I'd bet a donut that the mixture won't change more than a wee tad between
running a spring, and not. The point is that the vacuum signal that raises
the piston really wants to get equalized so it raises the piston until it
is. the difference between the weight of the piston and the additional
force of the spring isn't all that much. Now, for part throttle, street
type use, where the vacuum signal is less aggressive than wide open
throttle, the spring probably does have an effect, and the carb probably
would run somewhat randomly richer.
If t'was me, I'd run some springs. They would probably help keep the
mixture lean during off-throttle or part throttle running, and would clean
up your transition back to full throttle coming out of corners, etc.
Brian
At 11:16 AM 7/7/99 -0400, you wrote:
>Brian Evans wrote:
>>
>> I'd also like to see a couple more Lbs of fuel pressure, but if the volumes
>> OK the pressure might not be a real problem. Float level is important, too.
>> And look for clear fuel lines - I once spent a very frustrating afternoon
>> discovering that one of my fuel connections had a little flap of rubber hose
>> that partially blocked the line only after it was tightened.
>>
>> While the oil in the damper does help control part throttle stuff, it's most
>> important function in a race application is to provide an accelerator pump
>> equivalent function as you get back on the throttle. The throttle plate
>> goes wide open so the volume of air going thru the carb goes up, the damper
>> slows the rise of the slide so the velocity of the air thru the carb goes
>> up, causing vacuum at the jet to go higher, and more fuel gets sucked out
>> than would normally be the case.
>>
>> This idea of signal strength at the jet orifice is also why the 948 A-series
>> and the early 1275 Cooper S used the same needles in their carbs for a
>> period of time. The extra capacity of the 1275 caused the same needle in
>> the carb to run rich enough to fuel the bigger motor.
>>
>> The other thing that comes to mind is that you only need a very small amount
>> of oil - maybe a teaspoonful - in the center tube of the dashpot to cause
>> the thing to work. The test I use is to unscrew the dashpot from the cover
>> and just lift it a bit. If there's some damping type resistance, then
>> there's enough oil. remember that since the slide rises, the dashpot
>> 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.
>>
>> Anyway, that's my take on SU's. Even after I spent a ton of time studying
>> the things I couldn't make them work as well as a Weber, so the SU's went
>> back on the shelf..
>>
>> Cheers, Brian
>>
>> At 11:40 PM 7/6/99 -0400, you wrote:
>> >my take on the oil in the S.U. is that it acts like a shock absorber to
>> >control part throttle flutter. my full race carbs do not have oil in them
>> >at all. correct me if I am wrong but S.U.s function well with 4 psi fuel
>> >pressure, best to run higher pressure (7psi) from the pump up to a regulator
>> >w/guage located under the bonnet set a 4 psi............this assures
adequate
>> >fuel to the float bowl in all "on track" situations
>> >I send this to all the list so my I may get corrected if not completely
>> >acurate
>> >.........................chuc
>> >
>> Brian Evans
>> Director, Global Sales
>> UUNET, An MCI WorldCom Company
>
>While we're on this subject, how many guys run with springs in the
>carbs, and how many without? With springs, will the mixture really be
>richer as my favorite guru says? I don't run with springs but maybe I
>should (wish I had a dyno!)
>--
>uncle jack
>TR4 racer -- yeehaa
>TR6 streeter -- hummahumma
>
Brian Evans
Director, Global Sales
UUNET, An MCI WorldCom Company
|