Marc,
You are on the mark regarding the problem. I have a book on SUs that goes
into detail regarding the reprofiling of needles to each engine's needs. He
also discusses piston spring rates and other modifications. I highly
recommend the book which is "How to Build and Power Tune SU Carburettors" by
Des Hammill. I bought it from Classic Motorbooks.
For now I will try the idea suggested by Mike to make sure that it is indeed
a needle profile problem and not some other problem with the fuel system
such as fuel delivery.
As you suggested I advanced my base setting to about 18 degrees advance and
checked the advance of the distributor by making additional marks on the
crankshaft pulley. While not super accurate it is fairly accurate and saves
me an additional cost until I can find someone who owns one.
BTW, I did some digging into all my extra SU parts and found that 2L SUs
came with 2 different piston springs, 1 set being about 1" longer than the
other. In comparision the 240 springs are about 1" shorter and 1600 springs
are almost 1.5" shorter. At the same given height they all exert a different
force to the piston.
In the book, Des recommended the first change to make is the spring rate. I
will try the various springs and see what happens. The stronger the spring,
the leaner the needles can be. And vice versa. A weaker spring will permit
quicker acceleration and more throttle response but the needles have to be
richer.
I may be learning more about SUs than I care to. I just want to go out and
play :(
Mike
----- Original Message -----
From: "Marc Sayer" <marcsayer@home.com>
To: "Mike Harper" <roadsterdude1600@yahoo.com>
Cc: "datsunmike" <datsunmike@nyc.rr.com>; "Datsun Roadster List"
<datsun-roadsters@autox.team.net>
Sent: Saturday, June 02, 2001 5:01 AM
Subject: Re: Runnin Outta Gas
> Problem is that the lean out isn't a sudden change, nor is it necessarily
only
> above a certain rpm/throttle opening. In fact it may affect only a certain
range
> then go away above that. The basic problem is that the taper profile of
the
> needle no longer matches the fuel requirements of the engine throughout
the full
> operating range. The needle taper is like a fuel map in a modern FI car,
it
> matches the fuel supply to the needs of the engine for any given rpm,
load, and
> throttle position. When the mixture at idle is wrong you can adjust it,
but that
> changes the mixture at all rpms. Unlike a Mikuni or Weber, where you can
adjust
> the mixture for each phase of operation separately, with an SU the only
things
> you can change are the needle profile and the damper action. The damper
action
> affects mixture during acceleration only and the needle profile affects
mixture
> during all phases of operation. This is the simplicity of the SU, which is
its
> strong point. But without sanding or filing on the needles, or changing to
a
> different needle profile, you can't change one phase of operation without
> changing all the others as well, which for specialized tuning needs, makes
it
> its weak spot too. Your suggestion would only work if the leanout occurred
only
> at a given rpm and above and at all throttle positions. And the leanout
would
> need to occur rapidly, which is not likely. Generally the leanout comes on
> gradually even though the driver may only notice it once it gets beyond a
given
> point. Besides having to fiddle with the choke all the time would not be
very
> safe. Your suggestion could get a person home, or as has happened to more
than
> one racer I know of, let you finish a race, but it is not really
addressing the
> core problem.
>
> Mike Harper wrote:
> >
> > Mike, maybe this is a stupid solution, if so I hope
> > the listers will explain why (I have some suspisions),
> > but why not give it one or 2 notches of choke at high
> > RPM's if that smooths it out? Thst leaves your low
> > rev mixture lean enough, and only enriches your high
> > rev mixture. Is this too simplistic??
> >
> > Mike Harper
> > '66 1600
> > Charleston SC
> > --- datsunmike <datsunmike@nyc.rr.com> wrote:
> > > The formulation thing makes sense. If they are
> > > running MBTE it is an oxygen
> > > enricher for gasoline and would displace the
> > > gasoline so it's less bang for
> > > each ounce which is why you get worse gas mileage
> > > using it.
> > >
> > > Makes sense but . . . that still leaves me with a
> > > problem I have to fix.
> > >
> > > It definitely runs lean under steady throttle yet
> > > it's rich under
> > > acceleration and idle.
> > >
> > >
> > >
> > > ----- Original Message -----
> > > From: "Marc Sayer" <marcsayer@home.com>
> > > To: "Datsun Roadster List"
> > > <datsun-roadsters@autox.team.net>
> > > Sent: Friday, June 01, 2001 2:34 PM
> > > Subject: Re: Runnin Outta Gas
> > >
> > >
> > > > Its the needles. The profile is not right for the
> > > engine and at steady
> > > throttle
> > > > you are going lean. Since it does not lean out on
> > > hard acceleration (which
> > > is
> > > > enriched by the damper), it is not a fuel feed
> > > problem, which is the only
> > > other
> > > > thing that could cause this. Just to be sure,
> > > check your fuel pressure and
> > > float
> > > > level settings, but I'll bet they are within
> > > specs.
> > > >
> > > > Lots of folks are starting to run into a lean-out
> > > condition like this,
> > > even with
> > > > a stock engine and new stock needles. I am not
> > > sure why. We miked several
> > > new
> > > > needles to see if they were different from the
> > > older versions and they
> > > were
> > > > identical, within tolerances. Same engines, same
> > > carbs, same needles, but
> > > now
> > > > the cars are running lean??
> > > >
> > > > All I can think of is that something they have
> > > done to the gas is causing
> > > the
> > > > problem. I know that on my FI cars a change in gas
> > > formulation is very
> > > > noticeable. When I drive down to SoCal, my cars
> > > always run worse on CA gas
> > > than
> > > > they do on the OR gas. We get a different
> > > formulation up here. So I am
> > > wondering
> > > > if one of the newer additives, like say the MBTE
> > > or other "oxygenators" is
> > > > causing the fuel to act "leaner" somehow. Any
> > > chemists on the list that
> > > want to
> > > > try and answer this? Anyone else got any ideas?
> > > >
> > > > Nathan Ruffcorn wrote:
> > > > >
> > > > > Mike-
> > > > > I have always experienced the same problem on my
> > > 2L (N17 needles). Mine
> > > > > will keep power while aggresively accelerating,
> > > but if I hold a steady
> > > > > speed, it tends to surge. I hate to do this,
> > > but if I'm on the highway
> > > I
> > > > > pull the choke out part way. You may try this
> > > as it will help determine
> > > if
> > > > > you are truly getting enough fuel. The choke
> > > cleans it right up, but I
> > > hate
> > > > > the chance of running too rich and washing my
> > > cylinder walls. I've
> > > > > experimented with richening the carbs
> > > dramatically, but I get the same
> > > > > result (also required radically increasing idle
> > > speed setting to run-not
> > > too
> > > > > practical). At first I thought the needles were
> > > causing the lean
> > > condition,
> > > > > now I'm not so sure. Any thoughts?!
> > > > > BTW, manifold vaccum is good through all RPM's.
> > > > >
> > > > > Nathan
> > > > >
> > > > > ----- Original Message -----
> > > > > From: datsunmike <datsunmike@nyc.rr.com>
> > > > > To: Datsun Roadster List
> > > <datsun-roadsters@autox.team.net>
> > > > > Sent: Thursday, May 31, 2001 8:08 PM
> > > > > Subject: Runnin Outta Gas
> > > > >
> > > > > > Well I finally took my car out for a spin and
> > > it was great! I haven't
> > > > > driven
> > > > > > my car in over a year and I forgot what it
> > > felt like.
> > > > > >
> > > > > > While the car generally runs well I have no
> > > power above 4K RPMs. It
> > > feels
> > > > > > like it's running outta gas. I have 2L carbs
> > > with N17 needles and
> > > one1/4"
> > > > > > fuel line supplying both carbs from the Facet
> > > electric fuel pump to
> > > the
> > > > > > carbs. Do you think this could be the problem?
> > > I am not running lean
> > > as if
> > > > > I
> > > > > > richen it my idle deteriorates.
> > > > > >
> > > > > > Thanks for any help you can provide.
> > > > > >
> > > > > > Mike
> > > >
> > > > --
> > > > Marc Sayer
> > > > 82 280ZXT
> > > > 71 510 2.5 Trans Am vintage racer
> > Get personalized email addresses from Yahoo! Mail - only $35
> > a year! http://personal.mail.yahoo.com/
>
> --
> Marc Sayer
> 82 280ZXT
> 71 510 2.5 Trans Am vintage racer
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