<VV> The YH Pump - THE final answer

[Frank DuVal]

In my memory my Spyder YH accellerator pump does slightly stroke with 
the throttle linkage. So I set out to confirm this.

I took apart a spare YH I purchased back in the early 80s, seeing as the 
one on my Spyder has not run in several years and will need cleaning to 
get apart.....

This YH performs just like Bob says it does! The accellerator pump shaft 
does not move down with the throttle linkage. The spring (lower) is very 
tight and the diaphram is slightly stiff. So the only thing that pulls 
the shaft down is vacuum on the lower side of the diaphram. If you push 
the shaft down with a finger (and a hard push is needed) then the 
throttle shaft will raise it, along with the metering rod.

I am crawling back under my rock so as not to provide any more wrong 
answers.....

Someday I must clean the Spyder YH and see if I am just getting senile. 
Maybe it has a really light lower spring, or something is out  of whack 
at the top of the pump rod.

Frank Duval

BobHelt at aol.com wrote:

>
>
> It's strange that such a simple device gathers such varying opinions 
> of operation.
>
>
> For the YH accelerator pump to work at all, manifold vacuum MUST be 
> present to suck the pump diaphragm down. There is no throttle action 
> that will cause this to happen, only manifold vacuum. Once the 
> diaphragm is down, fuel will fill the pump chamber waiting for the 
> throttle to be opened. The opening throttle will raise the pump link 
> which will allow the metering rod arm to raise the metering rod for a 
> richer mixture and will also allow the lower spring to force the 
> diaphragm to raise (overcoming the now lower vacuum holding the 
> diaphragm down) forcing the fuel in the pump chamber into the air stream.
>
> This is all with the engine running.
>
> When the engine is shut off, the pump diaphragm loses its vacuum and 
> raises up as far as it will go. Now any opening of the throttle will 
> simply raise the metering rod and its arm (with no effect) but the 
> pump link simply slides inside the metering rod arm and does not move 
> the diaphragm at all. Thus opening and closing the throttle with the 
> engine off will not actuate the diaphragm at all.
>
> Regards,
> Bob Helt
>
>
>