Hi, Frank.
It sounds very much like poor centreing to me. Later SUs had biased (Sprung)
needles but fixed needle carbs are not at all tolerant of out-of-centre
needles. However, check first that uyou don't have a dent in the dashpot.
This will be clearly visible as damage to the outside of the casting. If you
have this, relieving the clearance is only a cheap fix-a replacement is a
much better bet.
The trick I use when centreing is to run the jets up on the threads
(remember where they were if you want to keep the mixture setting) and
loosen the jet locking nut.. Then, with the damper out, find something like
a long, thin screwdriver, or rod, to push down firmly on the piston while
locking the jet nut. If you can't hear the piston drop with a soft 'click'
it's wrong. Try it both before and after resetting the jet height.
BTW, if you look very carefully, you will see that the largest diameter of
the piston is not a tight fit in the dashpot. That is a controlled air
bleed, which is why I never use metal polish or nay other abrasive inside
the dashpot or on that part of the piston. A long soak in cellulose thinners
should be enough to remove any carbon or dirt in the dashpot/piston assy.
Finally, you shouldn't need to convert to biased needles. In theory, the
biased needle, being spring loaded against the jet orifice, will wear both
parts more quickly.
Good Luck,
Dave H.
> Listers,
>
> First, thanks to all who helped with advice this weekend. Unfortunately
the
> answer to my float bowl flooding problem came to me in the middle of the
> night and Sunday I corrected the problem. It was my own stupidity (that I
> don't know how to correct <g>). I had placed the float needles in
backwards!
> My 68B had Grose Jets and I was replacing them with standard jets and I'm
a
> newbie, that will never happen again...
>
> Now the current problem! My rear piston is sticking. When I use the
lifting
> pin or a small screwdriver to raise the piston (as in tuning the carbs)
the
> piston has an initial point of resistance that must be overcome to rise.
> According to Haynes I should use metal polish to clean the area causing
the
> sticking. I have two questions:
>
> 1) How do I determine the spot that is causing the problem or should I
> simply make the whole piston and chamber insides shine?
>
> 2) As I rebuilt the carbs I had to center the jets, could poor centering
> cause the problem? In other words, is it possible that the piston is
sitting
> at an angle and thus does not rise correctly in relation to the chamber?
> Could this be tested by fitting the piston without the needle and lifting
it
> or would the absence of the needle cause the same condition?
>
> One last idea... I also have a set of 69 carbs and the piston bodies are
> both stamped AUC-1331 but the needle holding area is different, one is
fixed
> one has a spring. Is it possible to adapt these?
>
> Thanks for all the help. I can't wait to get my B back on the road!!!
>
> Frank Phillips
> Email: fwp@ufl.edu
> 1968 MGB - http://plaza.ufl.edu/fwp/mgb.htm
>
>
>
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