Spark plug too light after dyno

[Adam_Winstone]

Just to add a small correction, in the case mentioned it was actually the result of the float needle being slightly too broad, rather than too long (a common cause of fuel starvation). This was shown to be the case as another needle of equal length allowed the float to fully drop without restriction, whereas the broader needle would stop the float from dropping fully as it made contact with the fixed needle seat/valve passage wall.

NB: Standard original Dellorto SH2/22 so float needle solid, rather than sprung, and fixed needle valve seat/passage.

The needle in question was not a genuine Dellorto INC / diamond marked component but was obviously supplied at some stage as a replacement part. Indeed, it is actually the same size, flat to flat, as the same length needle that does allow the float to drop fully, however, the chamfers on the corners are not so wide and it is on these part-worked corners that this needle catches against the wall as it drops (rotational movement of float and insufficient clearance of float location to needle bottom to allow it to drop straight / self-correct its alignment).

Unlike trying to fit the wrong size needle to wrong size float seat (e.g. broader Dellorto 350 needle to 200 needle valve of other models), this is just a fractional manufacturing inaccuracy causing a big issue with float movement / limitation. A similar manufacturing issue became evident when everyone started to fit red tipped Jetex float needles to the Dellorto SH range of standard carbs as the Jetex needles were slightly longer and remained a restriction to fuel flow when the float dropped. This caught many people out as the length difference was also very minor but enough to result in starvation. It was also not being considered by people as many dealers were providing them as a direct replacement/upgrade so punters often didn't consider it as a possible problem.

^... I did manage to pick up on this when I too assumed that a swap to dealer supplied ethanol-resistant float needle would be a sensible upgrade. Thankfully, this put me in good stead to identify it when a number of clubmates did exactly the same and immediately found that their previously reliable tourers suffered fuel starvation on the open road... but not around town. When fuel demands were high, starvation became evident. When fuel demands were low, the bikes performed perfectly. This is exactly what we found with the bike that Alex has mentioned above, which we coaxed to Belfast and back. FYI - Now that the carb is back on the road with the replacement float needle, the bike no longer runs into issue on constant demand dual carriageways and the plug colour shows as it should (rich from upjetting on the way to Belfast), rather than the hot/weak that it previously showed on the same jetting and in same test conditions.

The lesson that we learnt from this is not to assume that any supplied part is actually a like for like swap, regardless of source, and that even needles that appear to be the same length and girth can suffer from minor production differences.

Adam

PS - We now have the chore of revisiting the jetting now that the fuel starvation has been resolved. Yes, we had previously addressed all the other possible causes of fuel starvation (tap rate, tank venting, pipe routing, vibration, body drillings, etc).

[Warkton Tornado No.1]

Thanks for the advice fellas
The kit is the casa ss225, with LI150 gears and a 46 x 15 crown wheel/ sprocket . 30mm dellorto TSR EVO pipe. All fairly common I would think.
Point taken regards plug brand and number, I'll go back to the 8 and look into changing brands for the equivalent.
Thanks again.

Thank you Adam for the correction to the previous explanation concerning the length. As you can see from the other information quoted above, the assumption of the float needle being sprung was based upon that.