I have just gone to an Edelbrock intake and brand new Holley 1848, 465 CFM carb on my stock 260 motor. At about 2500 RPM, when the throttle is opened there is a pronounced flat spot until finally the rpm's climb. I believe it to be an accelerator pump that is not in proper adjustment. Anyone been through this and how did you adjust the sqirt to alleviate the flat spot ? Would the flat spot indicate momentary fuel starvation ? No backfiring. Thanks.
Adjusting the 465 CFM Holley Carb.
- Thread starter Sgreg
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Holley Tuning
By far the best way to tune an accelerator pump circuit is with a wide band O2 meter. Record a run then examine it in quarter second intervals with a goal of having the mixture between 12.5 & 13 for the second after each shift. If you don't have one or can't borrow one, all is not lost.
If there's a flat spot at all rpms, you probably have an accelerator pump issue. If only at 2500, your vacuum secondaries might be opening too soon and needs a different spring.
(I'd also investigate for possible vacuum leaks and I'd check timing as well. Do you have full advance (total about 36 btc) before 2500? If not, you may want to install a Moroso spring kit.)
good luck,
bt
By far the best way to tune an accelerator pump circuit is with a wide band O2 meter. Record a run then examine it in quarter second intervals with a goal of having the mixture between 12.5 & 13 for the second after each shift. If you don't have one or can't borrow one, all is not lost.
If there's a flat spot at all rpms, you probably have an accelerator pump issue. If only at 2500, your vacuum secondaries might be opening too soon and needs a different spring.
(I'd also investigate for possible vacuum leaks and I'd check timing as well. Do you have full advance (total about 36 btc) before 2500? If not, you may want to install a Moroso spring kit.)
good luck,
bt
TigerBlue
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Flat spot 101
Almost always a lean condition. If rich there would be puff of black smoke.
A vacuum secondary carburetor uses two strategies to overcome the problem of the mix going extremely lean* for a split second when the throttle is opened quickly. This is the function of the accelerator pump for the first half of throttle movement. The second half strategy relates to opening the secondary throttle plates with a small delay by using a diaphragm to pull the the secondary open. Timing of this event is controlled by a spring in the vacuum pot on the carburetor. I stronger spring will hold the secondary plates closed until rpm (and manifold vacuum) rises to overcome the spring.
* "Lean" happens because when the throttle is snapped open the the very light air enters the combustion chambers much faster than the gasoline which responds after the flow of air in the carburetor due to inertia. The crude but effective way to avoid the stumble is to squirt gas at the moment of opening the throttle plates. Yes, Mechanical secondary carbs sometimes have two pumps. i.e. the double pumper.
Rick
Almost always a lean condition. If rich there would be puff of black smoke.
A vacuum secondary carburetor uses two strategies to overcome the problem of the mix going extremely lean* for a split second when the throttle is opened quickly. This is the function of the accelerator pump for the first half of throttle movement. The second half strategy relates to opening the secondary throttle plates with a small delay by using a diaphragm to pull the the secondary open. Timing of this event is controlled by a spring in the vacuum pot on the carburetor. I stronger spring will hold the secondary plates closed until rpm (and manifold vacuum) rises to overcome the spring.
* "Lean" happens because when the throttle is snapped open the the very light air enters the combustion chambers much faster than the gasoline which responds after the flow of air in the carburetor due to inertia. The crude but effective way to avoid the stumble is to squirt gas at the moment of opening the throttle plates. Yes, Mechanical secondary carbs sometimes have two pumps. i.e. the double pumper.
Rick