I had a similar problem, #2 carb (on #3&4 cyls) wouldn't respond to adjustment
the same as #1 would. After much swearing at carburettors I decided to remove
the carb for a closer look. The problem turned out to be a small fracture in
the brass pipe that is pressed into the underside of #2 carb that the vacuum
pipe goes onto. After I fixed the pipe all was back to normal again.
Alan Bent - Queensland , Australia
1966 Fairlady SP311 - 1965 Silvia CSP311
1966 Fairlady SP311 - 1964 Fairlady 3 seater SP310
1964 Fairlady 3 seater SP310 - 1964 Cedric Wagon WP31
1963 Bluebird P312 - 1963 Bluebird P312
1963 Cedric P31 - 1964 Cedric Wagon 1964
1969 Super Six G130
Early Datsun Homepage-
http://www.geocities.com/MotorCity/Pit/3823/index.html
Eric Armstrong wrote:
> Hi Folks -
>
> Having trouble adjusting my SU carbs...
>
> Some of you may remember that barbeque day at Mike Carion's place --
> several people tried to help me solve a problem where at about 4-4.5k rpm
> my roadster starts sputtering or backfiring through the carbs...
>
> Anyway, the net result of that assistance was a much richer running
> roadster. At idle, you could smell the gas, real strong, and while the
> backfiring problem seemed to have gotten a little better, the motor still
> misfired at a fast cruise. While at Shasta, it was running so rich that
> it was almost impossible to keep running at idle for the run to the rocks
> (at high altitude).
>
> So I picked up an airflow meter and a Colortune kit. The airflow matches
> between the two units, but I can't get the mixture right. My front carb
> mixture will adjust so I get the correct light blue flame at idle. The
> color stays the same light blue when I wind up the motor (colortune says
> this is the correct behavior). The other unit, however, will not allow me
> to achieve this light blue flame at idle.
>
> I can only get a yellow flame (too rich) no matter what setting I put my
> mixture screw at. Colortune says that this means I may have any of these
> conditions applying:
>
> 1) leak at float needle or float needle out of adjustment.( I'm flooding
> the carb all the time)
> 2) jet needle out of adjustment
> 3) worn needle seat
> 4) worn needle
>
> I've checked for 1 & 2, I don't know how to check for #3 or 4. The needle
> looked OK when I disassembled the suction chamber. I checked the float
> needle and lever -- the needle sealed the valve when depressed, and the
> lever measured per the specifications in the roadster manual (14-15mm
> space between the top of the lever and the underside of the float bowl
> cover)
>
> I've called a couple people -- they said check to make sure the choke
> isn't on all the time (checked and a-ok).
>
> As far as the backfiring goes, another person said check the oil level,
> your damper may be opening too quick and leaning out the rear carb. I
> don't see how a low oil level could affect me at idle, but went ahead and
> checked the oil anyway and the level was fine. Dampers on both carbs have
> good resistance, and now that I've taken apart the rear carb's suction
> chamber (had to pour out the oil), I know what a damper with a low oil
> level feels like....
>
> So anyway, anybody got some advice? Anybody want to take a trip over to
> help out, and get some free beer and free use of a Colortune kit as part
> of the deal? I'm reluctant to drive anywhere now, as I understand running
> too lean can lead to burned exhaust valves (if I haven't burned 'em
> already in my first 6 months on the road). I don't think I have burned
> valves because I only have 3500 miles on a rebuilt 1600, and on my only
> long drives (from CDM to Berkeley, and Berkeley to Solvang and back) I
> didn't really push the car too hard.
>
> Would messed up valves cause me to get these results with a Colortune kit?
> Would inadequate compression cause a problem like this on the rear
> cylinders?
>
> I've read some web pages, particularly the SU tech tip page on Ross's
> site. I'm hoping that adjusting the carbs will cure my problem instead of
> having to do all kinds of measurement if my needles are wrong or worn, or
> other things wrong.
>
> thx for any help...
>
> -Eric
> '66 1600 w/a '68 5-main motor
>
> Eric Armstrong
> ~~~~~~~~~~~~~~
> phone: (510) 502-5984
> e-mail: ericarm@alum.calberkeley.org
> http://alum.calberkeley.org/~ericarm
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