Britbike forum Forums British Motorcycle specific Brands Bsa bulletin board GP2 carbs

So i've ironed out some past issues and have test rode the bike ( 66 Spitfire ) . All is well except the carbs and am looking for some advise. As you may remember the bike had been restored 20 years ago, show winner @ Las Vegas,and only ridden 30 miles since as it was in storage, so i'm going under the assumption that everything on the rebuild is correct.

It starts easily, cold or warm and idles just fine. Runs very strong @ 3000 rpm's and above but very poorly under that when the throttle is 1/8 to 1/4 engaged, in any gear. I'm aware of some of the negative sides of these carbs , " they are not civilized " - " the distance from the needle jet to the spray nozzle gives rather poor low speed throttle response " etc. as described in the original Cycle World road test magazine that came with the Bike .

So before I start looking at the needle jet, clip etc. and the fact that the choke has no effect when starting cold & minimal while running I was wondering if this may be " normal " ? Yes I know upgrading to Monobloc or Concentric carbs would make life easier and I just may do so but am trying to get a feel for what to expect with stock ones.

Any input would be appreciated. Thanks !

When I got my '66 together and generally had the carb's more or less sorted, I had a flat spot off idle. As you probably know, the slides are pretty lean in comparison to Concentrics or Monoblocs (5.5s as I recall). I had been setting the pilot jet with a ColorTune. I ended up ignoring the ColorTune and richening up the pilots until the flat spot was gone. Runs fine although it can load up if it idles too long. Not sure if that is your problem or not.
Second issue. You say yours idles fine. Typically, they didn't come from the factory that way. BSA published a number of service bulletins on various ways to get an idle. If yours idles because the cables were simply adjusted too tight to lift the slides enough to get an idle, the result will be a jerky delivery off idle and at low speeds because the cables are tight and the slightest movement changes the slide position. In my opinion, the best solution for idling was special cables that BSA put out that had the screw end for the slide at the bottom and a a second ferrule a few inches above that. A slotted threaded hollow rod went into the top of the carb where the cable goes through. The second ferrule fit on top of that. By turning the threaded rod, you could raise and lower the slide. Meanwhile, a regular cable adjuster in the outer cable permitted you to adjust for free play and eliminate the too sensitive issue of a too tight throttle cable. Once again, not sure if this is your issue or not.
In answer to your basic question, no, they are not all like that. Mine are a bit like slightly worn out carbs (occasional blip at stoplights) at low end, but just fine above that.

Ed from NJ

Ed's comments are very good especially about finding the right compromise between best idle/best (or acceptable) response off idle. This assumes that the carbs are balanced perfectly as, at light throttle, this is critical. If you can get a good idle and good off idel response then try holding the throttle a bit over 1/8 and about 2,500 while underway and then turn off the fuel tap. If it speeds up before running out then the slides may be too rich. If it stumbles and does not pick up then too lean. If you want to get 'scientific' put and air/fuel meter on there .
My own bike was a bit fat where you describe. It came with #5. I was able to find a pair of 5.5 slides and that fixed it up nicely. Hope this helps, PRT

The colortune is a good bit of kit but what it tells you needs interpretation, for a good idle you aim for the Bunsen burner blue which equates to a 14.7 to 1 ratio. Fine for a steady no load idle, however for a just opening throttle then you need the colortune to show yellow ie a rich mixture for best power. So if you are having to enrichen the idle to get rid of the stagger just on throttle opening then your slide cutaway is too lean. Once your cutaway is correct then the pilot mixture can be reset back to Bunsen blue and on opening the throttle you will get the yellow rich mixture. When I first got one some decades ago I had this issue but the instructions they give you did not cover slides directly linked to the throttle cable but the common SU constant depression carb where the slide moves independently of the throttle cable so its not clear.

Thanks to all 3 of you !