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'70 98 off idle stumble, ideas?


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Before she was put in storage over 20 years ago, there was a stumble on very light acceleration say away from a stop sign. Sometimes enough that the engine would die.<P>Now that I've put her back in daily service the stumble is still there. Here is some info on what I've tried.<P>First off there are 84k original miles, everything is completely stock and unmolested. New plugs, wires, cap, points, condensor. Fresh rebuild of the Q-Jet to factory manual specs with special attention to the accelerator pump settings. Replacement of ALL vacuum lines.<P>Still the hesitation. It is better but still there. If you are more aggressive with the throttle, there is no problem...<P>Oh, and it doesn't backfire, which is what I'd expect if there was not enough fuel, ie accel pump not working correctly.<P>Any thoughts? Distributor/timing issue perhaps? I'm running out of ideas.<P>TIA,<BR>-dhl-

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If you didn't drain the fuel, you probably have a combination of partially plugged jets or air bleeds as well as timing problems. <P>Did you check all the parts that are getting vacuum? Vacuum advance canister on the distributor is one possiblity that could be leaking.<P>Remove the main vacuum take off on the car and see if the problem disappears. What is the vacuum reading you are getting at idle? Perhaps the carb needs to have the idle mixture adjusted?<P>There are so many things you have not checked that you have only scratched the surface of this problem.<P>Keep searching.

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Carb float level or distributor mechanical advance sticking. Sometimes the factory specs need to be tweaked just a bit on float level, but no backfire and no stumble under brisk acceleration points me more toward the distributor advance mechanism. WD40 will help here. Take out the point set and douse it liberally until you're sure things are free. If you can find someone with a Sun machine, have the distributor serviced and curved.<P>If it were an EGR car, I'd tell you to plug the EGR vacuum source line with a ball bearing. I keep old QuadraJet pump check balls for this very purpose.

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One of the biggest problems that I have encountered with carb rebuilds is that they are not soaked long enough. After the carb has soaked you need to blow some carb cleaner through all of the holes and follow it with a good blast of air. Then when you assemble the carb you will know that all of the passages are clear. It does sound like possibly the off idle ports are blocked. Have you looked down the carb with the engine OFF and watch the accelerator pump action? Usually though, under a light acceleration very little pump action and more transision port action. The advance action is also a good place to investigate. Like Chuck said, you need to look at all the possibilities, and there are many.<BR> Let us know what you find, Ron smile.gif" border="0

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Well, thanks for the ideas guys.<P>The Q-Jet soaked for a good 12 hrs in lacquer thinner before receiving good treatment from the blow gun at rebuild time.<P>My next step is going to be what Chuck suggested and start pulling vac. lines.<P>Thanks

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AftrBrn, I ve never had much luck with laquer thinner used as a carb cleaner, sure it will take off the superficial black deposits on the thottle plates but it won't remove the fuel gum that accumulates particularly in air bleed holes and other tiny orifices.<P>Industrial strength carb cleaner is the only stuff that really works well and must be neutralized with hot water afterward as it is a caustic soda and is very corrosive to organic soils and your hands.<P>The nasty stuff really stinks and should be used in a very well ventilated area. I suggest not even opening the can in your garage as the smell will not leave-ever. I went back to a old garage I used to rent a few days ago and it still smelled of carb cleaner, I gave up the garage 5 years ago(yikes!). Now the can full of carb cleaner sits outside with the solvent gloves I use with it sitting on top. I still can smell it when the wind is right.<P>Make sure to have safety glasses on as well as the stuff burns skin on contact and no amount of flushing with water will stop it, keep some vinegar around for any incidental contact as it neutralizes the base. Don't wait for it to start burning before you flush with vinegar, it will be to late by then. Good luck if you get it in your eyes.<P>You can usually find it at quality automotive shops around the globe in 1 gallon containers called Gunk.<P>It is possible that your carb cleaning only removed surface deposits and the real problems are hidden in some tiny inaccessible channel that only this nasty stuff can get at. Beware, if you have a nice gold alodized holley carb it won't be gold any longer after using this stuff. <P>This is really the only method I know of that dissolves fuel gum effectively, the carb cleaner in a spray can that is sold is crap don't bother with it.<P>Have fun!

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  • 8 months later...

If the car has a carb,,as opposed to FI,,I would suspect the accelorator pump is the culprit..Look down the carb as you engage the throttle linkage,,You should see two healthy streams of raw fuel shoot into the throat,,as though from twin water pistols,,,If NOT,,,you are lacking gas as the car asks for it on takeoff...

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  • 2 weeks later...

I had a problem similar to this. I have a 72 Cutlass Supreme, off idle it would stumble, but if the idle was a little higher then you stab it, it runs fine. I had converted to an HEI distributor and I pulled off the cap, coil and rotor. The contact on the coil was a little rusted, and the spring above the carbon brush somehow squished down and started to cut a groove in the cap. Doing that only gave it spark every now and then. So I replaced it all cap, coil and rotor...runs sweet now.

Hope this info helps!

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