[Roadsters] Z carb help

[John F Sandhoff]

[copied to list since it's an SU question]

Tim asks:
> "...The problem is with the slow speed circuit on the rear carb. It will
> run at high speed, but appears to not provide fuel to the rear three
> cylinders at slow speed...

There is no slow speed circuit on an SU - all fuel flow is controlled
by the variable venturi opening and the needle position.

I suspect the problem is the rear carb is either set way too lean,
or there is a needle problem, or the fuel (float) level is way low:

1) Idle mixture is set by the 'thumbwheel' that moves the needle
seat up and down. He needs to try richening the mixture by turning
the thumbwheel so it lowers the seat

2) If the needle has come loose and dropped down, it will be plugging
the opening at idle. He needs to remove the dome, carefully remove
the piston (remember the oil that's in it!), and make sure the needle is
properly seated. There's a set screw that holds it in place; it should
be inserted so the shoulder is flush with the face. ALSO, if the
wrong needle has been installed, that'll play havoc!

3) If the fuel level is way low, there won't be enough air flow (vacuum)
across the needle/seat to suck fuel at idle, but at higher engine
speeds you'll start drawing fuel. Personally, I'd look at this possibility
first.

-- John
     John F Sandhoff   sandhoff at csus.edu   Sacramento, CA