[gto19]

A while back I posted some hot start issues with my car . Someone had noticed my accelerator pump rod position on the carb and said that the rod might be in the wrong position and I might not be getting full pump shot .see picture... I know very little about carbs but I thought I’d ask what would happen if I move the rod to the next hole . I honestly can’t find any evidence the carb has ever been off the car .

Thanks

[77 TRASHCAN]

For more shot volume, arm needs to be in inner hole, on any car...usually.

[carbking]

The position of the pump operating rod is not going to effect hot starts in a positive manner, although it might help cold starts.

To understand the workings of the accelerator pump, think of a round cylinder (the pump cylinder) with a hole in the bottom, and a movable plug (the accelerator pump) in the top.

The pump shot is a function of the volume of the pump cylinder displaced by the movement of the pump. The higher the accelerator pump in the cylinder, the more fuel may be displaced by the action of the pump.

One needs to look at two issues when determining which hole in the arm will produce the largest shot.

(1) The height of the pump shaft where the pump rod contacts the shaft. The accelerator pump is articulated, and will eventually bottom out in the pump cylinder. The higher the pump in the cylinder, the more fuel that will be displaced until the pump bottoms. The articulation is necessary to allow throttle movement past the point where the pump bottoms. The articulation allow the pump shaft to continue to move, and the length of the pump will compress as the pump is bottomed in the pump cylinder.

(2) The linear movement of the pump shaft due to the movement of the pump arm. The pump arm is held in place by a pin that acts as a fulcrum. The travel of the pump operating rod (the one attached to the throttle arm) is constant; so the hole closest to the fulcrum will provide the greatest travel. With the articulated pump, this dimension generally effects the time that the shot is delivered, rather than the overall volume of the shot.

Generally, with hot start issues, very little pump OR NO PUMP action is desired; as the intake manifold mixture is already too rich due to vaporization of the fuel.

http://www.thecarburetorshop.com/Tro...m#Hardstarthot

Jon.

[gto19]

Thanks Jon

Now I understand it won’t make any difference in hot start issues and I figured that . I was just referencing to the thread the person noticed the arm in the lower hole . By moving it to the other hole will I notice anything performance wise or should I just leave the arm in the hole it’s in ?

Thanks

[Cliff R]

EXCELLENT post Jon.

With a Q-jet in particular there are also at least a dozen different pump lengths so the height of the pump seal in the chamber will vary some depending on which one is being used and which hole in the pump arm the link is in.

The accl pumps initial purpose is to cover up a lean conditions with relatively quick throttle movements so an important part of daily driving. Without good pump shot or any delay in it there can and typically will be some hesitation/stumble/bog with many driving scenarios.

It's secondary purpose is to provide a strong/steady pump shot going to full throttle and why there is a duration spring on it as the pump arm move quickly to it's lowest position for full accl pump delivery. In the going quickly to full throttle scenario the accl pump will continue to supply fuel to help cover up any potential lean condition when the huge secondaries come in and before fuel gets flowing good from the POE system and main nozzles. The duration spring combined with the size of the exit holes in the airhorn determine how much and how long the fuel will be delivered until all the fuel trapped under the plunger is gone.

A really BIG deal here is that we find at least 50 percent of the carbs that come in here using accl pumps with the WRONG spring on them, or the wrong pump for the spring. The have too many coils and too compressed so they "coil bind" going to full throttle, which is NOT a good scenario.

This is why we ALWAYS supply a complete pump assembly with the right springs on it in all of our rebuild kits. This takes mixing/matching and incorrect function OUT of the equation.........plus we offer a lifetime warranty on the pump seal as I toss the ones supplied on them and install one that will not fail in this new fuel, or in ethanol, methanol, E-85 or most anything else you may choose to run thru your carburetor.......Cliff

[carbking]

Cliff's post above about the lengths of the pumps is something to consider.

The pump length is just one of many "tuning" items available when modifying a Q-Jet.

And I totally concur about the pump articulation spring. The company that supplies wholesale (in large quantity) carb parts obsoleted the good spring, and kept the one with way too many coils. We had to reproduce the good spring ourselves. The commercial carb rebuilders are going to get the spring that coil-binds.

Back on the correct hole; there really isn't one correct for all conditions.

At least as early as 1932 Carter offered as many as 2 holes (later carbs would have 3 and 4 holes) for the pump operating rod. The reason was fuel economy! Vehicles used in city driving with lots of stoplight to stoplight could waste a lot of fuel with the accelerator pump. Many carburetor companies called this adjustment a summer/winter adjustment. In fact Holley placed the letter "S" and "W" on the throttle arm which is where the adjustment was located on the old Holley type AA-1 carbs used on Ford V-8's in the late 1930's. A larger volume pump shot is often desirable in the winter.

One popular carburetor modification using during WWII (at least in rural Missouri) was to REMOVE THE ACCELERATOR PUMP COMPLETELY! A vehicle can be driven without one. And while some here may have experienced gas rationing in the 1970's; that was nowhere near as stringent as the rationing that occurred during WWII.

Jon.