16-08-2024, 03:46 PM
Dashpot drop test figures are dependant the SU model not the car so there is no reason that Burlen can't give you the details for the particular SU carb number that you are using.
The last car that I had an SU on developed the sort of mid range hesitation that has been mentioned on this thread. Cleaning the dashpot interior with methylated spirit showed several bright spots where there had been contact with the piston. I removed these with wet and dry 2000, lubricated with thin oil, applied very gently to just the bright spots until there was no evidence of contact the you spun the piston (with piston rod lubricated with WD40) with the dashpot inverted and allowed it to fall under gravity. It is easy to hear if there is any piston to dashpot contact. This resolved the problem.
The carb had originally been set up by my local rolling road by an SU expert. His method was firstly to ensure that the SU piston would rise to nearly the top under full power adjusting piston weight, (or spring strength if carb had one). He then started with a needle that was weak over the full range and then proceeded to delicately file a flat progressively along the taper starting at the top of the needle and checking the mixture was right at that point and then increasing engine load to raise the needle about an 1/8th off an inch and repeating the filing till the mixture was correct for the new needle position. In this way he tuned in the needle along its whole length with a variable filed flat along the length. That the needle had a flat is not important as it is the relevant cross sectional area difference between needle and jet that controls the flow of fuel. Apart from when the piston/damper pick up occurred I never needed to adjust the carb again whilst I owned the car
The last car that I had an SU on developed the sort of mid range hesitation that has been mentioned on this thread. Cleaning the dashpot interior with methylated spirit showed several bright spots where there had been contact with the piston. I removed these with wet and dry 2000, lubricated with thin oil, applied very gently to just the bright spots until there was no evidence of contact the you spun the piston (with piston rod lubricated with WD40) with the dashpot inverted and allowed it to fall under gravity. It is easy to hear if there is any piston to dashpot contact. This resolved the problem.
The carb had originally been set up by my local rolling road by an SU expert. His method was firstly to ensure that the SU piston would rise to nearly the top under full power adjusting piston weight, (or spring strength if carb had one). He then started with a needle that was weak over the full range and then proceeded to delicately file a flat progressively along the taper starting at the top of the needle and checking the mixture was right at that point and then increasing engine load to raise the needle about an 1/8th off an inch and repeating the filing till the mixture was correct for the new needle position. In this way he tuned in the needle along its whole length with a variable filed flat along the length. That the needle had a flat is not important as it is the relevant cross sectional area difference between needle and jet that controls the flow of fuel. Apart from when the piston/damper pick up occurred I never needed to adjust the carb again whilst I owned the car