According to the SU carburator literature, the spring is in the carburator
as one way of insuring that the mixture is correct at wide open throtle at
all engine speeds. Ie., it works with the taper on the needle and the vacum
generated by the engine. Ideally, the carburator should be sized so that
the piston only is all the way up to the top when the engine is a peak revs.
at WOT. Anywhere else in the rev range the piston should be partially down
and at the same time the spring should be selected,(there are 5 different
strength springs) to give a continous smooth travel of the piston at the
desired intake manifold vacum. I am not sure what that would be but it is
probably around 1.5 inches of mercury. The "blue" springs are the lightest
and the "red " springs are next in line. For optimum racing performance the
carbs that come on any given engine are most likely one size to small. Ie.
a TR-4 that came with 1 3/4 inch carbs can usefully be fitted with 2 inch
carbs and an MG-B would probably like at least 1 3/4 inch carbs. The real
answers are to be found on the dyno. I have only dynoed 1 3/4 inch carbs on
an 2187cc TR-4 engine back to back with 45 dcoe Weber carbs. The engine had
12.8: CR, a 300 degree full race cam, ported head, standard size valves,
header, etc. With the Weber carbs the engine made 174 hp at 6,000 rpm.
With the SU carbs it made 163 hp at 6,000 rpm. The power curves began to
diverge as soon as the engine came on the cam at around 4,000 rpm. It
appeared to me as though the Su carbs just wouldn't flow as much air as the
Webers. I probably could have gotten a litle more power with the SUs by
advancing the ignition timing more than the optimum figure with the Weber
carbs, but I did not try to do that as the power disadvantage was so large
that I didn't think it could be overcome entirely.
Regards, Greg Solow
----- Original Message -----
From: Neil Cotty <neilc@tradesrv.com.au>
To: <vintage-race@autox.team.net>
Sent: Thursday, July 08, 1999 12:12 AM
Subject: Re: SU Carbs
> Hi all,
>
> >effectively goes lower in the tube than when it's at rest, and more oil
> >doesn't make it work any better than just enough.
>
> Yep but just make sure there is enough oil to initially cover the damper
> when the throttle is closed, because if there isn't and with the piston
> resting on the bridge - on acceleration the piston will rise and
effectively
> foam/splash the oil and induce a slight hesitation. You can see the oil
> leave via the cap vent! Apparently you see the remains on the bonnet
felt..
> <G> Don't ask how I know. :)
>
> Jim - I will have to try the method you outline, however it only takes a
> minute to remove the dashpots and while I'm there I give the carbs a quick
> squirt of carb cleaner to remove any buildup. I don't find I lose hardly
any
> oil so I don't have to go in there very often. Bill at SU Midel here in
> Sydney advised me that the springs do SFA and never be concerned if one of
> them is slightly shorter etc than the other one. Either way I'd prefer to
> have oil in the dampers and springs fitted when racing for the exact
reason
> Brian mentions, you aren't *always* at full throttle, and I think you
would
> notice some hesitation without oil & springs.
>
> BTW, SU Midel is now offering Solid State SU Fuel pumps that are
*identical*
> to the original points types, they're also selling kits to upgrade
existing
> points type SU Fuel pumps to solid state. Sounds cool, no interest yadda
> yadda yadda - Web address is: http://www.sumidel.com/ Take a look on the
> News page.
>
> Cheers,
> Neil.
> --
> Neil Cotty - Sydney, Australia
> 1970 MG B GT / 1961 MG A 1600 Mk1 / 1956 AH 100 BN2
> GHD5UA200702G / YGHN2 100102-1399 / BN2L-232789
>
>
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