I agree about the race car part - that's my experience on my two G Production
cars also. I suspect for a motor that lives at 5K plus it's a moot point. I
have yet to verify this for the Datsun, but when I was running a G Spitfire,
another Spitfire owner had a chance to run his car on a portable dyno that was
at the track - he observed that the slides were all the way up from 4K up.
Anyone know if this applies to the 1600s (in race tune)? Or was it that his
carbs were all screwed up and the slides should have been working? Comments?
Also, I have never had any real sputtering/hesitation problems with either car
coming off idle. Of course idle is 1500 to 2000 and so rough that maybe I just
never noticed any sputtering or hesitation.
Pete Dehmler
Gregg Robinson wrote:
> You are right Jon, this is like trying to decide what oil is best for your
> engine, and EVERYONE has an opinion! I like to use ATF in the cooler
> months.. It seems to give faster response. In the summer motor oil may be
> best. The whole theory behind SUs is to give you a variable venturi... On
> the 2L race car when the rules required SUs for D Production we ran nothing
> in them at all. This made them cough a sneeze at slow speeds, but so did the
> race cam... But oh the throttle response at 4800 and above (we wound it to
> 7800) The Solex carbs and cam have a reputation for being less drivable on
> the street, because there is too much carburetor for that size engine until
> you get the RPM up. When you have a large bore carb on a car and mash the
> throttle open you get a huge drop in induction system vacuum. This almost
> completely stops the carburetor from drawing fuel into the air stream, hence
> they have accelerator pumps to squirt extra fuel into the airstream until
> the engine speed picks up and the system starts running normal again. With
> SU carbs there is no accelerator pump. When you mash the throttle the slides
> that are dampened by the oil essentially varies the size of the carburetor
> keeping the system drawing air and fuel at a more constant velocity. On the
> race car we had enough compression and we wound the engine high enough that
> the solex carbs gave better drivability.
>
> So.... If when you mash in the throttle at a fairly slow speed and the car
> falls on it's face a little, try a thicker oil. If it just takes off and
> screams.. you might get quicker response with thinner oil.... just my humble
> opinion and no scientific experience!
>
> Gregg
> '70 1600
> McMinnville, OR
>
> -----Original Message-----
> From: owner-datsun-roadsters@autox.team.net
> [mailto:owner-datsun-roadsters@autox.team.net]On Behalf Of Jon Wissler
> Sent: Sunday, March 19, 2000 08:40 AM
> To: datsun-roadsters@autox.team.net
> Subject: SU Carb Oil Question
>
> Here is another classic neophyte (aka stupid) question concerning
> which oil to use in SU carbs. I have told to use 10W, 30W, and ATF from
> various sources. What does this learned list have to say on the subject,
> and are there different conditions to warrant the use of different fluids.
> Thanks in advance for the advice.
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