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Re: SU needle adjustment question...

To: ken__pearce@hotmail.com, datsun-roadsters@autox.team.net
Subject: Re: SU needle adjustment question...
From: Keith0alan@aol.com
Date: Mon, 7 Apr 2003 20:48:28 EDT
Hello,

    Lifting the piston leans the mixture. While the nozzle is more open 
because the needle is pulled farther out there is very little velocity across 
it. It is the velocity across the nozzle opening that draws the fuel out of 
it. The design of these carbs are such that the piston floats up and down 
with engine demand keeping the velocity across the nozzle fairly constant. So 
lift the piston to add more air and reduce the fuel, lower the piston to 
reduce the air and increase fuel.

    Classic indications of worn out needle and nozzle. Pull off the dome and 
remove the piston. Check the nozzle hole. It should be round. I have seen 
them pear shaped. This is caused by the nozzle being not centered properly. 
Check the needle. The original finish on the needle is slightly satin. If it 
has shiny spots, usually near the base, it has been rubbing and needs to be 
replaced. The dimentions of the needle diameter are precise to .0001 inch. A 
little wear in the idle area can make a big differance. Sliding the needle 
out may make things better but definatly is a stop gap measure. A sixteenth 
of an inch is quite a lot. Get out the feeler gauges and try moving it .010" 
at a time.

                             keith williams 



> 
> Two questions:
> 
> 1.)I've been told, and I believe it's in the various reference manuals that 
> 
> lifting the piston of the carb with your finger effectively disables that 
> carb.  Doesn't this take the needle completely out of the jet and leave it 
> wide open?  Or is it too open and therefore not have enough air pressure to 
> 
> suck fuel in?
> 
> 2.)When I turn my mixture adjustment all the way lean on this carb, the car 
> 
> will still run on that carb alone.  In an effort to further lean it out, 
> I'm 
> adjusting the needle so that it sticks out further from the piston, about 
> 1/16".  Will this help me lean it out further or am I barking up he wrong 
> tree?
> 
> Thanks,
> 
> Ken Pearce 68-2000
> Bellingham, WA

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