Pump travel vs Duration spring travel.

[Peer81]

Hello everybody!

Small question,
I'm looking into the accelerator pump travel theorie and I came across something.
I want to increase the pump travel (shortening the shaft) but here is the thing.
The max pump lever travel from idle to full throttle is 11.1mm but the max travel of the duration spring on the pump is 10.0mm. At this moment my max pump travel is 8mm.

As known the duration spring is there because fuel will not compress and better a duration spring compressed then the pump lever bend! But if I increase the pump travel to the max 11.1mm there will be 1.1mm that will not compress and put force on the fuel. Is this a smart thing to do or is it better to stick with the 10.0mm travel so the durations spring can take the beating?

I also know Doug Roe says to max shorten the shaft between 1/16 (1.6mm) and 3/32 (2.4mm) but this is also a theoretical question.

Are there also known negative sides on shortening the pump shaft? I don't mean that the pump seal will go above the refill hole but if there is to much fuel entering the carb.

btw. To make this easier for myself I put everything in CAD so it is easy to see what will happen.
Thanks!

Greetings Peter

[Cliff Ruggles]

The accellerator pump used on the q-jet is a very sturdy and realiable part.  They came in dozens of different configurations, length, retainer location, spring tension, type of seal, etc.

The need to modify the pump should match the job the carburetor is being asked to do.  Simply increasing pump capacity by shortening the shaft doesn't necessarily mean that the engine will want or need the addtional fuel.

This is one of those areas where one pretty much has to test the carburetor on the application it is being used one, make changes, test again, etc, to determine the exact amount of fuel the pump needs to supply when it is called upon to do so......Cliff

[Peer81]

Thanks for the reply Cliff,

Of course with my near stock engine there is no need to make the change but looking into this I think it's never wise to the travel bigger then the duration spring can handle but I could be wrong.

Groeten Peter

[Cliff Ruggles]

The duration spring is very important.  In the mid-1970's the accellerator pumps started showing up with a lower retainer position, and stronger spring.  They also added a garter spring under the seal to keep it in contact with the bore. 

Our pumps use custom springs wound to our specifications with a matching spring that goes under the pump.  We use the later style pump with garter spring, and install a good ethanol resistant seal on them.  We also provide a new retainer with them.  They are available separately from our kits, and we will cut the groove on the end for the rubber boot upon request.......Cliff

[Peer81]

Thanks, the missing parts arrived this week and I hope to finish the carb tomorrow!
The secondairy airflaps are now at 1.3" opening and 3/4 spring so I think that is perfect to begin with.

Groeten Peter