04-04-2025, 07:43 AM
Hello Nick
As I am sure you know, the SU is designed so that the piston rises to accommodate increased airflow, whilst simultaneously withdrawing the tapered needle from the jet to give increased fuel flow. The needle taper is chosen to give a more or less constant Air/Fuel ratio over the whiole range.
With a downdraught SU (Horizontal Dashpot) there has to be a light spring for the piston to work against, unlike the more commonplace sidedraught SU (Vertical Dashpot) which can use gravity or combined gravity and spring. The horizontal piston would seem to me to be more at the mercy of slight wear which might cause it to stick somewhere in its travel, restricting the airflow to the engine and thus its top end power. The clearance between the upper part of the piston and its dashpot is very small, and the two are a matched assembly which cannot easily be swapped with others
I suggest you check the opening with your fingers and make sure it is genuinely moving freely over the whole travel. Burlen have quite a lot of technical info and drawings on their website, but I have attached SU's 1958 Manual which may be useful.
As I am sure you know, the SU is designed so that the piston rises to accommodate increased airflow, whilst simultaneously withdrawing the tapered needle from the jet to give increased fuel flow. The needle taper is chosen to give a more or less constant Air/Fuel ratio over the whiole range.
With a downdraught SU (Horizontal Dashpot) there has to be a light spring for the piston to work against, unlike the more commonplace sidedraught SU (Vertical Dashpot) which can use gravity or combined gravity and spring. The horizontal piston would seem to me to be more at the mercy of slight wear which might cause it to stick somewhere in its travel, restricting the airflow to the engine and thus its top end power. The clearance between the upper part of the piston and its dashpot is very small, and the two are a matched assembly which cannot easily be swapped with others
I suggest you check the opening with your fingers and make sure it is genuinely moving freely over the whole travel. Burlen have quite a lot of technical info and drawings on their website, but I have attached SU's 1958 Manual which may be useful.