Hello Ed,
Thank you for a very detailed and believable answer. People who have
recently gone through it are a great source of information.
I am pretty convinced, from a good sized group of people who took the
trouble to reply, that loose shafts are the primary culprit, (I can feel
them wobble) and perhaps loose return springs are the secondary culprit. I
will try tightening up the springs this week. I have opted to rebush the
shafts this winter when the car is off the road and I can completely
rebuild the carbs. I will also put in new shafts because these are
probably out of round as yours were. Thanks for the help.
Thanks to EVERYBODY who offered assistance.
Sumner
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> From: Edward Hamilton <hamilton@ptd.net>
> To: Sumner Weisman <sweisman@gis.net>
> Subject: Re: Carb Assistance Wanted!
> Date: Friday, July 24, 1998 1:09 PM
>
> Sumner,
> Whilst I surely do not qualify as an SU guru, I can offer a few
suggestions --
> having just experienced what your going through. I rebuilt and installed
HS4's
> on my 1500 spitfire. The absolute lowest idle I could achieve was, as
you, in
> the range of 800 -- with the idle adjusting screws backed all the way
out.
> When working the throttle levers, it would not return to the original
idle
> without pressing on the levers.
> This is what worked for me. I had a bit of "play" in the throttle
shafts,
> which I thought I could get away with. Secondly, the butterflies were
fitted
> with the springed "pop" valves. The standard, as manufactured, throttle
shafts
> (HS4's) are .310 diameter. The carb measurements should be .312-.313.
Thetas
> the normal amount of "play." Upon reexamination, I found one shaft to be
a bit
> out-of-round, measuring .306-.309. If you don't have the proper
measurements,
> you can check the amount of play you have, compared to what it "most
likely"
> should be as follows: remove the butterflies and slide the throttle
shafts out
> so an area (unworn) is operating in the carb bushing. Open the throttle
shaft
> about 30% or so, which is a normal operating point and check the amount
of free
> play. If the "play" is much different when the shaft is in its normal
> position, then the wear is most likely too excessive. Most people would
opt
> for rebushing the carbs as opposed to installing oversize shafts. In my
case,
> I replaced the worn shaft, with a standard size one, refitted the shafts
with
> solid butterflies and the idle problem went away. With regard to the
"pop"
> valves on the butterflies, a lot of people routinely seal them shut. The
> thinking being that the springs have worn and allow air to enter.
> I had the exact same problem with regard to "return-to-idle" and
experimented
> with various return springs. You want to use the softest workable spring
to
> reduce or diminish the throttle shaft wear. As a last resort, I
installed a
> spring, which to the feel, I felt to be too stiff and the carbs now
return and
> operate beautifully.
> I did all of this at the same time and may now go back to the previous
spring
> arrangement. It may very well be that the egg-shapped shaft was causing
some
> resistance. You may find the same thing.
>
> Regards,
> Ed Hamilton
>
> Sumner Weisman wrote:
>
> > OK all you SU carb gurus, what suggestions do you have for me?
> >
> > I cannot adjust the idle speed less than 800 rpm. That's fine --
that's
> > where I want it. The idle adjustment screws are backed out all the
way. I
> > can feel a little wobble in the shafts. I will get the carbs rebushed
and
> > I am planning to completely rebuild them this winter. I imagine that
there
> > is a little air leaking in.
> >
> > That's the background, now here's my question:
> >
> > Frequently, when I take my foot off the gas pedal, it hangs up at about
> > 1100 -- 1200 rpm. If I gun it, the idle speed will come back down.
> > Pressing down on each idle screw will bring it about half way back
down.
> > (Pressing down on both brings it all the way down). If I pull up on
the
> > linkage (above the generator) that controls both shafts, it comes all
the
> > way down. The return springs seem OK.
> >
> > I have lubricated every link, bearing, and bushing I could find, all
the
> > way back to the gas pedal. It still hangs up. Any suggestions? Could
a
> > butterfly be hanging up? I really don't want to tie up the car in a
major
> > repair until winter. I got it on the road too late this summer as it
is.
> > Any suggestions appreciated.
> >
> > Sumner Weisman
> > 62 TR-3B
>
>
>
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