Accelerator pump travel with carb. Assembled study

[Mr.Pontiac]

17056240 race car, had some time to kill, studying accelerator pump travel.If I take a small screwdriver with arm connected to pump and push down on pump it actually go,s down inside of airhorn travel. Showing no stop or end. Am I correct in thinking travel should be 21/32 to the end? Disconect the arm rod and total amount sticking up out of my carb. Is 24/32. Have I been running the wrong accelerator pump in my carb. All this time? Great time for me to learn about this.I bought this kit from you maybe 3 yrs ago and has been running like a dream

[old cars]

I believe the preload on the pump arm needs to be at least 1/64 inch  . Meaning when you hook up the linkage it has moved the accelerator pump down at least 1/64

[Cliff Ruggles]

The pump travel is limited by where you put the link in the pump arm.  On the inner hole it will have a longer stroke or more travel.  So basically moving the link closer to center on the pump arm increases the stroke of the pump.

The length of the pump also limits how much fuel the engine gets because no matter how much travel the pump arm has it will bottom out the pump after the spring fully compresses.  The spring on the pump is very important to fuel delivery and the duration of the pump shot.  I see a LOT of pumps that have the wrong spring on them and they coil bind when fully compressed (very bad).  This is why we sell complete pump assemblies with the correct spring on them.  There have been so many incorrect pumps and pump springs installed in these carbs over the years it is always best to replace the entire assembly vs just putting a new seal on an old pump.

Shorter pumps delivery more fuel per stroke simply because they locate the seal higher in the pump bore leaving more fuel under it  A shorter pump isn't needed for most applications as the pump is there solely help prevent a lean condition caused by very quick throttle movements right off idle or from very light throttle.  This is why the duration spring is so important.  A stronger spring ensures good fuel delivery early in the cycle before fuel is flowing well from the boosters.  A strong pump shot also helps when you go quickly to full throttle as it puts a LOT of fuel into the engine when the huge secondaries open.  The POE system will be lagging behind just a tad so I can't stress enough how important the accl pump is to good throttle response in all scenarios.

With a boat or racing application a shorter pump can be beneficial as often some of the pump shot is used up because a good bit of throttle may already be applied when we push the throttle quickly to speed up the vehicle, boat, etc.  This is important for auto trans cars that are drag raced as most racers come up on the converter some on the starting line.  If the pump is too short you've used a lot of the pump shot and can experience a hesitation/stumble/bog going to full throttle on the launch.

For your boat imagine that you are cruising along about 20 mph and need to very quickly go to full throttle to come up on a plain.  If the pump is too short and no further pump shot available the engine may "lay down" for a second and in extreme cases stumble and stall out.  This is why it is imperative to install the correct pump in your Marine Qjet and use a shorter pump for certain racing and high performance applications.........Cliff

[old cars]

To further look at this. When pushing the accelerator pump down it is pretty hard to tell when we have finished collapsing the return spring (under the accelerator pump) and begin collapsing the duration spring on the accelerator pump. But if you were to remove the return spring and set the accelerator pump back into position you can easily see how much is protruding above the air horn. This would be the pump at the bottom of its bore.
To measure travel, would be the measurement from where it is sitting at the bottom of its bore (fullest travel) to where it sits pulled up to the arm. Aproximately 21/32 or .656 depending on the length of accelerator pump you used. Shorter means more travel in bore, equals a better shot. But we can not use an accelerator pump to short or you risk raising the cup above the fill slot in the bore. This would make the pump shot inefficient. I believe early body carbs are more sensitive to this.
Hope this helps

[Mr.Pontiac]

Not, sure if my question was answered but I guess it

[Mr.Pontiac]

Don`t know why my answer was not posted completely but again not sure if my question was answered to what I was concerned about. Read in detail Doug Roe`s book and while I have some time I thought I would study why I fill have to much stem sticking up from airhorn.Pump shot and pump travel was what I was concerned about.Getting every bit of travel and performance from my pump was my concern. Love to see more written about this area of the carb.find it very interesting.

[Cliff Ruggles]

I don't really understand what you are asking for, but my first response covers accl pump function and delivery options.

What is important to keep in mind is that the pump "stroke" is the same depending on what hole in the pump arm you are using. 

If you want more stroke you'll need to drill an additional hole or holes closer to the pivot point.

Really no need for all that, especially since you mentioned that the carb has been "flawless".

The other two ways to increase delivery, or the total amount of fuel trapped under the plunger is to either shorter the pump shaft or the link going up to the pump arm.  Doing either one RAISES the height of the pump plunder in the bore trapping more fuel under it per stroke.

Keep in mind that you are limited with some pumps as the retainer is too high and hits the bottom of the airhorn before the plunger would reach its maximum height.

One must also make sure when modifying pumps, links or arms for increased fuel delivery that the seal must be in the bore to prevent a delay in pump shot when it first starts moving down.

I will also add here that the accl pump is not a weak point on the Q-jet, and most applications will NOT require a short pump to increase how much fuel is added per stroke. 
 
As explained above in my first response shorter pumps were application specific, like a Marine carb for example.  They require a short pump not to increase fuel delivery per stroke as much as they need some pump shot left at higher throttle angles since those engines are heavily loaded in most scenarios.  For example, a boat cruising along steadily at 20 MPH or so will need a lot of throttle angle to maintain that speed.  If the operator chooses to go ahead quickly there may not be any pump shot left if the wrong pump were used and the engine is likely to stumble instead of increasing RPM smoothly.

So bottom line here is that if/when you want to increase accl pump delivery you need to shorten the pump shaft or shorten the link up to the pump arm.  Doing either one raises the pump in the bore.  To increase pump stroke you will need to move the link closer to the pivot point by adding an additional hole in the pump arm.....hope this helps some......Cliff