-start.
Comments: This is becoming a common problem. The alcohol additives in modern fuels are not compatible with the materials used in many accelerator pumps (even some of the pumps in brand new carb kits). This causes the pump plungers to swell up and to seize in the pump bore. The spring on the pump shaft still allows the shaft to move up and down, making it look as if the pump is functioning. But the pump itself can be seized up solid in the bore with the shaft working just fine. To fix this, you must install a pump that is specifically compatible with alcohol.

Worn accelerator pump
Results in: Off-idle stumble or hesitation, hard cold-start.
Comments: Common on older carbs and on carbs that have been allowed to dry out (vehicles stored without being started through the entire winter season). The rubber plunger material will dry out and shrivel up, making the accelerator pump completely ineffective. The pump must be replaced.

Edited by WayneC
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I agree, the accelerator pump is not squirting adequate fuel into the primary venturi s

Lars is the best especially on timing/ignition.

Joe

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Thanks for Your input on this problem guys. The history of this Quadrajet is that it was overhauled with a kit about three years ago (did the work myself) and this overhaul included a new composite float. It performed quite well after that.....and still does in every way except for that pesky 'off idle' stumble or bog. I hear You about the questionable accelerator pump....but the twin squirts of fuel are forceful and instantaneous as soon as the throttle is moved....I'm really confused on this one!....Any other thoughts? THANKS!

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I've had a similar problem with my electronic qjet (82) and taken it apart and replaced all the fiddly parts including the accelerator pump and that didn't seem to help it. Considering you just replaced the fuel pump and filter is it possible the pump is faulty and the fuel pressure is low? Another this is that the qjet off idle circuit could be clogged or maybe there is a vacuum leak that causes for a low vacuum as the throttle plates start to open. Is this only occurring when the engine is cold and goes away once the engine is warmed up? Are cold starts harder than they were before this issue started?

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CaffeineRacer...Now that You mention it, there really is no off idle stumble when the engine is cold and warming up. The problem is there once the engine is up to normal (195) temperature. Hmmmm!

Also, I never really thought of testing the pressure output of the new fuel pump...but I could try that, as I have one of those vac-press gauges. Food for thought....Thanks!

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  • 6 years later...

I noticed this old post and figured I'd let folks know what the off idle hesitation problem actually was (in case someone else finds themselves in a similar situation)

The problem turned out to be leaking well plugs at the base of that Quadrajet...an invisible issue with the carb installed on the engine....Apparently the engine 'stumbled' with the accelerator pump 'squirt', PLUS the additional fuel leaking out of the well plugs. (a bad original design in my opinion)

I long ago replaced this carb....and all has been well!:)......Ed

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