Here is what I have... - 1973 Buick 350 4-barrel - Bottom end stock with 70k miles - Top end rebuilt (slight cam) with 20k miles - Stock intake, carb, exhaust manifolds - GM HEI ignition Here is the problem I'm having... It has a slight stumble / hesitation just off idle under normal driving conditions. If you really grandma it, there is NO hesitation. If you goose it a little bit with slightly more aggressive throttle (primaries only) there is NO hesitation. It's only in that "sweet spot" when driving the car normal. Doesn't matter if your at a stop sign or coasting down the road at 45mph, it will do it every time. Here is the details... The fuel system, ignition system, charging system, vacuum system, cooling system is all 100%. Everything is new or in perfect working order. I rebuilt the correct carb for this engine. This engine runs PERFECT in ALL other conditions. - EGR seated and disconnected - no vacuum leaks - no nossle dripping - strong accel pump squirt - get good mixture between 2 and 3 turns out I even tried a bunch of different things to try an help isolate it... - ran ignition vacuum advance manifold - ran ignition vacuum advance ported - ran timing retarded - ran timing advanced - ran low idle (throttle plate position) - ran high idle (throttle plate position) - ran it on the lean side - ran it on the rich side - changed accel pump linkage to outer hole No matter what, that slight hesitation is still there. uzzled: Does ANYONE have ANY ideas or suggestions? o No: It's REALLY bumming me out. PS: Just for the record... I was an Engine Performance Specialist for Precision Tune in the 80's. (engine doctor) I must have rebuilt at least 100 Q-jets, so I know my way around these engines / carbs. This one has me stumped.
A couple of things: (A) If you are using ethanol, it might cause this issue (B) Even though you have a strong pump squirt, what kind of pump squirt do you have when you have the stumble? Some of the aftermarket pumps are better than others (C) How "slight" is the cam? It may be necessary to install a shorter accelerator pump for more volume of squirt. One modification begats 5 more :TU: Jon.
Accel pump squirt... I keep going back to that. Visually it looks fine. Maybe the squirt is just ever so slightly delayed only under that specific throttle amount. If you look at all the other changes I made, yet the hesitation remained, and doesn't do it any other time, what else could it be? I'm going to have to find a different accel pump and try it.
Sounds like the accelerator pump to me. Whens the last time that Q-jet was rebuilt? I just rebuilt the 800cfm Q-jet on my Wildcat and had a nasty bog too. Turns out the accelerator pump rubber in the kit was of poor quality. It would come off the plastic all together. A good used one from my parts box solved the problem.
Rebuilt the carb a month ago. Tonight I decided to take the carb off again. I had some used accel pump parts in my carb parts bin. I installed a used GM accel pump, no idea what year it's from. It was roughly the same lenght, but different in design. I didn't have the rubber for it, so I had to switch it over from the new pump. The new aftermarket pump used only the rubber, where the GM pump used a circular spring under the rubber. I even changed the upper pump spring. Put it back on the car, covered it with a shop rag, and called it a night. I'll test drive it tomorrow, let you know how it turned out. Say a prayer
Something to consider as well has to do with the construction of the Q jet carb. These carbs have an idle transfer slot, which allows slightly leaner accelerator pump calibrations, and off-idle calibrations. The correct operation of this slot depends directly on the position of the throttle blades at idle. If the "mild" cam requires the opening of the throttle blades farther to allow the engine to idle correctly, the transfer slot will have depleted its fuel supply, caising a transition lean out, at exactly the point where you feel it. This slot may be emptying out, and the normal fuel feed is therefore lean, causing the tip-in sag. Ray
Thanks for the tips Ray. That is generally why I was trying so many different setups (manifold advance / ported advance / rich lean / high low idle / etc) because it will drasticly change base idle throttle blade position. All of it had zero effect, which kept me coming back to the accel pump.
Well... There is no more hesitation, changing to a different accel pump fixed it. :TU: All that time & energy wasted because these companies can't put a decent accel pump in their kit. Aftermarket replacement parts are a joke anymore. There is still one problem... The used accel pump I used, being a different design, having that circular spring under the rubber, it is slightly too fat i believe. So the accel pump return is not instant, it doesn't stay with the accel pump arm / linkage 100%, it's slightly delayed upon return. It ran perfect. I'm not sure if that will have any affect on anything. The only thing I can think of is... if you were running the car very hard, like in NASCAR or a car chase, heavy off & on the throttle, the pump wouldn't be there for you when you got back on the gas. But that would have to be very extreme driving conditions. Thoughts? Any other possible effects? At least the hesitation is fixed for now, and I know what the problem is. I can try to get another accel pump in the near future.
