If the revs are increasing when you hold the syncro-check against the carb body its a classic sign of a weak mixture. If you have increased the gas flow through the ports you will need to alter the mixture and this may or may not require a new needle. SU jets (old memory now so check these details) of that era came in 2 sizes, plus left and right handed for single and twin installation, its unlikely you will need to rejet unless they are worn but what you describe does not suggest jet wear. Step one is to try and reset the mixture, remember to follow the Plateau tuning method and set to the rich side then back one flat. Short version is to go rich until she stutters when you raise the piston 1/16" to 1/8" and then the revs increase then fall back, from that position weaken one flat only. If you lift the piston from there you should get a very slight 100-150 RPM increase and then it drops back. Much smoke and mirrors about Dashpot oil but before I taught the subject at BL Service Training I spent 4 weeks working at SU Butec in the workshops fitting and setting up the carbs and they ALWAYS used 20/50 engine oil, anything else is too thin unless your in an extreme cold climate then use ATF Fluid. Some will bitch about this and claim 3 in 1 is the way to go, make your choice but engine oil is what SU and BL used in production (the training school was attached to the Triumph plant at Canley and I know what they used on the line!). Syncro both carbs air flow rates and balance them so you don't have a 'lead' carb, I used to use tissue paper under the throttle stops (idle screws) as you pull the throttle cable the airflow from the cooling fan should blow both pieces of tissue away together if both linkages are lifting at the same time.....simple and a whole lot cheaper than a pair of air flow gauges ;-).
Last edited by Spanner Juggler; 05/01/15 11:32 AM.
